diff --git a/exercises/lesson03/1/bl4ckout31/Makefile b/exercises/lesson03/1/bl4ckout31/Makefile
new file mode 100644
index 00000000..4f92a490
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/Makefile
@@ -0,0 +1,31 @@
+ARMGNU ?= aarch64-linux-gnu
+
+COPS = -Wall -nostdlib -nostartfiles -ffreestanding -Iinclude -mgeneral-regs-only
+ASMOPS = -Iinclude 
+
+BUILD_DIR = build
+SRC_DIR = src
+
+all : kernel8.img
+
+clean :
+	rm -rf $(BUILD_DIR) *.img 
+
+$(BUILD_DIR)/%_c.o: $(SRC_DIR)/%.c
+	mkdir -p $(@D)
+	$(ARMGNU)-gcc $(COPS) -MMD -c $< -o $@
+
+$(BUILD_DIR)/%_s.o: $(SRC_DIR)/%.S
+	$(ARMGNU)-gcc $(ASMOPS) -MMD -c $< -o $@
+
+C_FILES = $(wildcard $(SRC_DIR)/*.c)
+ASM_FILES = $(wildcard $(SRC_DIR)/*.S)
+OBJ_FILES = $(C_FILES:$(SRC_DIR)/%.c=$(BUILD_DIR)/%_c.o)
+OBJ_FILES += $(ASM_FILES:$(SRC_DIR)/%.S=$(BUILD_DIR)/%_s.o)
+
+DEP_FILES = $(OBJ_FILES:%.o=%.d)
+-include $(DEP_FILES)
+
+kernel8.img: $(SRC_DIR)/linker.ld $(OBJ_FILES)
+	$(ARMGNU)-ld -T $(SRC_DIR)/linker.ld -o $(BUILD_DIR)/kernel8.elf  $(OBJ_FILES)
+	$(ARMGNU)-objcopy $(BUILD_DIR)/kernel8.elf -O binary kernel8.img
diff --git a/exercises/lesson03/1/bl4ckout31/build.bat b/exercises/lesson03/1/bl4ckout31/build.bat
new file mode 100755
index 00000000..26eb1b5e
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/build.bat
@@ -0,0 +1 @@
+docker run --rm -v %cd%:/app -w /app smatyukevich/raspberry-pi-os-builder make %1
diff --git a/exercises/lesson03/1/bl4ckout31/build.sh b/exercises/lesson03/1/bl4ckout31/build.sh
new file mode 100755
index 00000000..56cbff29
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/build.sh
@@ -0,0 +1,3 @@
+#!/bin/bash
+
+docker run --rm -v $(pwd):/app -w /app smatyukevich/raspberry-pi-os-builder make $1
diff --git a/exercises/lesson03/1/bl4ckout31/include/arm/sysregs.h b/exercises/lesson03/1/bl4ckout31/include/arm/sysregs.h
new file mode 100644
index 00000000..90853534
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/include/arm/sysregs.h
@@ -0,0 +1,42 @@
+#ifndef _SYSREGS_H
+#define _SYSREGS_H
+
+// ***************************************
+// SCTLR_EL1, System Control Register (EL1), Page 2654 of AArch64-Reference-Manual.
+// ***************************************
+
+#define SCTLR_RESERVED                  (3 << 28) | (3 << 22) | (1 << 20) | (1 << 11)
+#define SCTLR_EE_LITTLE_ENDIAN          (0 << 25)
+#define SCTLR_EOE_LITTLE_ENDIAN         (0 << 24)
+#define SCTLR_I_CACHE_DISABLED          (0 << 12)
+#define SCTLR_D_CACHE_DISABLED          (0 << 2)
+#define SCTLR_MMU_DISABLED              (0 << 0)
+#define SCTLR_MMU_ENABLED               (1 << 0)
+
+#define SCTLR_VALUE_MMU_DISABLED	(SCTLR_RESERVED | SCTLR_EE_LITTLE_ENDIAN | SCTLR_I_CACHE_DISABLED | SCTLR_D_CACHE_DISABLED | SCTLR_MMU_DISABLED)
+
+// ***************************************
+// HCR_EL2, Hypervisor Configuration Register (EL2), Page 2487 of AArch64-Reference-Manual.
+// ***************************************
+
+#define HCR_RW	    			(1 << 31)
+#define HCR_VALUE			HCR_RW
+
+// ***************************************
+// SCR_EL3, Secure Configuration Register (EL3), Page 2648 of AArch64-Reference-Manual.
+// ***************************************
+
+#define SCR_RESERVED	    		(3 << 4)
+#define SCR_RW				(1 << 10)
+#define SCR_NS				(1 << 0)
+#define SCR_VALUE	    	    	(SCR_RESERVED | SCR_RW | SCR_NS)
+
+// ***************************************
+// SPSR_EL3, Saved Program Status Register (EL3) Page 389 of AArch64-Reference-Manual.
+// ***************************************
+
+#define SPSR_MASK_ALL 			(7 << 6)
+#define SPSR_EL1h			(5 << 0)
+#define SPSR_VALUE			(SPSR_MASK_ALL | SPSR_EL1h)
+
+#endif
diff --git a/exercises/lesson03/1/bl4ckout31/include/entry.h b/exercises/lesson03/1/bl4ckout31/include/entry.h
new file mode 100644
index 00000000..cb800cfe
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/include/entry.h
@@ -0,0 +1,26 @@
+#ifndef _ENTRY_H
+#define _ENTRY_H
+
+#define S_FRAME_SIZE			256 		// size of all saved registers 
+
+#define SYNC_INVALID_EL1t		0 
+#define IRQ_INVALID_EL1t		1 
+#define FIQ_INVALID_EL1t		2 
+#define ERROR_INVALID_EL1t		3 
+
+#define SYNC_INVALID_EL1h		4 
+#define IRQ_INVALID_EL1h		5 
+#define FIQ_INVALID_EL1h		6 
+#define ERROR_INVALID_EL1h		7 
+
+#define SYNC_INVALID_EL0_64	    	8 
+#define IRQ_INVALID_EL0_64	    	9 
+#define FIQ_INVALID_EL0_64		10 
+#define ERROR_INVALID_EL0_64		11 
+
+#define SYNC_INVALID_EL0_32		12 
+#define IRQ_INVALID_EL0_32		13 
+#define FIQ_INVALID_EL0_32		14 
+#define ERROR_INVALID_EL0_32		15 
+
+#endif
diff --git a/exercises/lesson03/1/bl4ckout31/include/irq.h b/exercises/lesson03/1/bl4ckout31/include/irq.h
new file mode 100644
index 00000000..0e35437b
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/include/irq.h
@@ -0,0 +1,10 @@
+#ifndef	_IRQ_H
+#define	_IRQ_H
+
+void enable_interrupt_controller( void );
+
+void irq_vector_init( void );
+void enable_irq( void );
+void disable_irq( void );
+
+#endif  /*_IRQ_H */
diff --git a/exercises/lesson03/1/bl4ckout31/include/mini_uart.h b/exercises/lesson03/1/bl4ckout31/include/mini_uart.h
new file mode 100644
index 00000000..e3bd1372
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/include/mini_uart.h
@@ -0,0 +1,9 @@
+#ifndef	_MINI_UART_H
+#define	_MINI_UART_H
+
+void uart_init ( void );
+char uart_recv ( void );
+void uart_send ( char c );
+void putc ( void* p, char c );
+
+#endif  /*_MINI_UART_H */
diff --git a/exercises/lesson03/1/bl4ckout31/include/mm.h b/exercises/lesson03/1/bl4ckout31/include/mm.h
new file mode 100644
index 00000000..784ac4b8
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/include/mm.h
@@ -0,0 +1,19 @@
+#ifndef	_MM_H
+#define	_MM_H
+
+#define PAGE_SHIFT	 		12
+#define TABLE_SHIFT 			9
+#define SECTION_SHIFT			(PAGE_SHIFT + TABLE_SHIFT)
+
+#define PAGE_SIZE   			(1 << PAGE_SHIFT)	
+#define SECTION_SIZE			(1 << SECTION_SHIFT)	
+
+#define LOW_MEMORY              	(2 * SECTION_SIZE)
+
+#ifndef __ASSEMBLER__
+
+void memzero(unsigned long src, unsigned long n);
+
+#endif
+
+#endif  /*_MM_H */
diff --git a/exercises/lesson03/1/bl4ckout31/include/peripherals/base.h b/exercises/lesson03/1/bl4ckout31/include/peripherals/base.h
new file mode 100644
index 00000000..2ef4c57b
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/include/peripherals/base.h
@@ -0,0 +1,7 @@
+#ifndef	_P_BASE_H
+#define	_P_BASE_H
+
+#define PBASE  0x3F000000
+#define LPBASE 0x40000000
+
+#endif  /*_P_BASE_H */
diff --git a/exercises/lesson03/1/bl4ckout31/include/peripherals/gpio.h b/exercises/lesson03/1/bl4ckout31/include/peripherals/gpio.h
new file mode 100644
index 00000000..5e9af26a
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/include/peripherals/gpio.h
@@ -0,0 +1,12 @@
+#ifndef	_P_GPIO_H
+#define	_P_GPIO_H
+
+#include "peripherals/base.h"
+
+#define GPFSEL1         (PBASE+0x00200004)
+#define GPSET0          (PBASE+0x0020001C)
+#define GPCLR0          (PBASE+0x00200028)
+#define GPPUD           (PBASE+0x00200094)
+#define GPPUDCLK0       (PBASE+0x00200098)
+
+#endif  /*_P_GPIO_H */
diff --git a/exercises/lesson03/1/bl4ckout31/include/peripherals/irq.h b/exercises/lesson03/1/bl4ckout31/include/peripherals/irq.h
new file mode 100644
index 00000000..1d35eb51
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/include/peripherals/irq.h
@@ -0,0 +1,28 @@
+#ifndef	_P_IRQ_H
+#define	_P_IRQ_H
+
+#include "peripherals/base.h"
+
+#define IRQ_BASIC_PENDING	(PBASE+0x0000B200)
+#define IRQ_PENDING_1		(PBASE+0x0000B204)
+#define IRQ_PENDING_2		(PBASE+0x0000B208)
+#define FIQ_CONTROL		    (PBASE+0x0000B20C)
+#define ENABLE_IRQS_1		(PBASE+0x0000B210)
+#define ENABLE_IRQS_2		(PBASE+0x0000B214)
+#define ENABLE_BASIC_IRQS	(PBASE+0x0000B218)
+#define DISABLE_IRQS_1		(PBASE+0x0000B21C)
+#define DISABLE_IRQS_2		(PBASE+0x0000B220)
+#define DISABLE_BASIC_IRQS	(PBASE+0x0000B224)
+
+#define SYSTEM_TIMER_IRQ_0	(1 << 0)
+#define SYSTEM_TIMER_IRQ_1	(1 << 1)
+#define SYSTEM_TIMER_IRQ_2	(1 << 2)
+#define SYSTEM_TIMER_IRQ_3	(1 << 3)
+
+// See BCM2836 ARM-local peripherals at
+// https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2836/QA7_rev3.4.pdf
+
+#define CORE0_INTERRUPT_SOURCES (LPBASE+0x60)
+#define LTIMER_INTERRUPT (1 << 11)
+
+#endif  /*_P_IRQ_H */
diff --git a/exercises/lesson03/1/bl4ckout31/include/peripherals/mini_uart.h b/exercises/lesson03/1/bl4ckout31/include/peripherals/mini_uart.h
new file mode 100644
index 00000000..af95a3be
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/include/peripherals/mini_uart.h
@@ -0,0 +1,19 @@
+#ifndef	_P_MINI_UART_H
+#define	_P_MINI_UART_H
+
+#include "peripherals/base.h"
+
+#define AUX_ENABLES     (PBASE+0x00215004)
+#define AUX_MU_IO_REG   (PBASE+0x00215040)
+#define AUX_MU_IER_REG  (PBASE+0x00215044)
+#define AUX_MU_IIR_REG  (PBASE+0x00215048)
+#define AUX_MU_LCR_REG  (PBASE+0x0021504C)
+#define AUX_MU_MCR_REG  (PBASE+0x00215050)
+#define AUX_MU_LSR_REG  (PBASE+0x00215054)
+#define AUX_MU_MSR_REG  (PBASE+0x00215058)
+#define AUX_MU_SCRATCH  (PBASE+0x0021505C)
+#define AUX_MU_CNTL_REG (PBASE+0x00215060)
+#define AUX_MU_STAT_REG (PBASE+0x00215064)
+#define AUX_MU_BAUD_REG (PBASE+0x00215068)
+
+#endif  /*_P_MINI_UART_H */
diff --git a/exercises/lesson03/1/bl4ckout31/include/peripherals/timer.h b/exercises/lesson03/1/bl4ckout31/include/peripherals/timer.h
new file mode 100644
index 00000000..6e078fcc
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/include/peripherals/timer.h
@@ -0,0 +1,31 @@
+#ifndef	_P_TIMER_H
+#define	_P_TIMER_H
+
+#include "peripherals/base.h"
+
+#define TIMER_CS        (PBASE+0x00003000)
+#define TIMER_CLO       (PBASE+0x00003004)
+#define TIMER_CHI       (PBASE+0x00003008)
+#define TIMER_C0        (PBASE+0x0000300C)
+#define TIMER_C1        (PBASE+0x00003010)
+#define TIMER_C2        (PBASE+0x00003014)
+#define TIMER_C3        (PBASE+0x00003018)
+
+#define TIMER_CS_M0	(1 << 0)
+#define TIMER_CS_M1	(1 << 1)
+#define TIMER_CS_M2	(1 << 2)
+#define TIMER_CS_M3	(1 << 3)
+
+// See BCM2836 ARM-local peripherals at
+// https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2836/QA7_rev3.4.pdf
+
+#define LTIMER_CTRL    (LPBASE+0x34)
+#define LTIMER_CLR     (LPBASE+0x38)
+
+#define LTIMER_CTRL_EN     (1 << 28)
+#define LTIMER_CTRL_INT_EN (1 << 29)
+#define LTIMER_CTRL_VALUE  (LTIMER_CTRL_EN | LTIMER_CTRL_INT_EN)
+
+#define LTIMER_CLR_ACK (1 << 31)
+
+#endif  /*_P_TIMER_H */
diff --git a/exercises/lesson03/1/bl4ckout31/include/printf.h b/exercises/lesson03/1/bl4ckout31/include/printf.h
new file mode 100644
index 00000000..f37c1bf2
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/include/printf.h
@@ -0,0 +1,106 @@
+/*
+File: printf.h
+
+Copyright (C) 2004  Kustaa Nyholm
+
+This library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Lesser General Public
+License as published by the Free Software Foundation; either
+version 2.1 of the License, or (at your option) any later version.
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+See the GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public
+License along with this library; if not, write to the Free Software
+Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+
+This library is realy just two files: 'printf.h' and 'printf.c'.
+
+They provide a simple and small (+200 loc) printf functionality to
+be used in embedded systems.
+
+I've found them so usefull in debugging that I do not bother with a
+debugger at all.
+
+They are distributed in source form, so to use them, just compile them
+into your project.
+
+Two printf variants are provided: printf and sprintf.
+
+The formats supported by this implementation are: 'd' 'u' 'c' 's' 'x' 'X'.
+
+Zero padding and field width are also supported.
+
+If the library is compiled with 'PRINTF_SUPPORT_LONG' defined then the
+long specifier is also
+supported. Note that this will pull in some long math routines (pun intended!)
+and thus make your executable noticably longer.
+
+The memory foot print of course depends on the target cpu, compiler and
+compiler options, but a rough guestimate (based on a H8S target) is about
+1.4 kB for code and some twenty 'int's and 'char's, say 60 bytes of stack space.
+Not too bad. Your milage may vary. By hacking the source code you can
+get rid of some hunred bytes, I'm sure, but personally I feel the balance of
+functionality and flexibility versus  code size is close to optimal for
+many embedded systems.
+
+To use the printf you need to supply your own character output function,
+something like :
+
+	void putc ( void* p, char c)
+		{
+		while (!SERIAL_PORT_EMPTY) ;
+		SERIAL_PORT_TX_REGISTER = c;
+		}
+
+Before you can call printf you need to initialize it to use your
+character output function with something like:
+
+	init_printf(NULL,putc);
+
+Notice the 'NULL' in 'init_printf' and the parameter 'void* p' in 'putc',
+the NULL (or any pointer) you pass into the 'init_printf' will eventually be
+passed to your 'putc' routine. This allows you to pass some storage space (or
+anything realy) to the character output function, if necessary.
+This is not often needed but it was implemented like that because it made
+implementing the sprintf function so neat (look at the source code).
+
+The code is re-entrant, except for the 'init_printf' function, so it
+is safe to call it from interupts too, although this may result in mixed output.
+If you rely on re-entrancy, take care that your 'putc' function is re-entrant!
+
+The printf and sprintf functions are actually macros that translate to
+'tfp_printf' and 'tfp_sprintf'. This makes it possible
+to use them along with 'stdio.h' printf's in a single source file.
+You just need to undef the names before you include the 'stdio.h'.
+Note that these are not function like macros, so if you have variables
+or struct members with these names, things will explode in your face.
+Without variadic macros this is the best we can do to wrap these
+fucnction. If it is a problem just give up the macros and use the
+functions directly or rename them.
+
+For further details see source code.
+
+regs Kusti, 23.10.2004
+*/
+
+
+#ifndef __TFP_PRINTF__
+#define __TFP_PRINTF__
+
+#include <stdarg.h>
+
+void init_printf(void* putp,void (*putf) (void*,char));
+
+void tfp_printf(char *fmt, ...);
+void tfp_sprintf(char* s,char *fmt, ...);
+
+void tfp_format(void* putp,void (*putf) (void*,char),char *fmt, va_list va);
+
+#define printf tfp_printf
+#define sprintf tfp_sprintf
+
+#endif
diff --git a/exercises/lesson03/1/bl4ckout31/include/timer.h b/exercises/lesson03/1/bl4ckout31/include/timer.h
new file mode 100644
index 00000000..f8088c5e
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/include/timer.h
@@ -0,0 +1,10 @@
+#ifndef	_TIMER_H
+#define	_TIMER_H
+
+void timer_init ( void );
+void handle_timer_irq ( void );
+
+void local_timer_init ( void );
+void handle_local_timer_irq ( void );
+
+#endif  /*_TIMER_H */
diff --git a/exercises/lesson03/1/bl4ckout31/include/utils.h b/exercises/lesson03/1/bl4ckout31/include/utils.h
new file mode 100644
index 00000000..20fb61f2
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/include/utils.h
@@ -0,0 +1,9 @@
+#ifndef	_UTILS_H
+#define	_UTILS_H
+
+extern void delay ( unsigned long);
+extern void put32 ( unsigned long, unsigned int );
+extern unsigned int get32 ( unsigned long );
+extern int get_el ( void );
+
+#endif  /*_UTILS_H */
diff --git a/exercises/lesson03/1/bl4ckout31/src/boot.S b/exercises/lesson03/1/bl4ckout31/src/boot.S
new file mode 100644
index 00000000..6db1006e
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/src/boot.S
@@ -0,0 +1,43 @@
+#include "arm/sysregs.h"
+
+#include "mm.h"
+
+.section ".text.boot"
+
+.globl _start
+_start:
+	mrs	x0, mpidr_el1		
+	and	x0, x0,#0xFF		// Check processor id
+	cbz	x0, master		// Hang for all non-primary CPU
+	b	proc_hang
+
+proc_hang: 
+	b 	proc_hang
+
+master:
+	ldr	x0, =SCTLR_VALUE_MMU_DISABLED
+	msr	sctlr_el1, x0		
+
+	ldr	x0, =HCR_VALUE
+	msr	hcr_el2, x0
+
+	ldr	x0, =SCR_VALUE
+	msr	scr_el3, x0
+
+	ldr	x0, =SPSR_VALUE
+	msr	spsr_el3, x0
+
+	adr	x0, el1_entry		
+	msr	elr_el3, x0
+
+	eret				
+
+el1_entry:
+	adr	x0, bss_begin
+	adr	x1, bss_end
+	sub	x1, x1, x0
+	bl 	memzero
+
+	mov	sp, #LOW_MEMORY
+	bl	kernel_main
+	b 	proc_hang		// should never come here
diff --git a/exercises/lesson03/1/bl4ckout31/src/config.txt b/exercises/lesson03/1/bl4ckout31/src/config.txt
new file mode 100644
index 00000000..27934969
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/src/config.txt
@@ -0,0 +1,2 @@
+kernel_old=1
+disable_commandline_tags=1
diff --git a/exercises/lesson03/1/bl4ckout31/src/entry.S b/exercises/lesson03/1/bl4ckout31/src/entry.S
new file mode 100644
index 00000000..18a5e0c3
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/src/entry.S
@@ -0,0 +1,136 @@
+#include "entry.h"
+
+	.macro handle_invalid_entry type
+	kernel_entry
+	mov	x0, #\type
+	mrs	x1, esr_el1
+	mrs	x2, elr_el1
+	bl	show_invalid_entry_message
+	b	err_hang
+	.endm
+
+	.macro	ventry	label
+	.align	7
+	b	\label
+	.endm
+
+	.macro	kernel_entry
+	sub	sp, sp, #S_FRAME_SIZE
+	stp	x0, x1, [sp, #16 * 0]
+	stp	x2, x3, [sp, #16 * 1]
+	stp	x4, x5, [sp, #16 * 2]
+	stp	x6, x7, [sp, #16 * 3]
+	stp	x8, x9, [sp, #16 * 4]
+	stp	x10, x11, [sp, #16 * 5]
+	stp	x12, x13, [sp, #16 * 6]
+	stp	x14, x15, [sp, #16 * 7]
+	stp	x16, x17, [sp, #16 * 8]
+	stp	x18, x19, [sp, #16 * 9]
+	stp	x20, x21, [sp, #16 * 10]
+	stp	x22, x23, [sp, #16 * 11]
+	stp	x24, x25, [sp, #16 * 12]
+	stp	x26, x27, [sp, #16 * 13]
+	stp	x28, x29, [sp, #16 * 14]
+	str	x30, [sp, #16 * 15] 
+	.endm
+
+	.macro	kernel_exit
+	ldp	x0, x1, [sp, #16 * 0]
+	ldp	x2, x3, [sp, #16 * 1]
+	ldp	x4, x5, [sp, #16 * 2]
+	ldp	x6, x7, [sp, #16 * 3]
+	ldp	x8, x9, [sp, #16 * 4]
+	ldp	x10, x11, [sp, #16 * 5]
+	ldp	x12, x13, [sp, #16 * 6]
+	ldp	x14, x15, [sp, #16 * 7]
+	ldp	x16, x17, [sp, #16 * 8]
+	ldp	x18, x19, [sp, #16 * 9]
+	ldp	x20, x21, [sp, #16 * 10]
+	ldp	x22, x23, [sp, #16 * 11]
+	ldp	x24, x25, [sp, #16 * 12]
+	ldp	x26, x27, [sp, #16 * 13]
+	ldp	x28, x29, [sp, #16 * 14]
+	ldr	x30, [sp, #16 * 15] 
+	add	sp, sp, #S_FRAME_SIZE		
+	eret
+	.endm
+
+
+/*
+ * Exception vectors.
+ */
+.align	11
+.globl vectors 
+vectors:
+	ventry	sync_invalid_el1t			// Synchronous EL1t
+	ventry	irq_invalid_el1t			// IRQ EL1t
+	ventry	fiq_invalid_el1t			// FIQ EL1t
+	ventry	error_invalid_el1t			// Error EL1t
+
+	ventry	sync_invalid_el1h			// Synchronous EL1h
+	ventry	el1_irq					// IRQ EL1h
+	ventry	fiq_invalid_el1h			// FIQ EL1h
+	ventry	error_invalid_el1h			// Error EL1h
+
+	ventry	sync_invalid_el0_64			// Synchronous 64-bit EL0
+	ventry	irq_invalid_el0_64			// IRQ 64-bit EL0
+	ventry	fiq_invalid_el0_64			// FIQ 64-bit EL0
+	ventry	error_invalid_el0_64			// Error 64-bit EL0
+
+	ventry	sync_invalid_el0_32			// Synchronous 32-bit EL0
+	ventry	irq_invalid_el0_32			// IRQ 32-bit EL0
+	ventry	fiq_invalid_el0_32			// FIQ 32-bit EL0
+	ventry	error_invalid_el0_32			// Error 32-bit EL0
+
+sync_invalid_el1t:
+	handle_invalid_entry  SYNC_INVALID_EL1t
+
+irq_invalid_el1t:
+	handle_invalid_entry  IRQ_INVALID_EL1t
+
+fiq_invalid_el1t:
+	handle_invalid_entry  FIQ_INVALID_EL1t
+
+error_invalid_el1t:
+	handle_invalid_entry  ERROR_INVALID_EL1t
+
+sync_invalid_el1h:
+	handle_invalid_entry  SYNC_INVALID_EL1h
+
+fiq_invalid_el1h:
+	handle_invalid_entry  FIQ_INVALID_EL1h
+
+error_invalid_el1h:
+	handle_invalid_entry  ERROR_INVALID_EL1h
+
+sync_invalid_el0_64:
+	handle_invalid_entry  SYNC_INVALID_EL0_64
+
+irq_invalid_el0_64:
+	handle_invalid_entry  IRQ_INVALID_EL0_64
+
+fiq_invalid_el0_64:
+	handle_invalid_entry  FIQ_INVALID_EL0_64
+
+error_invalid_el0_64:
+	handle_invalid_entry  ERROR_INVALID_EL0_64
+
+sync_invalid_el0_32:
+	handle_invalid_entry  SYNC_INVALID_EL0_32
+
+irq_invalid_el0_32:
+	handle_invalid_entry  IRQ_INVALID_EL0_32
+
+fiq_invalid_el0_32:
+	handle_invalid_entry  FIQ_INVALID_EL0_32
+
+error_invalid_el0_32:
+	handle_invalid_entry  ERROR_INVALID_EL0_32
+
+el1_irq:
+	kernel_entry 
+	bl	handle_irq
+	kernel_exit 
+
+.globl err_hang
+err_hang: b err_hang
diff --git a/exercises/lesson03/1/bl4ckout31/src/irq.S b/exercises/lesson03/1/bl4ckout31/src/irq.S
new file mode 100644
index 00000000..1c10ed2b
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/src/irq.S
@@ -0,0 +1,15 @@
+.globl irq_vector_init
+irq_vector_init:
+	adr	x0, vectors		// load VBAR_EL1 with virtual
+	msr	vbar_el1, x0		// vector table address
+	ret
+
+.globl enable_irq
+enable_irq:
+	msr    daifclr, #2 
+	ret
+
+.globl disable_irq
+disable_irq:
+	msr	daifset, #2
+	ret
diff --git a/exercises/lesson03/1/bl4ckout31/src/irq.c b/exercises/lesson03/1/bl4ckout31/src/irq.c
new file mode 100644
index 00000000..d4772d7d
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/src/irq.c
@@ -0,0 +1,57 @@
+#include "utils.h"
+#include "printf.h"
+#include "timer.h"
+#include "entry.h"
+#include "peripherals/irq.h"
+
+const char *entry_error_messages[] = {
+    "SYNC_INVALID_EL1t",
+    "IRQ_INVALID_EL1t",		
+    "FIQ_INVALID_EL1t",		
+    "ERROR_INVALID_EL1T",		
+
+    "SYNC_INVALID_EL1h",		
+    "IRQ_INVALID_EL1h",		
+    "FIQ_INVALID_EL1h",		
+    "ERROR_INVALID_EL1h",		
+
+    "SYNC_INVALID_EL0_64",		
+    "IRQ_INVALID_EL0_64",		
+    "FIQ_INVALID_EL0_64",		
+    "ERROR_INVALID_EL0_64",	
+
+    "SYNC_INVALID_EL0_32",		
+    "IRQ_INVALID_EL0_32",		
+    "FIQ_INVALID_EL0_32",		
+    "ERROR_INVALID_EL0_32"	
+};
+
+void enable_interrupt_controller()
+{
+    /** The local interrupt controller can't be enable or disable.
+     *  Instead, we are supposed to rely on the interrupt sources
+     *  registers to enable or disable their interrupts
+     *  
+     *  Neverless, we can still change wich core will receive the
+     *  interrupts. They go to Core 0 IRQ by default, which is what
+     *  we want. This function becomes effectively useless. 
+     */
+}
+
+void show_invalid_entry_message(int type, unsigned long esr, unsigned long address)
+{
+    printf("%s, ESR: %x, address: %x\r\n", entry_error_messages[type], esr, address);
+}
+
+void handle_irq(void)
+{
+    // Each Core has its own pending local intrrupts register
+    unsigned int irq = get32(CORE0_INTERRUPT_SOURCES);
+    switch (irq) {
+        case (LTIMER_INTERRUPT):
+            handle_local_timer_irq();
+            break;
+        default:
+            printf("Unknown pending irq: %x\r\n", irq);
+    }
+}
\ No newline at end of file
diff --git a/exercises/lesson03/1/bl4ckout31/src/kernel.c b/exercises/lesson03/1/bl4ckout31/src/kernel.c
new file mode 100644
index 00000000..4b69af56
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/src/kernel.c
@@ -0,0 +1,19 @@
+#include "printf.h"
+#include "timer.h"
+#include "irq.h"
+#include "mini_uart.h"
+
+
+void kernel_main(void)
+{
+	uart_init();
+	init_printf(0, putc);
+	irq_vector_init();
+	local_timer_init();
+	enable_interrupt_controller();
+	enable_irq();
+
+	while (1){
+		uart_send(uart_recv());
+	}	
+}
diff --git a/exercises/lesson03/1/bl4ckout31/src/linker.ld b/exercises/lesson03/1/bl4ckout31/src/linker.ld
new file mode 100644
index 00000000..16fe2acc
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/src/linker.ld
@@ -0,0 +1,11 @@
+SECTIONS
+{
+	.text.boot : { *(.text.boot) }
+	.text : { *(.text) }
+	.rodata : { *(.rodata) }
+	.data : { *(.data) }
+	. = ALIGN(0x8);
+	bss_begin = .;
+	.bss : { *(.bss*) } 
+	bss_end = .;
+}
diff --git a/exercises/lesson03/1/bl4ckout31/src/mini_uart.c b/exercises/lesson03/1/bl4ckout31/src/mini_uart.c
new file mode 100644
index 00000000..988f4782
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/src/mini_uart.c
@@ -0,0 +1,62 @@
+#include "utils.h"
+#include "peripherals/mini_uart.h"
+#include "peripherals/gpio.h"
+
+void uart_send ( char c )
+{
+	while(1) {
+		if(get32(AUX_MU_LSR_REG)&0x20) 
+			break;
+	}
+	put32(AUX_MU_IO_REG,c);
+}
+
+char uart_recv ( void )
+{
+	while(1) {
+		if(get32(AUX_MU_LSR_REG)&0x01) 
+			break;
+	}
+	return(get32(AUX_MU_IO_REG)&0xFF);
+}
+
+void uart_send_string(char* str)
+{
+	for (int i = 0; str[i] != '\0'; i ++) {
+		uart_send((char)str[i]);
+	}
+}
+
+void uart_init ( void )
+{
+	unsigned int selector;
+
+	selector = get32(GPFSEL1);
+	selector &= ~(7<<12);                   // clean gpio14
+	selector |= 2<<12;                      // set alt5 for gpio14
+	selector &= ~(7<<15);                   // clean gpio15
+	selector |= 2<<15;                      // set alt5 for gpio15
+	put32(GPFSEL1,selector);
+
+	put32(GPPUD,0);
+	delay(150);
+	put32(GPPUDCLK0,(1<<14)|(1<<15));
+	delay(150);
+	put32(GPPUDCLK0,0);
+
+	put32(AUX_ENABLES,1);                   //Enable mini uart (this also enables access to it registers)
+	put32(AUX_MU_CNTL_REG,0);               //Disable auto flow control and disable receiver and transmitter (for now)
+	put32(AUX_MU_IER_REG,0);                //Disable receive and transmit interrupts
+	put32(AUX_MU_LCR_REG,3);                //Enable 8 bit mode
+	put32(AUX_MU_MCR_REG,0);                //Set RTS line to be always high
+	put32(AUX_MU_BAUD_REG,270);             //Set baud rate to 115200
+
+	put32(AUX_MU_CNTL_REG,3);               //Finally, enable transmitter and receiver
+}
+
+
+// This function is required by printf function
+void putc ( void* p, char c)
+{
+	uart_send(c);
+}
diff --git a/exercises/lesson03/1/bl4ckout31/src/mm.S b/exercises/lesson03/1/bl4ckout31/src/mm.S
new file mode 100644
index 00000000..1bd32ff3
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/src/mm.S
@@ -0,0 +1,6 @@
+.globl memzero
+memzero:
+	str xzr, [x0], #8
+	subs x1, x1, #8
+	b.gt memzero
+	ret
diff --git a/exercises/lesson03/1/bl4ckout31/src/printf.c b/exercises/lesson03/1/bl4ckout31/src/printf.c
new file mode 100644
index 00000000..1383039a
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/src/printf.c
@@ -0,0 +1,233 @@
+/*
+File: printf.c
+
+Copyright (C) 2004  Kustaa Nyholm
+
+This library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Lesser General Public
+License as published by the Free Software Foundation; either
+version 2.1 of the License, or (at your option) any later version.
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public
+License along with this library; if not, write to the Free Software
+Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+
+*/
+
+#include "printf.h"
+
+typedef void (*putcf) (void*,char);
+static putcf stdout_putf;
+static void* stdout_putp;
+
+
+#ifdef PRINTF_LONG_SUPPORT
+
+static void uli2a(unsigned long int num, unsigned int base, int uc,char * bf)
+    {
+    int n=0;
+    unsigned int d=1;
+    while (num/d >= base)
+        d*=base;
+    while (d!=0) {
+        int dgt = num / d;
+        num%=d;
+        d/=base;
+        if (n || dgt>0|| d==0) {
+            *bf++ = dgt+(dgt<10 ? '0' : (uc ? 'A' : 'a')-10);
+            ++n;
+            }
+        }
+    *bf=0;
+    }
+
+static void li2a (long num, char * bf)
+    {
+    if (num<0) {
+        num=-num;
+        *bf++ = '-';
+        }
+    uli2a(num,10,0,bf);
+    }
+
+#endif
+
+static void ui2a(unsigned int num, unsigned int base, int uc,char * bf)
+    {
+    int n=0;
+    unsigned int d=1;
+    while (num/d >= base)
+        d*=base;
+    while (d!=0) {
+        int dgt = num / d;
+        num%= d;
+        d/=base;
+        if (n || dgt>0 || d==0) {
+            *bf++ = dgt+(dgt<10 ? '0' : (uc ? 'A' : 'a')-10);
+            ++n;
+            }
+        }
+    *bf=0;
+    }
+
+static void i2a (int num, char * bf)
+    {
+    if (num<0) {
+        num=-num;
+        *bf++ = '-';
+        }
+    ui2a(num,10,0,bf);
+    }
+
+static int a2d(char ch)
+    {
+    if (ch>='0' && ch<='9')
+        return ch-'0';
+    else if (ch>='a' && ch<='f')
+        return ch-'a'+10;
+    else if (ch>='A' && ch<='F')
+        return ch-'A'+10;
+    else return -1;
+    }
+
+static char a2i(char ch, char** src,int base,int* nump)
+    {
+    char* p= *src;
+    int num=0;
+    int digit;
+    while ((digit=a2d(ch))>=0) {
+        if (digit>base) break;
+        num=num*base+digit;
+        ch=*p++;
+        }
+    *src=p;
+    *nump=num;
+    return ch;
+    }
+
+static void putchw(void* putp,putcf putf,int n, char z, char* bf)
+    {
+    char fc=z? '0' : ' ';
+    char ch;
+    char* p=bf;
+    while (*p++ && n > 0)
+        n--;
+    while (n-- > 0)
+        putf(putp,fc);
+    while ((ch= *bf++))
+        putf(putp,ch);
+    }
+
+void tfp_format(void* putp,putcf putf,char *fmt, va_list va)
+    {
+    char bf[12];
+
+    char ch;
+
+
+    while ((ch=*(fmt++))) {
+        if (ch!='%')
+            putf(putp,ch);
+        else {
+            char lz=0;
+#ifdef  PRINTF_LONG_SUPPORT
+            char lng=0;
+#endif
+            int w=0;
+            ch=*(fmt++);
+            if (ch=='0') {
+                ch=*(fmt++);
+                lz=1;
+                }
+            if (ch>='0' && ch<='9') {
+                ch=a2i(ch,&fmt,10,&w);
+                }
+#ifdef  PRINTF_LONG_SUPPORT
+            if (ch=='l') {
+                ch=*(fmt++);
+                lng=1;
+            }
+#endif
+            switch (ch) {
+                case 0:
+                    goto abort;
+                case 'u' : {
+#ifdef  PRINTF_LONG_SUPPORT
+                    if (lng)
+                        uli2a(va_arg(va, unsigned long int),10,0,bf);
+                    else
+#endif
+                    ui2a(va_arg(va, unsigned int),10,0,bf);
+                    putchw(putp,putf,w,lz,bf);
+                    break;
+                    }
+                case 'd' :  {
+#ifdef  PRINTF_LONG_SUPPORT
+                    if (lng)
+                        li2a(va_arg(va, unsigned long int),bf);
+                    else
+#endif
+                    i2a(va_arg(va, int),bf);
+                    putchw(putp,putf,w,lz,bf);
+                    break;
+                    }
+                case 'x': case 'X' :
+#ifdef  PRINTF_LONG_SUPPORT
+                    if (lng)
+                        uli2a(va_arg(va, unsigned long int),16,(ch=='X'),bf);
+                    else
+#endif
+                    ui2a(va_arg(va, unsigned int),16,(ch=='X'),bf);
+                    putchw(putp,putf,w,lz,bf);
+                    break;
+                case 'c' :
+                    putf(putp,(char)(va_arg(va, int)));
+                    break;
+                case 's' :
+                    putchw(putp,putf,w,0,va_arg(va, char*));
+                    break;
+                case '%' :
+                    putf(putp,ch);
+                default:
+                    break;
+                }
+            }
+        }
+    abort:;
+    }
+
+
+void init_printf(void* putp,void (*putf) (void*,char))
+    {
+    stdout_putf=putf;
+    stdout_putp=putp;
+    }
+
+void tfp_printf(char *fmt, ...)
+    {
+    va_list va;
+    va_start(va,fmt);
+    tfp_format(stdout_putp,stdout_putf,fmt,va);
+    va_end(va);
+    }
+
+static void putcp(void* p,char c)
+    {
+    *(*((char**)p))++ = c;
+    }
+
+
+
+void tfp_sprintf(char* s,char *fmt, ...)
+    {
+    va_list va;
+    va_start(va,fmt);
+    tfp_format(&s,putcp,fmt,va);
+    putcp(&s,0);
+    va_end(va);
+    }
diff --git a/exercises/lesson03/1/bl4ckout31/src/timer.c b/exercises/lesson03/1/bl4ckout31/src/timer.c
new file mode 100644
index 00000000..ade15196
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/src/timer.c
@@ -0,0 +1,32 @@
+#include "utils.h"
+#include "printf.h"
+#include "peripherals/timer.h"
+
+const unsigned int interval = 9600000;
+unsigned int curVal = 0;
+
+void timer_init ( void )
+{
+	curVal = get32(TIMER_CLO);
+	curVal += interval;
+	put32(TIMER_C1, curVal);
+}
+
+void handle_timer_irq( void ) 
+{
+	curVal += interval;
+	put32(TIMER_C1, curVal);
+	put32(TIMER_CS, TIMER_CS_M1);
+	printf("Timer interrupt received\n\r");
+}
+
+void local_timer_init ( void )
+{
+	put32(LTIMER_CTRL, (interval | LTIMER_CTRL_VALUE));
+}
+
+void handle_local_timer_irq( void ) 
+{
+	printf("Timer interrupt received\n\r");
+	put32(LTIMER_CLR, LTIMER_CLR_ACK);
+}
\ No newline at end of file
diff --git a/exercises/lesson03/1/bl4ckout31/src/utils.S b/exercises/lesson03/1/bl4ckout31/src/utils.S
new file mode 100644
index 00000000..44be4857
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/src/utils.S
@@ -0,0 +1,21 @@
+.globl get_el
+get_el:
+	mrs x0, CurrentEL
+	lsr x0, x0, #2
+	ret
+
+.globl put32
+put32:
+	str w1,[x0]
+	ret
+
+.globl get32
+get32:
+	ldr w0,[x0]
+	ret
+
+.globl delay
+delay:
+	subs x0, x0, #1
+	bne delay
+	ret
diff --git a/exercises/lesson03/1/bl4ckout31/start.sh b/exercises/lesson03/1/bl4ckout31/start.sh
new file mode 100755
index 00000000..522e5563
--- /dev/null
+++ b/exercises/lesson03/1/bl4ckout31/start.sh
@@ -0,0 +1,10 @@
+#!/bin/bash
+
+if [ $# -ne 1 ]; then
+    echo "usage: $0 kernel"
+    exit 1
+fi
+
+mount -L rpiboot /mnt
+cp "$1" /mnt
+umount /mnt
diff --git a/exercises/lesson03/2/bl4ckout31/Makefile b/exercises/lesson03/2/bl4ckout31/Makefile
new file mode 100644
index 00000000..4f92a490
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/Makefile
@@ -0,0 +1,31 @@
+ARMGNU ?= aarch64-linux-gnu
+
+COPS = -Wall -nostdlib -nostartfiles -ffreestanding -Iinclude -mgeneral-regs-only
+ASMOPS = -Iinclude 
+
+BUILD_DIR = build
+SRC_DIR = src
+
+all : kernel8.img
+
+clean :
+	rm -rf $(BUILD_DIR) *.img 
+
+$(BUILD_DIR)/%_c.o: $(SRC_DIR)/%.c
+	mkdir -p $(@D)
+	$(ARMGNU)-gcc $(COPS) -MMD -c $< -o $@
+
+$(BUILD_DIR)/%_s.o: $(SRC_DIR)/%.S
+	$(ARMGNU)-gcc $(ASMOPS) -MMD -c $< -o $@
+
+C_FILES = $(wildcard $(SRC_DIR)/*.c)
+ASM_FILES = $(wildcard $(SRC_DIR)/*.S)
+OBJ_FILES = $(C_FILES:$(SRC_DIR)/%.c=$(BUILD_DIR)/%_c.o)
+OBJ_FILES += $(ASM_FILES:$(SRC_DIR)/%.S=$(BUILD_DIR)/%_s.o)
+
+DEP_FILES = $(OBJ_FILES:%.o=%.d)
+-include $(DEP_FILES)
+
+kernel8.img: $(SRC_DIR)/linker.ld $(OBJ_FILES)
+	$(ARMGNU)-ld -T $(SRC_DIR)/linker.ld -o $(BUILD_DIR)/kernel8.elf  $(OBJ_FILES)
+	$(ARMGNU)-objcopy $(BUILD_DIR)/kernel8.elf -O binary kernel8.img
diff --git a/exercises/lesson03/2/bl4ckout31/build.bat b/exercises/lesson03/2/bl4ckout31/build.bat
new file mode 100755
index 00000000..26eb1b5e
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/build.bat
@@ -0,0 +1 @@
+docker run --rm -v %cd%:/app -w /app smatyukevich/raspberry-pi-os-builder make %1
diff --git a/exercises/lesson03/2/bl4ckout31/build.sh b/exercises/lesson03/2/bl4ckout31/build.sh
new file mode 100755
index 00000000..56cbff29
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/build.sh
@@ -0,0 +1,3 @@
+#!/bin/bash
+
+docker run --rm -v $(pwd):/app -w /app smatyukevich/raspberry-pi-os-builder make $1
diff --git a/exercises/lesson03/2/bl4ckout31/include/arm/sysregs.h b/exercises/lesson03/2/bl4ckout31/include/arm/sysregs.h
new file mode 100644
index 00000000..90853534
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/include/arm/sysregs.h
@@ -0,0 +1,42 @@
+#ifndef _SYSREGS_H
+#define _SYSREGS_H
+
+// ***************************************
+// SCTLR_EL1, System Control Register (EL1), Page 2654 of AArch64-Reference-Manual.
+// ***************************************
+
+#define SCTLR_RESERVED                  (3 << 28) | (3 << 22) | (1 << 20) | (1 << 11)
+#define SCTLR_EE_LITTLE_ENDIAN          (0 << 25)
+#define SCTLR_EOE_LITTLE_ENDIAN         (0 << 24)
+#define SCTLR_I_CACHE_DISABLED          (0 << 12)
+#define SCTLR_D_CACHE_DISABLED          (0 << 2)
+#define SCTLR_MMU_DISABLED              (0 << 0)
+#define SCTLR_MMU_ENABLED               (1 << 0)
+
+#define SCTLR_VALUE_MMU_DISABLED	(SCTLR_RESERVED | SCTLR_EE_LITTLE_ENDIAN | SCTLR_I_CACHE_DISABLED | SCTLR_D_CACHE_DISABLED | SCTLR_MMU_DISABLED)
+
+// ***************************************
+// HCR_EL2, Hypervisor Configuration Register (EL2), Page 2487 of AArch64-Reference-Manual.
+// ***************************************
+
+#define HCR_RW	    			(1 << 31)
+#define HCR_VALUE			HCR_RW
+
+// ***************************************
+// SCR_EL3, Secure Configuration Register (EL3), Page 2648 of AArch64-Reference-Manual.
+// ***************************************
+
+#define SCR_RESERVED	    		(3 << 4)
+#define SCR_RW				(1 << 10)
+#define SCR_NS				(1 << 0)
+#define SCR_VALUE	    	    	(SCR_RESERVED | SCR_RW | SCR_NS)
+
+// ***************************************
+// SPSR_EL3, Saved Program Status Register (EL3) Page 389 of AArch64-Reference-Manual.
+// ***************************************
+
+#define SPSR_MASK_ALL 			(7 << 6)
+#define SPSR_EL1h			(5 << 0)
+#define SPSR_VALUE			(SPSR_MASK_ALL | SPSR_EL1h)
+
+#endif
diff --git a/exercises/lesson03/2/bl4ckout31/include/entry.h b/exercises/lesson03/2/bl4ckout31/include/entry.h
new file mode 100644
index 00000000..cb800cfe
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/include/entry.h
@@ -0,0 +1,26 @@
+#ifndef _ENTRY_H
+#define _ENTRY_H
+
+#define S_FRAME_SIZE			256 		// size of all saved registers 
+
+#define SYNC_INVALID_EL1t		0 
+#define IRQ_INVALID_EL1t		1 
+#define FIQ_INVALID_EL1t		2 
+#define ERROR_INVALID_EL1t		3 
+
+#define SYNC_INVALID_EL1h		4 
+#define IRQ_INVALID_EL1h		5 
+#define FIQ_INVALID_EL1h		6 
+#define ERROR_INVALID_EL1h		7 
+
+#define SYNC_INVALID_EL0_64	    	8 
+#define IRQ_INVALID_EL0_64	    	9 
+#define FIQ_INVALID_EL0_64		10 
+#define ERROR_INVALID_EL0_64		11 
+
+#define SYNC_INVALID_EL0_32		12 
+#define IRQ_INVALID_EL0_32		13 
+#define FIQ_INVALID_EL0_32		14 
+#define ERROR_INVALID_EL0_32		15 
+
+#endif
diff --git a/exercises/lesson03/2/bl4ckout31/include/irq.h b/exercises/lesson03/2/bl4ckout31/include/irq.h
new file mode 100644
index 00000000..0e35437b
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/include/irq.h
@@ -0,0 +1,10 @@
+#ifndef	_IRQ_H
+#define	_IRQ_H
+
+void enable_interrupt_controller( void );
+
+void irq_vector_init( void );
+void enable_irq( void );
+void disable_irq( void );
+
+#endif  /*_IRQ_H */
diff --git a/exercises/lesson03/2/bl4ckout31/include/mini_uart.h b/exercises/lesson03/2/bl4ckout31/include/mini_uart.h
new file mode 100644
index 00000000..d2dd4cbc
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/include/mini_uart.h
@@ -0,0 +1,11 @@
+#ifndef	_MINI_UART_H
+#define	_MINI_UART_H
+
+void uart_init ( void );
+char uart_recv ( void );
+void uart_send ( char c );
+void putc ( void* p, char c );
+
+void handle_uart_irq ( void );
+
+#endif  /*_MINI_UART_H */
diff --git a/exercises/lesson03/2/bl4ckout31/include/mm.h b/exercises/lesson03/2/bl4ckout31/include/mm.h
new file mode 100644
index 00000000..784ac4b8
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/include/mm.h
@@ -0,0 +1,19 @@
+#ifndef	_MM_H
+#define	_MM_H
+
+#define PAGE_SHIFT	 		12
+#define TABLE_SHIFT 			9
+#define SECTION_SHIFT			(PAGE_SHIFT + TABLE_SHIFT)
+
+#define PAGE_SIZE   			(1 << PAGE_SHIFT)	
+#define SECTION_SIZE			(1 << SECTION_SHIFT)	
+
+#define LOW_MEMORY              	(2 * SECTION_SIZE)
+
+#ifndef __ASSEMBLER__
+
+void memzero(unsigned long src, unsigned long n);
+
+#endif
+
+#endif  /*_MM_H */
diff --git a/exercises/lesson03/2/bl4ckout31/include/peripherals/base.h b/exercises/lesson03/2/bl4ckout31/include/peripherals/base.h
new file mode 100644
index 00000000..63f9c038
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/include/peripherals/base.h
@@ -0,0 +1,6 @@
+#ifndef	_P_BASE_H
+#define	_P_BASE_H
+
+#define PBASE 0x3F000000
+
+#endif  /*_P_BASE_H */
diff --git a/exercises/lesson03/2/bl4ckout31/include/peripherals/gpio.h b/exercises/lesson03/2/bl4ckout31/include/peripherals/gpio.h
new file mode 100644
index 00000000..5e9af26a
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/include/peripherals/gpio.h
@@ -0,0 +1,12 @@
+#ifndef	_P_GPIO_H
+#define	_P_GPIO_H
+
+#include "peripherals/base.h"
+
+#define GPFSEL1         (PBASE+0x00200004)
+#define GPSET0          (PBASE+0x0020001C)
+#define GPCLR0          (PBASE+0x00200028)
+#define GPPUD           (PBASE+0x00200094)
+#define GPPUDCLK0       (PBASE+0x00200098)
+
+#endif  /*_P_GPIO_H */
diff --git a/exercises/lesson03/2/bl4ckout31/include/peripherals/irq.h b/exercises/lesson03/2/bl4ckout31/include/peripherals/irq.h
new file mode 100644
index 00000000..731d47a6
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/include/peripherals/irq.h
@@ -0,0 +1,27 @@
+#ifndef	_P_IRQ_H
+#define	_P_IRQ_H
+
+#include "peripherals/base.h"
+
+#define IRQ_BASIC_PENDING	(PBASE+0x0000B200)
+#define IRQ_PENDING_1		(PBASE+0x0000B204)
+#define IRQ_PENDING_2		(PBASE+0x0000B208)
+#define FIQ_CONTROL		    (PBASE+0x0000B20C)
+#define ENABLE_IRQS_1		(PBASE+0x0000B210)
+#define ENABLE_IRQS_2		(PBASE+0x0000B214)
+#define ENABLE_BASIC_IRQS	(PBASE+0x0000B218)
+#define DISABLE_IRQS_1		(PBASE+0x0000B21C)
+#define DISABLE_IRQS_2		(PBASE+0x0000B220)
+#define DISABLE_BASIC_IRQS	(PBASE+0x0000B224)
+
+#define SYSTEM_TIMER_IRQ_0	(1 << 0)
+#define SYSTEM_TIMER_IRQ_1	(1 << 1)
+#define SYSTEM_TIMER_IRQ_2	(1 << 2)
+#define SYSTEM_TIMER_IRQ_3	(1 << 3)
+
+// See BCM2537 ARM Peripherals p113 at 
+// https://github.com/raspberrypi/documentation/files/1888662/BCM2837-ARM-Peripherals.-.Revised.-.V2-1.pdf 
+
+#define AUX_IRQ (1 << 29)
+
+#endif  /*_P_IRQ_H */
diff --git a/exercises/lesson03/2/bl4ckout31/include/peripherals/mini_uart.h b/exercises/lesson03/2/bl4ckout31/include/peripherals/mini_uart.h
new file mode 100644
index 00000000..5341dc86
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/include/peripherals/mini_uart.h
@@ -0,0 +1,29 @@
+#ifndef	_P_MINI_UART_H
+#define	_P_MINI_UART_H
+
+#include "peripherals/base.h"
+
+#define AUX_IRQ_REG     (PBASE+0x00215000)
+#define AUX_ENABLES     (PBASE+0x00215004)
+#define AUX_MU_IO_REG   (PBASE+0x00215040)
+#define AUX_MU_IER_REG  (PBASE+0x00215044)
+#define AUX_MU_IIR_REG  (PBASE+0x00215048)
+#define AUX_MU_LCR_REG  (PBASE+0x0021504C)
+#define AUX_MU_MCR_REG  (PBASE+0x00215050)
+#define AUX_MU_LSR_REG  (PBASE+0x00215054)
+#define AUX_MU_MSR_REG  (PBASE+0x00215058)
+#define AUX_MU_SCRATCH  (PBASE+0x0021505C)
+#define AUX_MU_CNTL_REG (PBASE+0x00215060)
+#define AUX_MU_STAT_REG (PBASE+0x00215064)
+#define AUX_MU_BAUD_REG (PBASE+0x00215068)
+
+// See BCM2537 ARM Peripherals p12 at 
+// https://github.com/raspberrypi/documentation/files/1888662/BCM2837-ARM-Peripherals.-.Revised.-.V2-1.pdf 
+
+#define IER_REG_EN_REC_INT (1 << 0)
+#define IER_REG_INT        (3 << 2) // Must be set to receive interrupts
+#define IER_REG_VALUE      (IER_REG_EN_REC_INT | IER_REG_INT)
+
+#define IIR_REG_REC_NON_EMPTY (2 << 1)
+
+#endif  /*_P_MINI_UART_H */
diff --git a/exercises/lesson03/2/bl4ckout31/include/peripherals/timer.h b/exercises/lesson03/2/bl4ckout31/include/peripherals/timer.h
new file mode 100644
index 00000000..8a52085d
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/include/peripherals/timer.h
@@ -0,0 +1,19 @@
+#ifndef	_P_TIMER_H
+#define	_P_TIMER_H
+
+#include "peripherals/base.h"
+
+#define TIMER_CS        (PBASE+0x00003000)
+#define TIMER_CLO       (PBASE+0x00003004)
+#define TIMER_CHI       (PBASE+0x00003008)
+#define TIMER_C0        (PBASE+0x0000300C)
+#define TIMER_C1        (PBASE+0x00003010)
+#define TIMER_C2        (PBASE+0x00003014)
+#define TIMER_C3        (PBASE+0x00003018)
+
+#define TIMER_CS_M0	(1 << 0)
+#define TIMER_CS_M1	(1 << 1)
+#define TIMER_CS_M2	(1 << 2)
+#define TIMER_CS_M3	(1 << 3)
+
+#endif  /*_P_TIMER_H */
diff --git a/exercises/lesson03/2/bl4ckout31/include/printf.h b/exercises/lesson03/2/bl4ckout31/include/printf.h
new file mode 100644
index 00000000..f37c1bf2
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/include/printf.h
@@ -0,0 +1,106 @@
+/*
+File: printf.h
+
+Copyright (C) 2004  Kustaa Nyholm
+
+This library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Lesser General Public
+License as published by the Free Software Foundation; either
+version 2.1 of the License, or (at your option) any later version.
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+See the GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public
+License along with this library; if not, write to the Free Software
+Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+
+This library is realy just two files: 'printf.h' and 'printf.c'.
+
+They provide a simple and small (+200 loc) printf functionality to
+be used in embedded systems.
+
+I've found them so usefull in debugging that I do not bother with a
+debugger at all.
+
+They are distributed in source form, so to use them, just compile them
+into your project.
+
+Two printf variants are provided: printf and sprintf.
+
+The formats supported by this implementation are: 'd' 'u' 'c' 's' 'x' 'X'.
+
+Zero padding and field width are also supported.
+
+If the library is compiled with 'PRINTF_SUPPORT_LONG' defined then the
+long specifier is also
+supported. Note that this will pull in some long math routines (pun intended!)
+and thus make your executable noticably longer.
+
+The memory foot print of course depends on the target cpu, compiler and
+compiler options, but a rough guestimate (based on a H8S target) is about
+1.4 kB for code and some twenty 'int's and 'char's, say 60 bytes of stack space.
+Not too bad. Your milage may vary. By hacking the source code you can
+get rid of some hunred bytes, I'm sure, but personally I feel the balance of
+functionality and flexibility versus  code size is close to optimal for
+many embedded systems.
+
+To use the printf you need to supply your own character output function,
+something like :
+
+	void putc ( void* p, char c)
+		{
+		while (!SERIAL_PORT_EMPTY) ;
+		SERIAL_PORT_TX_REGISTER = c;
+		}
+
+Before you can call printf you need to initialize it to use your
+character output function with something like:
+
+	init_printf(NULL,putc);
+
+Notice the 'NULL' in 'init_printf' and the parameter 'void* p' in 'putc',
+the NULL (or any pointer) you pass into the 'init_printf' will eventually be
+passed to your 'putc' routine. This allows you to pass some storage space (or
+anything realy) to the character output function, if necessary.
+This is not often needed but it was implemented like that because it made
+implementing the sprintf function so neat (look at the source code).
+
+The code is re-entrant, except for the 'init_printf' function, so it
+is safe to call it from interupts too, although this may result in mixed output.
+If you rely on re-entrancy, take care that your 'putc' function is re-entrant!
+
+The printf and sprintf functions are actually macros that translate to
+'tfp_printf' and 'tfp_sprintf'. This makes it possible
+to use them along with 'stdio.h' printf's in a single source file.
+You just need to undef the names before you include the 'stdio.h'.
+Note that these are not function like macros, so if you have variables
+or struct members with these names, things will explode in your face.
+Without variadic macros this is the best we can do to wrap these
+fucnction. If it is a problem just give up the macros and use the
+functions directly or rename them.
+
+For further details see source code.
+
+regs Kusti, 23.10.2004
+*/
+
+
+#ifndef __TFP_PRINTF__
+#define __TFP_PRINTF__
+
+#include <stdarg.h>
+
+void init_printf(void* putp,void (*putf) (void*,char));
+
+void tfp_printf(char *fmt, ...);
+void tfp_sprintf(char* s,char *fmt, ...);
+
+void tfp_format(void* putp,void (*putf) (void*,char),char *fmt, va_list va);
+
+#define printf tfp_printf
+#define sprintf tfp_sprintf
+
+#endif
diff --git a/exercises/lesson03/2/bl4ckout31/include/timer.h b/exercises/lesson03/2/bl4ckout31/include/timer.h
new file mode 100644
index 00000000..eb9ef6bd
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/include/timer.h
@@ -0,0 +1,7 @@
+#ifndef	_TIMER_H
+#define	_TIMER_H
+
+void timer_init ( void );
+void handle_timer_irq ( void );
+
+#endif  /*_TIMER_H */
diff --git a/exercises/lesson03/2/bl4ckout31/include/utils.h b/exercises/lesson03/2/bl4ckout31/include/utils.h
new file mode 100644
index 00000000..20fb61f2
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/include/utils.h
@@ -0,0 +1,9 @@
+#ifndef	_UTILS_H
+#define	_UTILS_H
+
+extern void delay ( unsigned long);
+extern void put32 ( unsigned long, unsigned int );
+extern unsigned int get32 ( unsigned long );
+extern int get_el ( void );
+
+#endif  /*_UTILS_H */
diff --git a/exercises/lesson03/2/bl4ckout31/src/boot.S b/exercises/lesson03/2/bl4ckout31/src/boot.S
new file mode 100644
index 00000000..6db1006e
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/src/boot.S
@@ -0,0 +1,43 @@
+#include "arm/sysregs.h"
+
+#include "mm.h"
+
+.section ".text.boot"
+
+.globl _start
+_start:
+	mrs	x0, mpidr_el1		
+	and	x0, x0,#0xFF		// Check processor id
+	cbz	x0, master		// Hang for all non-primary CPU
+	b	proc_hang
+
+proc_hang: 
+	b 	proc_hang
+
+master:
+	ldr	x0, =SCTLR_VALUE_MMU_DISABLED
+	msr	sctlr_el1, x0		
+
+	ldr	x0, =HCR_VALUE
+	msr	hcr_el2, x0
+
+	ldr	x0, =SCR_VALUE
+	msr	scr_el3, x0
+
+	ldr	x0, =SPSR_VALUE
+	msr	spsr_el3, x0
+
+	adr	x0, el1_entry		
+	msr	elr_el3, x0
+
+	eret				
+
+el1_entry:
+	adr	x0, bss_begin
+	adr	x1, bss_end
+	sub	x1, x1, x0
+	bl 	memzero
+
+	mov	sp, #LOW_MEMORY
+	bl	kernel_main
+	b 	proc_hang		// should never come here
diff --git a/exercises/lesson03/2/bl4ckout31/src/config.txt b/exercises/lesson03/2/bl4ckout31/src/config.txt
new file mode 100644
index 00000000..27934969
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/src/config.txt
@@ -0,0 +1,2 @@
+kernel_old=1
+disable_commandline_tags=1
diff --git a/exercises/lesson03/2/bl4ckout31/src/entry.S b/exercises/lesson03/2/bl4ckout31/src/entry.S
new file mode 100644
index 00000000..18a5e0c3
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/src/entry.S
@@ -0,0 +1,136 @@
+#include "entry.h"
+
+	.macro handle_invalid_entry type
+	kernel_entry
+	mov	x0, #\type
+	mrs	x1, esr_el1
+	mrs	x2, elr_el1
+	bl	show_invalid_entry_message
+	b	err_hang
+	.endm
+
+	.macro	ventry	label
+	.align	7
+	b	\label
+	.endm
+
+	.macro	kernel_entry
+	sub	sp, sp, #S_FRAME_SIZE
+	stp	x0, x1, [sp, #16 * 0]
+	stp	x2, x3, [sp, #16 * 1]
+	stp	x4, x5, [sp, #16 * 2]
+	stp	x6, x7, [sp, #16 * 3]
+	stp	x8, x9, [sp, #16 * 4]
+	stp	x10, x11, [sp, #16 * 5]
+	stp	x12, x13, [sp, #16 * 6]
+	stp	x14, x15, [sp, #16 * 7]
+	stp	x16, x17, [sp, #16 * 8]
+	stp	x18, x19, [sp, #16 * 9]
+	stp	x20, x21, [sp, #16 * 10]
+	stp	x22, x23, [sp, #16 * 11]
+	stp	x24, x25, [sp, #16 * 12]
+	stp	x26, x27, [sp, #16 * 13]
+	stp	x28, x29, [sp, #16 * 14]
+	str	x30, [sp, #16 * 15] 
+	.endm
+
+	.macro	kernel_exit
+	ldp	x0, x1, [sp, #16 * 0]
+	ldp	x2, x3, [sp, #16 * 1]
+	ldp	x4, x5, [sp, #16 * 2]
+	ldp	x6, x7, [sp, #16 * 3]
+	ldp	x8, x9, [sp, #16 * 4]
+	ldp	x10, x11, [sp, #16 * 5]
+	ldp	x12, x13, [sp, #16 * 6]
+	ldp	x14, x15, [sp, #16 * 7]
+	ldp	x16, x17, [sp, #16 * 8]
+	ldp	x18, x19, [sp, #16 * 9]
+	ldp	x20, x21, [sp, #16 * 10]
+	ldp	x22, x23, [sp, #16 * 11]
+	ldp	x24, x25, [sp, #16 * 12]
+	ldp	x26, x27, [sp, #16 * 13]
+	ldp	x28, x29, [sp, #16 * 14]
+	ldr	x30, [sp, #16 * 15] 
+	add	sp, sp, #S_FRAME_SIZE		
+	eret
+	.endm
+
+
+/*
+ * Exception vectors.
+ */
+.align	11
+.globl vectors 
+vectors:
+	ventry	sync_invalid_el1t			// Synchronous EL1t
+	ventry	irq_invalid_el1t			// IRQ EL1t
+	ventry	fiq_invalid_el1t			// FIQ EL1t
+	ventry	error_invalid_el1t			// Error EL1t
+
+	ventry	sync_invalid_el1h			// Synchronous EL1h
+	ventry	el1_irq					// IRQ EL1h
+	ventry	fiq_invalid_el1h			// FIQ EL1h
+	ventry	error_invalid_el1h			// Error EL1h
+
+	ventry	sync_invalid_el0_64			// Synchronous 64-bit EL0
+	ventry	irq_invalid_el0_64			// IRQ 64-bit EL0
+	ventry	fiq_invalid_el0_64			// FIQ 64-bit EL0
+	ventry	error_invalid_el0_64			// Error 64-bit EL0
+
+	ventry	sync_invalid_el0_32			// Synchronous 32-bit EL0
+	ventry	irq_invalid_el0_32			// IRQ 32-bit EL0
+	ventry	fiq_invalid_el0_32			// FIQ 32-bit EL0
+	ventry	error_invalid_el0_32			// Error 32-bit EL0
+
+sync_invalid_el1t:
+	handle_invalid_entry  SYNC_INVALID_EL1t
+
+irq_invalid_el1t:
+	handle_invalid_entry  IRQ_INVALID_EL1t
+
+fiq_invalid_el1t:
+	handle_invalid_entry  FIQ_INVALID_EL1t
+
+error_invalid_el1t:
+	handle_invalid_entry  ERROR_INVALID_EL1t
+
+sync_invalid_el1h:
+	handle_invalid_entry  SYNC_INVALID_EL1h
+
+fiq_invalid_el1h:
+	handle_invalid_entry  FIQ_INVALID_EL1h
+
+error_invalid_el1h:
+	handle_invalid_entry  ERROR_INVALID_EL1h
+
+sync_invalid_el0_64:
+	handle_invalid_entry  SYNC_INVALID_EL0_64
+
+irq_invalid_el0_64:
+	handle_invalid_entry  IRQ_INVALID_EL0_64
+
+fiq_invalid_el0_64:
+	handle_invalid_entry  FIQ_INVALID_EL0_64
+
+error_invalid_el0_64:
+	handle_invalid_entry  ERROR_INVALID_EL0_64
+
+sync_invalid_el0_32:
+	handle_invalid_entry  SYNC_INVALID_EL0_32
+
+irq_invalid_el0_32:
+	handle_invalid_entry  IRQ_INVALID_EL0_32
+
+fiq_invalid_el0_32:
+	handle_invalid_entry  FIQ_INVALID_EL0_32
+
+error_invalid_el0_32:
+	handle_invalid_entry  ERROR_INVALID_EL0_32
+
+el1_irq:
+	kernel_entry 
+	bl	handle_irq
+	kernel_exit 
+
+.globl err_hang
+err_hang: b err_hang
diff --git a/exercises/lesson03/2/bl4ckout31/src/irq.S b/exercises/lesson03/2/bl4ckout31/src/irq.S
new file mode 100644
index 00000000..1c10ed2b
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/src/irq.S
@@ -0,0 +1,15 @@
+.globl irq_vector_init
+irq_vector_init:
+	adr	x0, vectors		// load VBAR_EL1 with virtual
+	msr	vbar_el1, x0		// vector table address
+	ret
+
+.globl enable_irq
+enable_irq:
+	msr    daifclr, #2 
+	ret
+
+.globl disable_irq
+disable_irq:
+	msr	daifset, #2
+	ret
diff --git a/exercises/lesson03/2/bl4ckout31/src/irq.c b/exercises/lesson03/2/bl4ckout31/src/irq.c
new file mode 100644
index 00000000..10b311ae
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/src/irq.c
@@ -0,0 +1,59 @@
+#include "utils.h"
+#include "printf.h"
+#include "timer.h"
+#include "mini_uart.h"
+#include "entry.h"
+#include "peripherals/irq.h"
+
+const char *entry_error_messages[] = {
+	"SYNC_INVALID_EL1t",
+	"IRQ_INVALID_EL1t",		
+	"FIQ_INVALID_EL1t",		
+	"ERROR_INVALID_EL1T",		
+
+	"SYNC_INVALID_EL1h",		
+	"IRQ_INVALID_EL1h",		
+	"FIQ_INVALID_EL1h",		
+	"ERROR_INVALID_EL1h",		
+
+	"SYNC_INVALID_EL0_64",		
+	"IRQ_INVALID_EL0_64",		
+	"FIQ_INVALID_EL0_64",		
+	"ERROR_INVALID_EL0_64",	
+
+	"SYNC_INVALID_EL0_32",		
+	"IRQ_INVALID_EL0_32",		
+	"FIQ_INVALID_EL0_32",		
+	"ERROR_INVALID_EL0_32"	
+};
+
+void enable_interrupt_controller()
+{
+	put32(ENABLE_IRQS_1, SYSTEM_TIMER_IRQ_1 | AUX_IRQ);
+}
+
+void show_invalid_entry_message(int type, unsigned long esr, unsigned long address)
+{
+	printf("%s, ESR: %x, address: %x\r\n", entry_error_messages[type], esr, address);
+}
+
+void handle_irq(void)
+{
+	unsigned int irq = get32(IRQ_PENDING_1);
+
+	// loop in case multiple interrupts have been raised
+	while (irq) {
+		// each bitflag is only 1 bit so we do not 
+		// need to test equality against themself
+		if (irq & SYSTEM_TIMER_IRQ_1) {
+			handle_timer_irq();
+			irq &= ~SYSTEM_TIMER_IRQ_1;
+		} else if (irq & AUX_IRQ) {
+			handle_uart_irq();
+			irq &= ~AUX_IRQ;
+		} else {
+			printf("Unknown pending irq: %x\r\n", irq);
+			irq = 0;
+		}
+	}
+}
diff --git a/exercises/lesson03/2/bl4ckout31/src/kernel.c b/exercises/lesson03/2/bl4ckout31/src/kernel.c
new file mode 100644
index 00000000..d2e64375
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/src/kernel.c
@@ -0,0 +1,19 @@
+#include "printf.h"
+#include "timer.h"
+#include "irq.h"
+#include "mini_uart.h"
+
+
+void kernel_main(void)
+{
+	uart_init();
+	init_printf(0, putc);
+	irq_vector_init();
+	timer_init();
+	enable_interrupt_controller();
+	enable_irq();
+
+	while (1){
+		//uart_send(uart_recv());
+	}	
+}
diff --git a/exercises/lesson03/2/bl4ckout31/src/linker.ld b/exercises/lesson03/2/bl4ckout31/src/linker.ld
new file mode 100644
index 00000000..16fe2acc
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/src/linker.ld
@@ -0,0 +1,11 @@
+SECTIONS
+{
+	.text.boot : { *(.text.boot) }
+	.text : { *(.text) }
+	.rodata : { *(.rodata) }
+	.data : { *(.data) }
+	. = ALIGN(0x8);
+	bss_begin = .;
+	.bss : { *(.bss*) } 
+	bss_end = .;
+}
diff --git a/exercises/lesson03/2/bl4ckout31/src/mini_uart.c b/exercises/lesson03/2/bl4ckout31/src/mini_uart.c
new file mode 100644
index 00000000..8f74cfc3
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/src/mini_uart.c
@@ -0,0 +1,71 @@
+#include "utils.h"
+#include "peripherals/mini_uart.h"
+#include "peripherals/gpio.h"
+
+void uart_send ( char c )
+{
+	while(1) {
+		if(get32(AUX_MU_LSR_REG)&0x20) 
+			break;
+	}
+	put32(AUX_MU_IO_REG,c);
+}
+
+char uart_recv ( void )
+{
+	while(1) {
+		if(get32(AUX_MU_LSR_REG)&0x01) 
+			break;
+	}
+	return(get32(AUX_MU_IO_REG)&0xFF);
+}
+
+void uart_send_string(char* str)
+{
+	for (int i = 0; str[i] != '\0'; i ++) {
+		uart_send((char)str[i]);
+	}
+}
+
+void uart_init ( void )
+{
+	unsigned int selector;
+
+	selector = get32(GPFSEL1);
+	selector &= ~(7<<12);                   // clean gpio14
+	selector |= 2<<12;                      // set alt5 for gpio14
+	selector &= ~(7<<15);                   // clean gpio15
+	selector |= 2<<15;                      // set alt5 for gpio15
+	put32(GPFSEL1,selector);
+
+	put32(GPPUD,0);
+	delay(150);
+	put32(GPPUDCLK0,(1<<14)|(1<<15));
+	delay(150);
+	put32(GPPUDCLK0,0);
+
+	put32(AUX_ENABLES,1);                   //Enable mini uart (this also enables access to it registers)
+	put32(AUX_MU_CNTL_REG,0);               //Disable auto flow control and disable receiver and transmitter (for now)
+	put32(AUX_MU_IER_REG, IER_REG_VALUE);   //Enable receive interrupts
+	put32(AUX_MU_LCR_REG,3);                //Enable 8 bit mode
+	put32(AUX_MU_MCR_REG,0);                //Set RTS line to be always high
+	put32(AUX_MU_BAUD_REG,270);             //Set baud rate to 115200
+
+	put32(AUX_MU_CNTL_REG,3);               //Finally, enable transmitter and receiver
+}
+
+
+// This function is required by printf function
+void putc ( void* p, char c)
+{
+	uart_send(c);
+}
+
+void handle_uart_irq( void )
+{
+	// There may be more than one byte in the FIFO.
+	while((get32(AUX_MU_IIR_REG) & IIR_REG_REC_NON_EMPTY) ==
+	      IIR_REG_REC_NON_EMPTY) {
+		uart_send(uart_recv()); 
+	}
+}
\ No newline at end of file
diff --git a/exercises/lesson03/2/bl4ckout31/src/mm.S b/exercises/lesson03/2/bl4ckout31/src/mm.S
new file mode 100644
index 00000000..1bd32ff3
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/src/mm.S
@@ -0,0 +1,6 @@
+.globl memzero
+memzero:
+	str xzr, [x0], #8
+	subs x1, x1, #8
+	b.gt memzero
+	ret
diff --git a/exercises/lesson03/2/bl4ckout31/src/printf.c b/exercises/lesson03/2/bl4ckout31/src/printf.c
new file mode 100644
index 00000000..1383039a
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/src/printf.c
@@ -0,0 +1,233 @@
+/*
+File: printf.c
+
+Copyright (C) 2004  Kustaa Nyholm
+
+This library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Lesser General Public
+License as published by the Free Software Foundation; either
+version 2.1 of the License, or (at your option) any later version.
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public
+License along with this library; if not, write to the Free Software
+Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+
+*/
+
+#include "printf.h"
+
+typedef void (*putcf) (void*,char);
+static putcf stdout_putf;
+static void* stdout_putp;
+
+
+#ifdef PRINTF_LONG_SUPPORT
+
+static void uli2a(unsigned long int num, unsigned int base, int uc,char * bf)
+    {
+    int n=0;
+    unsigned int d=1;
+    while (num/d >= base)
+        d*=base;
+    while (d!=0) {
+        int dgt = num / d;
+        num%=d;
+        d/=base;
+        if (n || dgt>0|| d==0) {
+            *bf++ = dgt+(dgt<10 ? '0' : (uc ? 'A' : 'a')-10);
+            ++n;
+            }
+        }
+    *bf=0;
+    }
+
+static void li2a (long num, char * bf)
+    {
+    if (num<0) {
+        num=-num;
+        *bf++ = '-';
+        }
+    uli2a(num,10,0,bf);
+    }
+
+#endif
+
+static void ui2a(unsigned int num, unsigned int base, int uc,char * bf)
+    {
+    int n=0;
+    unsigned int d=1;
+    while (num/d >= base)
+        d*=base;
+    while (d!=0) {
+        int dgt = num / d;
+        num%= d;
+        d/=base;
+        if (n || dgt>0 || d==0) {
+            *bf++ = dgt+(dgt<10 ? '0' : (uc ? 'A' : 'a')-10);
+            ++n;
+            }
+        }
+    *bf=0;
+    }
+
+static void i2a (int num, char * bf)
+    {
+    if (num<0) {
+        num=-num;
+        *bf++ = '-';
+        }
+    ui2a(num,10,0,bf);
+    }
+
+static int a2d(char ch)
+    {
+    if (ch>='0' && ch<='9')
+        return ch-'0';
+    else if (ch>='a' && ch<='f')
+        return ch-'a'+10;
+    else if (ch>='A' && ch<='F')
+        return ch-'A'+10;
+    else return -1;
+    }
+
+static char a2i(char ch, char** src,int base,int* nump)
+    {
+    char* p= *src;
+    int num=0;
+    int digit;
+    while ((digit=a2d(ch))>=0) {
+        if (digit>base) break;
+        num=num*base+digit;
+        ch=*p++;
+        }
+    *src=p;
+    *nump=num;
+    return ch;
+    }
+
+static void putchw(void* putp,putcf putf,int n, char z, char* bf)
+    {
+    char fc=z? '0' : ' ';
+    char ch;
+    char* p=bf;
+    while (*p++ && n > 0)
+        n--;
+    while (n-- > 0)
+        putf(putp,fc);
+    while ((ch= *bf++))
+        putf(putp,ch);
+    }
+
+void tfp_format(void* putp,putcf putf,char *fmt, va_list va)
+    {
+    char bf[12];
+
+    char ch;
+
+
+    while ((ch=*(fmt++))) {
+        if (ch!='%')
+            putf(putp,ch);
+        else {
+            char lz=0;
+#ifdef  PRINTF_LONG_SUPPORT
+            char lng=0;
+#endif
+            int w=0;
+            ch=*(fmt++);
+            if (ch=='0') {
+                ch=*(fmt++);
+                lz=1;
+                }
+            if (ch>='0' && ch<='9') {
+                ch=a2i(ch,&fmt,10,&w);
+                }
+#ifdef  PRINTF_LONG_SUPPORT
+            if (ch=='l') {
+                ch=*(fmt++);
+                lng=1;
+            }
+#endif
+            switch (ch) {
+                case 0:
+                    goto abort;
+                case 'u' : {
+#ifdef  PRINTF_LONG_SUPPORT
+                    if (lng)
+                        uli2a(va_arg(va, unsigned long int),10,0,bf);
+                    else
+#endif
+                    ui2a(va_arg(va, unsigned int),10,0,bf);
+                    putchw(putp,putf,w,lz,bf);
+                    break;
+                    }
+                case 'd' :  {
+#ifdef  PRINTF_LONG_SUPPORT
+                    if (lng)
+                        li2a(va_arg(va, unsigned long int),bf);
+                    else
+#endif
+                    i2a(va_arg(va, int),bf);
+                    putchw(putp,putf,w,lz,bf);
+                    break;
+                    }
+                case 'x': case 'X' :
+#ifdef  PRINTF_LONG_SUPPORT
+                    if (lng)
+                        uli2a(va_arg(va, unsigned long int),16,(ch=='X'),bf);
+                    else
+#endif
+                    ui2a(va_arg(va, unsigned int),16,(ch=='X'),bf);
+                    putchw(putp,putf,w,lz,bf);
+                    break;
+                case 'c' :
+                    putf(putp,(char)(va_arg(va, int)));
+                    break;
+                case 's' :
+                    putchw(putp,putf,w,0,va_arg(va, char*));
+                    break;
+                case '%' :
+                    putf(putp,ch);
+                default:
+                    break;
+                }
+            }
+        }
+    abort:;
+    }
+
+
+void init_printf(void* putp,void (*putf) (void*,char))
+    {
+    stdout_putf=putf;
+    stdout_putp=putp;
+    }
+
+void tfp_printf(char *fmt, ...)
+    {
+    va_list va;
+    va_start(va,fmt);
+    tfp_format(stdout_putp,stdout_putf,fmt,va);
+    va_end(va);
+    }
+
+static void putcp(void* p,char c)
+    {
+    *(*((char**)p))++ = c;
+    }
+
+
+
+void tfp_sprintf(char* s,char *fmt, ...)
+    {
+    va_list va;
+    va_start(va,fmt);
+    tfp_format(&s,putcp,fmt,va);
+    putcp(&s,0);
+    va_end(va);
+    }
diff --git a/exercises/lesson03/2/bl4ckout31/src/timer.c b/exercises/lesson03/2/bl4ckout31/src/timer.c
new file mode 100644
index 00000000..ac01f70e
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/src/timer.c
@@ -0,0 +1,21 @@
+#include "utils.h"
+#include "printf.h"
+#include "peripherals/timer.h"
+
+const unsigned int interval = 200000;
+unsigned int curVal = 0;
+
+void timer_init ( void )
+{
+	curVal = get32(TIMER_CLO);
+	curVal += interval;
+	put32(TIMER_C1, curVal);
+}
+
+void handle_timer_irq( void ) 
+{
+	curVal += interval;
+	put32(TIMER_C1, curVal);
+	put32(TIMER_CS, TIMER_CS_M1);
+	printf("Timer interrupt received\n\r");
+}
diff --git a/exercises/lesson03/2/bl4ckout31/src/utils.S b/exercises/lesson03/2/bl4ckout31/src/utils.S
new file mode 100644
index 00000000..44be4857
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/src/utils.S
@@ -0,0 +1,21 @@
+.globl get_el
+get_el:
+	mrs x0, CurrentEL
+	lsr x0, x0, #2
+	ret
+
+.globl put32
+put32:
+	str w1,[x0]
+	ret
+
+.globl get32
+get32:
+	ldr w0,[x0]
+	ret
+
+.globl delay
+delay:
+	subs x0, x0, #1
+	bne delay
+	ret
diff --git a/exercises/lesson03/2/bl4ckout31/start.sh b/exercises/lesson03/2/bl4ckout31/start.sh
new file mode 100755
index 00000000..522e5563
--- /dev/null
+++ b/exercises/lesson03/2/bl4ckout31/start.sh
@@ -0,0 +1,10 @@
+#!/bin/bash
+
+if [ $# -ne 1 ]; then
+    echo "usage: $0 kernel"
+    exit 1
+fi
+
+mount -L rpiboot /mnt
+cp "$1" /mnt
+umount /mnt
diff --git a/exercises/lesson03/3/bl4ckout31/Makefile b/exercises/lesson03/3/bl4ckout31/Makefile
new file mode 100644
index 00000000..4f92a490
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/Makefile
@@ -0,0 +1,31 @@
+ARMGNU ?= aarch64-linux-gnu
+
+COPS = -Wall -nostdlib -nostartfiles -ffreestanding -Iinclude -mgeneral-regs-only
+ASMOPS = -Iinclude 
+
+BUILD_DIR = build
+SRC_DIR = src
+
+all : kernel8.img
+
+clean :
+	rm -rf $(BUILD_DIR) *.img 
+
+$(BUILD_DIR)/%_c.o: $(SRC_DIR)/%.c
+	mkdir -p $(@D)
+	$(ARMGNU)-gcc $(COPS) -MMD -c $< -o $@
+
+$(BUILD_DIR)/%_s.o: $(SRC_DIR)/%.S
+	$(ARMGNU)-gcc $(ASMOPS) -MMD -c $< -o $@
+
+C_FILES = $(wildcard $(SRC_DIR)/*.c)
+ASM_FILES = $(wildcard $(SRC_DIR)/*.S)
+OBJ_FILES = $(C_FILES:$(SRC_DIR)/%.c=$(BUILD_DIR)/%_c.o)
+OBJ_FILES += $(ASM_FILES:$(SRC_DIR)/%.S=$(BUILD_DIR)/%_s.o)
+
+DEP_FILES = $(OBJ_FILES:%.o=%.d)
+-include $(DEP_FILES)
+
+kernel8.img: $(SRC_DIR)/linker.ld $(OBJ_FILES)
+	$(ARMGNU)-ld -T $(SRC_DIR)/linker.ld -o $(BUILD_DIR)/kernel8.elf  $(OBJ_FILES)
+	$(ARMGNU)-objcopy $(BUILD_DIR)/kernel8.elf -O binary kernel8.img
diff --git a/exercises/lesson03/3/bl4ckout31/build.bat b/exercises/lesson03/3/bl4ckout31/build.bat
new file mode 100755
index 00000000..26eb1b5e
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/build.bat
@@ -0,0 +1 @@
+docker run --rm -v %cd%:/app -w /app smatyukevich/raspberry-pi-os-builder make %1
diff --git a/exercises/lesson03/3/bl4ckout31/build.sh b/exercises/lesson03/3/bl4ckout31/build.sh
new file mode 100755
index 00000000..56cbff29
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/build.sh
@@ -0,0 +1,3 @@
+#!/bin/bash
+
+docker run --rm -v $(pwd):/app -w /app smatyukevich/raspberry-pi-os-builder make $1
diff --git a/exercises/lesson03/3/bl4ckout31/include/arm/sysregs.h b/exercises/lesson03/3/bl4ckout31/include/arm/sysregs.h
new file mode 100644
index 00000000..90853534
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/include/arm/sysregs.h
@@ -0,0 +1,42 @@
+#ifndef _SYSREGS_H
+#define _SYSREGS_H
+
+// ***************************************
+// SCTLR_EL1, System Control Register (EL1), Page 2654 of AArch64-Reference-Manual.
+// ***************************************
+
+#define SCTLR_RESERVED                  (3 << 28) | (3 << 22) | (1 << 20) | (1 << 11)
+#define SCTLR_EE_LITTLE_ENDIAN          (0 << 25)
+#define SCTLR_EOE_LITTLE_ENDIAN         (0 << 24)
+#define SCTLR_I_CACHE_DISABLED          (0 << 12)
+#define SCTLR_D_CACHE_DISABLED          (0 << 2)
+#define SCTLR_MMU_DISABLED              (0 << 0)
+#define SCTLR_MMU_ENABLED               (1 << 0)
+
+#define SCTLR_VALUE_MMU_DISABLED	(SCTLR_RESERVED | SCTLR_EE_LITTLE_ENDIAN | SCTLR_I_CACHE_DISABLED | SCTLR_D_CACHE_DISABLED | SCTLR_MMU_DISABLED)
+
+// ***************************************
+// HCR_EL2, Hypervisor Configuration Register (EL2), Page 2487 of AArch64-Reference-Manual.
+// ***************************************
+
+#define HCR_RW	    			(1 << 31)
+#define HCR_VALUE			HCR_RW
+
+// ***************************************
+// SCR_EL3, Secure Configuration Register (EL3), Page 2648 of AArch64-Reference-Manual.
+// ***************************************
+
+#define SCR_RESERVED	    		(3 << 4)
+#define SCR_RW				(1 << 10)
+#define SCR_NS				(1 << 0)
+#define SCR_VALUE	    	    	(SCR_RESERVED | SCR_RW | SCR_NS)
+
+// ***************************************
+// SPSR_EL3, Saved Program Status Register (EL3) Page 389 of AArch64-Reference-Manual.
+// ***************************************
+
+#define SPSR_MASK_ALL 			(7 << 6)
+#define SPSR_EL1h			(5 << 0)
+#define SPSR_VALUE			(SPSR_MASK_ALL | SPSR_EL1h)
+
+#endif
diff --git a/exercises/lesson03/3/bl4ckout31/include/entry.h b/exercises/lesson03/3/bl4ckout31/include/entry.h
new file mode 100644
index 00000000..cb800cfe
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/include/entry.h
@@ -0,0 +1,26 @@
+#ifndef _ENTRY_H
+#define _ENTRY_H
+
+#define S_FRAME_SIZE			256 		// size of all saved registers 
+
+#define SYNC_INVALID_EL1t		0 
+#define IRQ_INVALID_EL1t		1 
+#define FIQ_INVALID_EL1t		2 
+#define ERROR_INVALID_EL1t		3 
+
+#define SYNC_INVALID_EL1h		4 
+#define IRQ_INVALID_EL1h		5 
+#define FIQ_INVALID_EL1h		6 
+#define ERROR_INVALID_EL1h		7 
+
+#define SYNC_INVALID_EL0_64	    	8 
+#define IRQ_INVALID_EL0_64	    	9 
+#define FIQ_INVALID_EL0_64		10 
+#define ERROR_INVALID_EL0_64		11 
+
+#define SYNC_INVALID_EL0_32		12 
+#define IRQ_INVALID_EL0_32		13 
+#define FIQ_INVALID_EL0_32		14 
+#define ERROR_INVALID_EL0_32		15 
+
+#endif
diff --git a/exercises/lesson03/3/bl4ckout31/include/irq.h b/exercises/lesson03/3/bl4ckout31/include/irq.h
new file mode 100644
index 00000000..0e35437b
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/include/irq.h
@@ -0,0 +1,10 @@
+#ifndef	_IRQ_H
+#define	_IRQ_H
+
+void enable_interrupt_controller( void );
+
+void irq_vector_init( void );
+void enable_irq( void );
+void disable_irq( void );
+
+#endif  /*_IRQ_H */
diff --git a/exercises/lesson03/3/bl4ckout31/include/mini_uart.h b/exercises/lesson03/3/bl4ckout31/include/mini_uart.h
new file mode 100644
index 00000000..e3bd1372
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/include/mini_uart.h
@@ -0,0 +1,9 @@
+#ifndef	_MINI_UART_H
+#define	_MINI_UART_H
+
+void uart_init ( void );
+char uart_recv ( void );
+void uart_send ( char c );
+void putc ( void* p, char c );
+
+#endif  /*_MINI_UART_H */
diff --git a/exercises/lesson03/3/bl4ckout31/include/mm.h b/exercises/lesson03/3/bl4ckout31/include/mm.h
new file mode 100644
index 00000000..784ac4b8
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/include/mm.h
@@ -0,0 +1,19 @@
+#ifndef	_MM_H
+#define	_MM_H
+
+#define PAGE_SHIFT	 		12
+#define TABLE_SHIFT 			9
+#define SECTION_SHIFT			(PAGE_SHIFT + TABLE_SHIFT)
+
+#define PAGE_SIZE   			(1 << PAGE_SHIFT)	
+#define SECTION_SIZE			(1 << SECTION_SHIFT)	
+
+#define LOW_MEMORY              	(2 * SECTION_SIZE)
+
+#ifndef __ASSEMBLER__
+
+void memzero(unsigned long src, unsigned long n);
+
+#endif
+
+#endif  /*_MM_H */
diff --git a/exercises/lesson03/3/bl4ckout31/include/peripherals/base.h b/exercises/lesson03/3/bl4ckout31/include/peripherals/base.h
new file mode 100644
index 00000000..2ef4c57b
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/include/peripherals/base.h
@@ -0,0 +1,7 @@
+#ifndef	_P_BASE_H
+#define	_P_BASE_H
+
+#define PBASE  0x3F000000
+#define LPBASE 0x40000000
+
+#endif  /*_P_BASE_H */
diff --git a/exercises/lesson03/3/bl4ckout31/include/peripherals/gpio.h b/exercises/lesson03/3/bl4ckout31/include/peripherals/gpio.h
new file mode 100644
index 00000000..5e9af26a
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/include/peripherals/gpio.h
@@ -0,0 +1,12 @@
+#ifndef	_P_GPIO_H
+#define	_P_GPIO_H
+
+#include "peripherals/base.h"
+
+#define GPFSEL1         (PBASE+0x00200004)
+#define GPSET0          (PBASE+0x0020001C)
+#define GPCLR0          (PBASE+0x00200028)
+#define GPPUD           (PBASE+0x00200094)
+#define GPPUDCLK0       (PBASE+0x00200098)
+
+#endif  /*_P_GPIO_H */
diff --git a/exercises/lesson03/3/bl4ckout31/include/peripherals/irq.h b/exercises/lesson03/3/bl4ckout31/include/peripherals/irq.h
new file mode 100644
index 00000000..2429a6a9
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/include/peripherals/irq.h
@@ -0,0 +1,31 @@
+#ifndef	_P_IRQ_H
+#define	_P_IRQ_H
+
+#include "peripherals/base.h"
+
+#define IRQ_BASIC_PENDING	(PBASE+0x0000B200)
+#define IRQ_PENDING_1		(PBASE+0x0000B204)
+#define IRQ_PENDING_2		(PBASE+0x0000B208)
+#define FIQ_CONTROL		    (PBASE+0x0000B20C)
+#define ENABLE_IRQS_1		(PBASE+0x0000B210)
+#define ENABLE_IRQS_2		(PBASE+0x0000B214)
+#define ENABLE_BASIC_IRQS	(PBASE+0x0000B218)
+#define DISABLE_IRQS_1		(PBASE+0x0000B21C)
+#define DISABLE_IRQS_2		(PBASE+0x0000B220)
+#define DISABLE_BASIC_IRQS	(PBASE+0x0000B224)
+
+#define SYSTEM_TIMER_IRQ_0	(1 << 0)
+#define SYSTEM_TIMER_IRQ_1	(1 << 1)
+#define SYSTEM_TIMER_IRQ_2	(1 << 2)
+#define SYSTEM_TIMER_IRQ_3	(1 << 3)
+
+// See BCM2836 ARM-local peripherals at
+// https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2836/QA7_rev3.4.pdf
+
+#define TIMER_INT_CTRL_0    (0x40000040)
+#define INT_SOURCE_0        (LPBASE+0x60)
+
+#define TIMER_INT_CTRL_0_VALUE  (1 << 1)
+#define GENERIC_TIMER_INTERRUPT (1 << 1)
+
+#endif  /*_P_IRQ_H */
diff --git a/exercises/lesson03/3/bl4ckout31/include/peripherals/mini_uart.h b/exercises/lesson03/3/bl4ckout31/include/peripherals/mini_uart.h
new file mode 100644
index 00000000..af95a3be
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/include/peripherals/mini_uart.h
@@ -0,0 +1,19 @@
+#ifndef	_P_MINI_UART_H
+#define	_P_MINI_UART_H
+
+#include "peripherals/base.h"
+
+#define AUX_ENABLES     (PBASE+0x00215004)
+#define AUX_MU_IO_REG   (PBASE+0x00215040)
+#define AUX_MU_IER_REG  (PBASE+0x00215044)
+#define AUX_MU_IIR_REG  (PBASE+0x00215048)
+#define AUX_MU_LCR_REG  (PBASE+0x0021504C)
+#define AUX_MU_MCR_REG  (PBASE+0x00215050)
+#define AUX_MU_LSR_REG  (PBASE+0x00215054)
+#define AUX_MU_MSR_REG  (PBASE+0x00215058)
+#define AUX_MU_SCRATCH  (PBASE+0x0021505C)
+#define AUX_MU_CNTL_REG (PBASE+0x00215060)
+#define AUX_MU_STAT_REG (PBASE+0x00215064)
+#define AUX_MU_BAUD_REG (PBASE+0x00215068)
+
+#endif  /*_P_MINI_UART_H */
diff --git a/exercises/lesson03/3/bl4ckout31/include/peripherals/timer.h b/exercises/lesson03/3/bl4ckout31/include/peripherals/timer.h
new file mode 100644
index 00000000..8a52085d
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/include/peripherals/timer.h
@@ -0,0 +1,19 @@
+#ifndef	_P_TIMER_H
+#define	_P_TIMER_H
+
+#include "peripherals/base.h"
+
+#define TIMER_CS        (PBASE+0x00003000)
+#define TIMER_CLO       (PBASE+0x00003004)
+#define TIMER_CHI       (PBASE+0x00003008)
+#define TIMER_C0        (PBASE+0x0000300C)
+#define TIMER_C1        (PBASE+0x00003010)
+#define TIMER_C2        (PBASE+0x00003014)
+#define TIMER_C3        (PBASE+0x00003018)
+
+#define TIMER_CS_M0	(1 << 0)
+#define TIMER_CS_M1	(1 << 1)
+#define TIMER_CS_M2	(1 << 2)
+#define TIMER_CS_M3	(1 << 3)
+
+#endif  /*_P_TIMER_H */
diff --git a/exercises/lesson03/3/bl4ckout31/include/printf.h b/exercises/lesson03/3/bl4ckout31/include/printf.h
new file mode 100644
index 00000000..f37c1bf2
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/include/printf.h
@@ -0,0 +1,106 @@
+/*
+File: printf.h
+
+Copyright (C) 2004  Kustaa Nyholm
+
+This library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Lesser General Public
+License as published by the Free Software Foundation; either
+version 2.1 of the License, or (at your option) any later version.
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+See the GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public
+License along with this library; if not, write to the Free Software
+Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+
+This library is realy just two files: 'printf.h' and 'printf.c'.
+
+They provide a simple and small (+200 loc) printf functionality to
+be used in embedded systems.
+
+I've found them so usefull in debugging that I do not bother with a
+debugger at all.
+
+They are distributed in source form, so to use them, just compile them
+into your project.
+
+Two printf variants are provided: printf and sprintf.
+
+The formats supported by this implementation are: 'd' 'u' 'c' 's' 'x' 'X'.
+
+Zero padding and field width are also supported.
+
+If the library is compiled with 'PRINTF_SUPPORT_LONG' defined then the
+long specifier is also
+supported. Note that this will pull in some long math routines (pun intended!)
+and thus make your executable noticably longer.
+
+The memory foot print of course depends on the target cpu, compiler and
+compiler options, but a rough guestimate (based on a H8S target) is about
+1.4 kB for code and some twenty 'int's and 'char's, say 60 bytes of stack space.
+Not too bad. Your milage may vary. By hacking the source code you can
+get rid of some hunred bytes, I'm sure, but personally I feel the balance of
+functionality and flexibility versus  code size is close to optimal for
+many embedded systems.
+
+To use the printf you need to supply your own character output function,
+something like :
+
+	void putc ( void* p, char c)
+		{
+		while (!SERIAL_PORT_EMPTY) ;
+		SERIAL_PORT_TX_REGISTER = c;
+		}
+
+Before you can call printf you need to initialize it to use your
+character output function with something like:
+
+	init_printf(NULL,putc);
+
+Notice the 'NULL' in 'init_printf' and the parameter 'void* p' in 'putc',
+the NULL (or any pointer) you pass into the 'init_printf' will eventually be
+passed to your 'putc' routine. This allows you to pass some storage space (or
+anything realy) to the character output function, if necessary.
+This is not often needed but it was implemented like that because it made
+implementing the sprintf function so neat (look at the source code).
+
+The code is re-entrant, except for the 'init_printf' function, so it
+is safe to call it from interupts too, although this may result in mixed output.
+If you rely on re-entrancy, take care that your 'putc' function is re-entrant!
+
+The printf and sprintf functions are actually macros that translate to
+'tfp_printf' and 'tfp_sprintf'. This makes it possible
+to use them along with 'stdio.h' printf's in a single source file.
+You just need to undef the names before you include the 'stdio.h'.
+Note that these are not function like macros, so if you have variables
+or struct members with these names, things will explode in your face.
+Without variadic macros this is the best we can do to wrap these
+fucnction. If it is a problem just give up the macros and use the
+functions directly or rename them.
+
+For further details see source code.
+
+regs Kusti, 23.10.2004
+*/
+
+
+#ifndef __TFP_PRINTF__
+#define __TFP_PRINTF__
+
+#include <stdarg.h>
+
+void init_printf(void* putp,void (*putf) (void*,char));
+
+void tfp_printf(char *fmt, ...);
+void tfp_sprintf(char* s,char *fmt, ...);
+
+void tfp_format(void* putp,void (*putf) (void*,char),char *fmt, va_list va);
+
+#define printf tfp_printf
+#define sprintf tfp_sprintf
+
+#endif
diff --git a/exercises/lesson03/3/bl4ckout31/include/timer.h b/exercises/lesson03/3/bl4ckout31/include/timer.h
new file mode 100644
index 00000000..0e32fad8
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/include/timer.h
@@ -0,0 +1,13 @@
+#ifndef	_TIMER_H
+#define	_TIMER_H
+
+void timer_init ( void );
+void handle_timer_irq ( void );
+
+void generic_timer_init ( void );
+void handle_generic_timer_irq ( void );
+
+extern void gen_timer_init();
+extern void gen_timer_reset();
+
+#endif  /*_TIMER_H */
diff --git a/exercises/lesson03/3/bl4ckout31/include/utils.h b/exercises/lesson03/3/bl4ckout31/include/utils.h
new file mode 100644
index 00000000..20fb61f2
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/include/utils.h
@@ -0,0 +1,9 @@
+#ifndef	_UTILS_H
+#define	_UTILS_H
+
+extern void delay ( unsigned long);
+extern void put32 ( unsigned long, unsigned int );
+extern unsigned int get32 ( unsigned long );
+extern int get_el ( void );
+
+#endif  /*_UTILS_H */
diff --git a/exercises/lesson03/3/bl4ckout31/src/boot.S b/exercises/lesson03/3/bl4ckout31/src/boot.S
new file mode 100644
index 00000000..2521a0bf
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/src/boot.S
@@ -0,0 +1,40 @@
+#include "arm/sysregs.h"
+
+#include "mm.h"
+
+.section ".text.boot"
+
+.globl _start
+_start:
+    mrs	x0, mpidr_el1		
+    and	x0, x0,#0xFF	// Check processor id
+    cbz	x0, master		// Hang for all non-primary CPU
+    b	proc_hang
+
+proc_hang: 
+    b 	proc_hang
+
+master:
+    ldr	x0, =SCTLR_VALUE_MMU_DISABLED
+    msr	sctlr_el1, x0		
+
+    ldr	x0, =HCR_VALUE
+    msr	hcr_el2, x0
+
+    ldr	x0, =SPSR_VALUE
+    msr	spsr_el2, x0
+
+    adr	x0, el1_entry		
+    msr	elr_el2, x0
+
+    eret				
+
+el1_entry:
+    adr	x0, bss_begin
+    adr	x1, bss_end
+    sub	x1, x1, x0
+    bl 	memzero
+
+    mov	sp, #LOW_MEMORY
+    bl	kernel_main
+    b 	proc_hang		// should never come here
diff --git a/exercises/lesson03/3/bl4ckout31/src/config.txt b/exercises/lesson03/3/bl4ckout31/src/config.txt
new file mode 100644
index 00000000..27934969
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/src/config.txt
@@ -0,0 +1,2 @@
+kernel_old=1
+disable_commandline_tags=1
diff --git a/exercises/lesson03/3/bl4ckout31/src/entry.S b/exercises/lesson03/3/bl4ckout31/src/entry.S
new file mode 100644
index 00000000..18a5e0c3
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/src/entry.S
@@ -0,0 +1,136 @@
+#include "entry.h"
+
+	.macro handle_invalid_entry type
+	kernel_entry
+	mov	x0, #\type
+	mrs	x1, esr_el1
+	mrs	x2, elr_el1
+	bl	show_invalid_entry_message
+	b	err_hang
+	.endm
+
+	.macro	ventry	label
+	.align	7
+	b	\label
+	.endm
+
+	.macro	kernel_entry
+	sub	sp, sp, #S_FRAME_SIZE
+	stp	x0, x1, [sp, #16 * 0]
+	stp	x2, x3, [sp, #16 * 1]
+	stp	x4, x5, [sp, #16 * 2]
+	stp	x6, x7, [sp, #16 * 3]
+	stp	x8, x9, [sp, #16 * 4]
+	stp	x10, x11, [sp, #16 * 5]
+	stp	x12, x13, [sp, #16 * 6]
+	stp	x14, x15, [sp, #16 * 7]
+	stp	x16, x17, [sp, #16 * 8]
+	stp	x18, x19, [sp, #16 * 9]
+	stp	x20, x21, [sp, #16 * 10]
+	stp	x22, x23, [sp, #16 * 11]
+	stp	x24, x25, [sp, #16 * 12]
+	stp	x26, x27, [sp, #16 * 13]
+	stp	x28, x29, [sp, #16 * 14]
+	str	x30, [sp, #16 * 15] 
+	.endm
+
+	.macro	kernel_exit
+	ldp	x0, x1, [sp, #16 * 0]
+	ldp	x2, x3, [sp, #16 * 1]
+	ldp	x4, x5, [sp, #16 * 2]
+	ldp	x6, x7, [sp, #16 * 3]
+	ldp	x8, x9, [sp, #16 * 4]
+	ldp	x10, x11, [sp, #16 * 5]
+	ldp	x12, x13, [sp, #16 * 6]
+	ldp	x14, x15, [sp, #16 * 7]
+	ldp	x16, x17, [sp, #16 * 8]
+	ldp	x18, x19, [sp, #16 * 9]
+	ldp	x20, x21, [sp, #16 * 10]
+	ldp	x22, x23, [sp, #16 * 11]
+	ldp	x24, x25, [sp, #16 * 12]
+	ldp	x26, x27, [sp, #16 * 13]
+	ldp	x28, x29, [sp, #16 * 14]
+	ldr	x30, [sp, #16 * 15] 
+	add	sp, sp, #S_FRAME_SIZE		
+	eret
+	.endm
+
+
+/*
+ * Exception vectors.
+ */
+.align	11
+.globl vectors 
+vectors:
+	ventry	sync_invalid_el1t			// Synchronous EL1t
+	ventry	irq_invalid_el1t			// IRQ EL1t
+	ventry	fiq_invalid_el1t			// FIQ EL1t
+	ventry	error_invalid_el1t			// Error EL1t
+
+	ventry	sync_invalid_el1h			// Synchronous EL1h
+	ventry	el1_irq					// IRQ EL1h
+	ventry	fiq_invalid_el1h			// FIQ EL1h
+	ventry	error_invalid_el1h			// Error EL1h
+
+	ventry	sync_invalid_el0_64			// Synchronous 64-bit EL0
+	ventry	irq_invalid_el0_64			// IRQ 64-bit EL0
+	ventry	fiq_invalid_el0_64			// FIQ 64-bit EL0
+	ventry	error_invalid_el0_64			// Error 64-bit EL0
+
+	ventry	sync_invalid_el0_32			// Synchronous 32-bit EL0
+	ventry	irq_invalid_el0_32			// IRQ 32-bit EL0
+	ventry	fiq_invalid_el0_32			// FIQ 32-bit EL0
+	ventry	error_invalid_el0_32			// Error 32-bit EL0
+
+sync_invalid_el1t:
+	handle_invalid_entry  SYNC_INVALID_EL1t
+
+irq_invalid_el1t:
+	handle_invalid_entry  IRQ_INVALID_EL1t
+
+fiq_invalid_el1t:
+	handle_invalid_entry  FIQ_INVALID_EL1t
+
+error_invalid_el1t:
+	handle_invalid_entry  ERROR_INVALID_EL1t
+
+sync_invalid_el1h:
+	handle_invalid_entry  SYNC_INVALID_EL1h
+
+fiq_invalid_el1h:
+	handle_invalid_entry  FIQ_INVALID_EL1h
+
+error_invalid_el1h:
+	handle_invalid_entry  ERROR_INVALID_EL1h
+
+sync_invalid_el0_64:
+	handle_invalid_entry  SYNC_INVALID_EL0_64
+
+irq_invalid_el0_64:
+	handle_invalid_entry  IRQ_INVALID_EL0_64
+
+fiq_invalid_el0_64:
+	handle_invalid_entry  FIQ_INVALID_EL0_64
+
+error_invalid_el0_64:
+	handle_invalid_entry  ERROR_INVALID_EL0_64
+
+sync_invalid_el0_32:
+	handle_invalid_entry  SYNC_INVALID_EL0_32
+
+irq_invalid_el0_32:
+	handle_invalid_entry  IRQ_INVALID_EL0_32
+
+fiq_invalid_el0_32:
+	handle_invalid_entry  FIQ_INVALID_EL0_32
+
+error_invalid_el0_32:
+	handle_invalid_entry  ERROR_INVALID_EL0_32
+
+el1_irq:
+	kernel_entry 
+	bl	handle_irq
+	kernel_exit 
+
+.globl err_hang
+err_hang: b err_hang
diff --git a/exercises/lesson03/3/bl4ckout31/src/irq.S b/exercises/lesson03/3/bl4ckout31/src/irq.S
new file mode 100644
index 00000000..1c10ed2b
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/src/irq.S
@@ -0,0 +1,15 @@
+.globl irq_vector_init
+irq_vector_init:
+	adr	x0, vectors		// load VBAR_EL1 with virtual
+	msr	vbar_el1, x0		// vector table address
+	ret
+
+.globl enable_irq
+enable_irq:
+	msr    daifclr, #2 
+	ret
+
+.globl disable_irq
+disable_irq:
+	msr	daifset, #2
+	ret
diff --git a/exercises/lesson03/3/bl4ckout31/src/irq.c b/exercises/lesson03/3/bl4ckout31/src/irq.c
new file mode 100644
index 00000000..1c9c98ec
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/src/irq.c
@@ -0,0 +1,51 @@
+#include "utils.h"
+#include "printf.h"
+#include "timer.h"
+#include "entry.h"
+#include "peripherals/irq.h"
+
+const char *entry_error_messages[] = {
+    "SYNC_INVALID_EL1t",
+    "IRQ_INVALID_EL1t",		
+    "FIQ_INVALID_EL1t",		
+    "ERROR_INVALID_EL1T",		
+
+    "SYNC_INVALID_EL1h",		
+    "IRQ_INVALID_EL1h",		
+    "FIQ_INVALID_EL1h",		
+    "ERROR_INVALID_EL1h",		
+
+    "SYNC_INVALID_EL0_64",		
+    "IRQ_INVALID_EL0_64",		
+    "FIQ_INVALID_EL0_64",		
+    "ERROR_INVALID_EL0_64",	
+
+    "SYNC_INVALID_EL0_32",		
+    "IRQ_INVALID_EL0_32",		
+    "FIQ_INVALID_EL0_32",		
+    "ERROR_INVALID_EL0_32"	
+};
+
+void enable_interrupt_controller()
+{
+    // Enables Core 0 Timers interrupt control for the generic timer 
+    put32(TIMER_INT_CTRL_0, TIMER_INT_CTRL_0_VALUE);
+}
+
+void show_invalid_entry_message(int type, unsigned long esr, unsigned long address)
+{
+    printf("%s, ESR: %x, address: %x\r\n", entry_error_messages[type], esr, address);
+}
+
+void handle_irq(void)
+{
+    // Each Core has its own pending local intrrupts register
+    unsigned int irq = get32(INT_SOURCE_0);
+    switch (irq) {
+        case (GENERIC_TIMER_INTERRUPT):
+            handle_generic_timer_irq();
+            break;
+        default:
+            printf("Unknown pending irq: %x\r\n", irq);
+    }
+}
\ No newline at end of file
diff --git a/exercises/lesson03/3/bl4ckout31/src/kernel.c b/exercises/lesson03/3/bl4ckout31/src/kernel.c
new file mode 100644
index 00000000..29661f3f
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/src/kernel.c
@@ -0,0 +1,20 @@
+#include "printf.h"
+#include "timer.h"
+#include "irq.h"
+#include "mini_uart.h"
+#include "utils.h"
+
+
+void kernel_main(void)
+{
+	uart_init();
+	init_printf(0, putc);
+	irq_vector_init();
+	generic_timer_init();
+	enable_interrupt_controller();
+	enable_irq();
+
+	while (1){
+		uart_send(uart_recv());
+	}	
+}
diff --git a/exercises/lesson03/3/bl4ckout31/src/linker.ld b/exercises/lesson03/3/bl4ckout31/src/linker.ld
new file mode 100644
index 00000000..e23b3161
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/src/linker.ld
@@ -0,0 +1,12 @@
+SECTIONS
+{
+    . = 0x80000;
+	.text.boot : { *(.text.boot) }
+	.text : { *(.text) }
+	.rodata : { *(.rodata) }
+	.data : { *(.data) }
+	. = ALIGN(0x8);
+	bss_begin = .;
+	.bss : { *(.bss*) } 
+	bss_end = .;
+}
diff --git a/exercises/lesson03/3/bl4ckout31/src/mini_uart.c b/exercises/lesson03/3/bl4ckout31/src/mini_uart.c
new file mode 100644
index 00000000..988f4782
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/src/mini_uart.c
@@ -0,0 +1,62 @@
+#include "utils.h"
+#include "peripherals/mini_uart.h"
+#include "peripherals/gpio.h"
+
+void uart_send ( char c )
+{
+	while(1) {
+		if(get32(AUX_MU_LSR_REG)&0x20) 
+			break;
+	}
+	put32(AUX_MU_IO_REG,c);
+}
+
+char uart_recv ( void )
+{
+	while(1) {
+		if(get32(AUX_MU_LSR_REG)&0x01) 
+			break;
+	}
+	return(get32(AUX_MU_IO_REG)&0xFF);
+}
+
+void uart_send_string(char* str)
+{
+	for (int i = 0; str[i] != '\0'; i ++) {
+		uart_send((char)str[i]);
+	}
+}
+
+void uart_init ( void )
+{
+	unsigned int selector;
+
+	selector = get32(GPFSEL1);
+	selector &= ~(7<<12);                   // clean gpio14
+	selector |= 2<<12;                      // set alt5 for gpio14
+	selector &= ~(7<<15);                   // clean gpio15
+	selector |= 2<<15;                      // set alt5 for gpio15
+	put32(GPFSEL1,selector);
+
+	put32(GPPUD,0);
+	delay(150);
+	put32(GPPUDCLK0,(1<<14)|(1<<15));
+	delay(150);
+	put32(GPPUDCLK0,0);
+
+	put32(AUX_ENABLES,1);                   //Enable mini uart (this also enables access to it registers)
+	put32(AUX_MU_CNTL_REG,0);               //Disable auto flow control and disable receiver and transmitter (for now)
+	put32(AUX_MU_IER_REG,0);                //Disable receive and transmit interrupts
+	put32(AUX_MU_LCR_REG,3);                //Enable 8 bit mode
+	put32(AUX_MU_MCR_REG,0);                //Set RTS line to be always high
+	put32(AUX_MU_BAUD_REG,270);             //Set baud rate to 115200
+
+	put32(AUX_MU_CNTL_REG,3);               //Finally, enable transmitter and receiver
+}
+
+
+// This function is required by printf function
+void putc ( void* p, char c)
+{
+	uart_send(c);
+}
diff --git a/exercises/lesson03/3/bl4ckout31/src/mm.S b/exercises/lesson03/3/bl4ckout31/src/mm.S
new file mode 100644
index 00000000..1bd32ff3
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/src/mm.S
@@ -0,0 +1,6 @@
+.globl memzero
+memzero:
+	str xzr, [x0], #8
+	subs x1, x1, #8
+	b.gt memzero
+	ret
diff --git a/exercises/lesson03/3/bl4ckout31/src/printf.c b/exercises/lesson03/3/bl4ckout31/src/printf.c
new file mode 100644
index 00000000..1383039a
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/src/printf.c
@@ -0,0 +1,233 @@
+/*
+File: printf.c
+
+Copyright (C) 2004  Kustaa Nyholm
+
+This library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Lesser General Public
+License as published by the Free Software Foundation; either
+version 2.1 of the License, or (at your option) any later version.
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public
+License along with this library; if not, write to the Free Software
+Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+
+*/
+
+#include "printf.h"
+
+typedef void (*putcf) (void*,char);
+static putcf stdout_putf;
+static void* stdout_putp;
+
+
+#ifdef PRINTF_LONG_SUPPORT
+
+static void uli2a(unsigned long int num, unsigned int base, int uc,char * bf)
+    {
+    int n=0;
+    unsigned int d=1;
+    while (num/d >= base)
+        d*=base;
+    while (d!=0) {
+        int dgt = num / d;
+        num%=d;
+        d/=base;
+        if (n || dgt>0|| d==0) {
+            *bf++ = dgt+(dgt<10 ? '0' : (uc ? 'A' : 'a')-10);
+            ++n;
+            }
+        }
+    *bf=0;
+    }
+
+static void li2a (long num, char * bf)
+    {
+    if (num<0) {
+        num=-num;
+        *bf++ = '-';
+        }
+    uli2a(num,10,0,bf);
+    }
+
+#endif
+
+static void ui2a(unsigned int num, unsigned int base, int uc,char * bf)
+    {
+    int n=0;
+    unsigned int d=1;
+    while (num/d >= base)
+        d*=base;
+    while (d!=0) {
+        int dgt = num / d;
+        num%= d;
+        d/=base;
+        if (n || dgt>0 || d==0) {
+            *bf++ = dgt+(dgt<10 ? '0' : (uc ? 'A' : 'a')-10);
+            ++n;
+            }
+        }
+    *bf=0;
+    }
+
+static void i2a (int num, char * bf)
+    {
+    if (num<0) {
+        num=-num;
+        *bf++ = '-';
+        }
+    ui2a(num,10,0,bf);
+    }
+
+static int a2d(char ch)
+    {
+    if (ch>='0' && ch<='9')
+        return ch-'0';
+    else if (ch>='a' && ch<='f')
+        return ch-'a'+10;
+    else if (ch>='A' && ch<='F')
+        return ch-'A'+10;
+    else return -1;
+    }
+
+static char a2i(char ch, char** src,int base,int* nump)
+    {
+    char* p= *src;
+    int num=0;
+    int digit;
+    while ((digit=a2d(ch))>=0) {
+        if (digit>base) break;
+        num=num*base+digit;
+        ch=*p++;
+        }
+    *src=p;
+    *nump=num;
+    return ch;
+    }
+
+static void putchw(void* putp,putcf putf,int n, char z, char* bf)
+    {
+    char fc=z? '0' : ' ';
+    char ch;
+    char* p=bf;
+    while (*p++ && n > 0)
+        n--;
+    while (n-- > 0)
+        putf(putp,fc);
+    while ((ch= *bf++))
+        putf(putp,ch);
+    }
+
+void tfp_format(void* putp,putcf putf,char *fmt, va_list va)
+    {
+    char bf[12];
+
+    char ch;
+
+
+    while ((ch=*(fmt++))) {
+        if (ch!='%')
+            putf(putp,ch);
+        else {
+            char lz=0;
+#ifdef  PRINTF_LONG_SUPPORT
+            char lng=0;
+#endif
+            int w=0;
+            ch=*(fmt++);
+            if (ch=='0') {
+                ch=*(fmt++);
+                lz=1;
+                }
+            if (ch>='0' && ch<='9') {
+                ch=a2i(ch,&fmt,10,&w);
+                }
+#ifdef  PRINTF_LONG_SUPPORT
+            if (ch=='l') {
+                ch=*(fmt++);
+                lng=1;
+            }
+#endif
+            switch (ch) {
+                case 0:
+                    goto abort;
+                case 'u' : {
+#ifdef  PRINTF_LONG_SUPPORT
+                    if (lng)
+                        uli2a(va_arg(va, unsigned long int),10,0,bf);
+                    else
+#endif
+                    ui2a(va_arg(va, unsigned int),10,0,bf);
+                    putchw(putp,putf,w,lz,bf);
+                    break;
+                    }
+                case 'd' :  {
+#ifdef  PRINTF_LONG_SUPPORT
+                    if (lng)
+                        li2a(va_arg(va, unsigned long int),bf);
+                    else
+#endif
+                    i2a(va_arg(va, int),bf);
+                    putchw(putp,putf,w,lz,bf);
+                    break;
+                    }
+                case 'x': case 'X' :
+#ifdef  PRINTF_LONG_SUPPORT
+                    if (lng)
+                        uli2a(va_arg(va, unsigned long int),16,(ch=='X'),bf);
+                    else
+#endif
+                    ui2a(va_arg(va, unsigned int),16,(ch=='X'),bf);
+                    putchw(putp,putf,w,lz,bf);
+                    break;
+                case 'c' :
+                    putf(putp,(char)(va_arg(va, int)));
+                    break;
+                case 's' :
+                    putchw(putp,putf,w,0,va_arg(va, char*));
+                    break;
+                case '%' :
+                    putf(putp,ch);
+                default:
+                    break;
+                }
+            }
+        }
+    abort:;
+    }
+
+
+void init_printf(void* putp,void (*putf) (void*,char))
+    {
+    stdout_putf=putf;
+    stdout_putp=putp;
+    }
+
+void tfp_printf(char *fmt, ...)
+    {
+    va_list va;
+    va_start(va,fmt);
+    tfp_format(stdout_putp,stdout_putf,fmt,va);
+    va_end(va);
+    }
+
+static void putcp(void* p,char c)
+    {
+    *(*((char**)p))++ = c;
+    }
+
+
+
+void tfp_sprintf(char* s,char *fmt, ...)
+    {
+    va_list va;
+    va_start(va,fmt);
+    tfp_format(&s,putcp,fmt,va);
+    putcp(&s,0);
+    va_end(va);
+    }
diff --git a/exercises/lesson03/3/bl4ckout31/src/timer.S b/exercises/lesson03/3/bl4ckout31/src/timer.S
new file mode 100644
index 00000000..1f334231
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/src/timer.S
@@ -0,0 +1,20 @@
+/** Here, the physical timer at EL1 is used with the TimerValue views.
+ *  Once the count-down has reach 0, the interrupt line is HIGH until
+ *  a new timer value > 0 is write into the CNTP_TVAL_EL0 system register.
+ *
+ *  See AArch64-referenc-manual p.2326 at
+ *  https://developer.arm.com/docs/ddi0487/ca/arm-architecture-reference-manual-armv8-for-armv8-a-architecture-profile
+ */
+
+.globl gen_timer_init
+gen_timer_init:
+	mov x0, #1
+	msr CNTP_CTL_EL0, x0
+	ret
+
+.globl gen_timer_reset
+gen_timer_reset:
+    mov x0, #1
+	lsl x0, x0, #24 
+	msr CNTP_TVAL_EL0, x0
+    ret
\ No newline at end of file
diff --git a/exercises/lesson03/3/bl4ckout31/src/timer.c b/exercises/lesson03/3/bl4ckout31/src/timer.c
new file mode 100644
index 00000000..b82f87c4
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/src/timer.c
@@ -0,0 +1,34 @@
+#include "utils.h"
+#include "printf.h"
+#include "peripherals/timer.h"
+#include "timer.h"
+
+const unsigned int interval = 9600000;
+unsigned int curVal = 0;
+
+void timer_init ( void )
+{
+	curVal = get32(TIMER_CLO);
+	curVal += interval;
+	put32(TIMER_C1, curVal);
+}
+
+void handle_timer_irq( void ) 
+{
+	curVal += interval;
+	put32(TIMER_C1, curVal);
+	put32(TIMER_CS, TIMER_CS_M1);
+	printf("Timer interrupt received\n\r");
+}
+
+void generic_timer_init ( void )
+{
+	gen_timer_init();
+	gen_timer_reset();
+}
+
+void handle_generic_timer_irq( void ) 
+{
+	printf("Timer interrupt received\n\r");
+	gen_timer_reset();
+}
\ No newline at end of file
diff --git a/exercises/lesson03/3/bl4ckout31/src/utils.S b/exercises/lesson03/3/bl4ckout31/src/utils.S
new file mode 100644
index 00000000..1838f1af
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/src/utils.S
@@ -0,0 +1,21 @@
+.globl get_el
+get_el:
+	mrs x0, CurrentEL
+	lsr x0, x0, #2
+	ret
+
+.globl put32
+put32:
+	str w1,[x0]
+	ret
+
+.globl get32
+get32:
+	ldr w0,[x0]
+	ret
+
+.globl delay
+delay:
+	subs x0, x0, #1
+	bne delay
+	ret
\ No newline at end of file
diff --git a/exercises/lesson03/3/bl4ckout31/start.sh b/exercises/lesson03/3/bl4ckout31/start.sh
new file mode 100755
index 00000000..64b2e174
--- /dev/null
+++ b/exercises/lesson03/3/bl4ckout31/start.sh
@@ -0,0 +1,8 @@
+#!/bin/bash
+
+echo "###"
+echo "### Use C-a h for help"
+echo "###"
+echo ""
+
+qemu-system-aarch64 -machine raspi3 -serial null -serial mon:stdio -nographic -kernel kernel8.img