kernel.c

#if !defined(__cplusplus)
#include <stdbool.h> /* C doesn't have booleans by default. */
#endif
#include <stddef.h>
#include <stdint.h>
 
/* Check if the compiler thinks we are targeting the wrong operating system. */
#if defined(__linux__)
#error "You are not using a cross-compiler, you will most certainly run into trouble"
#endif
 
/* This tutorial will only work for the 32-bit ix86 targets. */
#if !defined(__i386__)
#error "This tutorial needs to be compiled with a ix86-elf compiler"
#endif
 
/* Hardware text mode color constants. */
enum vga_color {
    COLOR_BLACK = 0,
    COLOR_BLUE = 1,
    COLOR_GREEN = 2,
    COLOR_CYAN = 3,
    COLOR_RED = 4,
    COLOR_MAGENTA = 5,
    COLOR_BROWN = 6,
    COLOR_LIGHT_GREY = 7,
    COLOR_DARK_GREY = 8,
    COLOR_LIGHT_BLUE = 9,
    COLOR_LIGHT_GREEN = 10,
    COLOR_LIGHT_CYAN = 11,
    COLOR_LIGHT_RED = 12,
    COLOR_LIGHT_MAGENTA = 13,
    COLOR_LIGHT_BROWN = 14,
    COLOR_WHITE = 15,
};
 
uint8_t make_color(enum vga_color fg, enum vga_color bg) {
    return fg | bg << 4;
}
 
uint16_t make_vgaentry(char c, uint8_t color) {
    uint16_t c16 = c;
    uint16_t color16 = color;
    return c16 | color16 << 8;
}
 
size_t strlen(const char* str) {
    size_t ret = 0;
    while ( str[ret] != 0 )
        ret++;
    return ret;
}
 
static const size_t VGA_WIDTH = 80;
static const size_t VGA_HEIGHT = 25;
 
size_t terminal_row;
size_t terminal_column;
uint8_t terminal_color;
uint16_t* terminal_buffer;
 
void terminal_initialize() {
    terminal_row = 0;
    terminal_column = 0;
    terminal_color = make_color(COLOR_LIGHT_GREY, COLOR_BLACK);
    terminal_buffer = (uint16_t*) 0xB8000;
    for (size_t y = 0; y < VGA_HEIGHT; y++) {
        for (size_t x = 0; x < VGA_WIDTH; x++) {
            const size_t index = y * VGA_WIDTH + x;
            terminal_buffer[index] = make_vgaentry(' ', terminal_color);
        }
    }
}
 
void terminal_setcolor(uint8_t color) {
    terminal_color = color;
}
 
void terminal_putentryat(char c, uint8_t color, size_t x, size_t y) {
    const size_t index = y * VGA_WIDTH + x;
    terminal_buffer[index] = make_vgaentry(c, color);
}
 
void terminal_putchar(char c) {
    terminal_putentryat(c, terminal_color, terminal_column, terminal_row);
    if (++terminal_column == VGA_WIDTH) {
        terminal_column = 0;
        if (++terminal_row == VGA_HEIGHT) {
            terminal_row = 0;
        }
    }
}
 
void terminal_writestring(const char* data) {
    size_t datalen = strlen(data);
    for (size_t i = 0; i < datalen; i++)
        terminal_putchar(data[i]);
}

/**
 * C++ version 0.4 char* style "itoa":
 * Written by Lukás Chmela
 * Released under GPLv3.
 */
char* itoa(int value, char* result, int base) {
    // check that the base if valid
    if (base < 2 || base > 36) { *result = '\0'; return result; }

    char* ptr = result, *ptr1 = result, tmp_char;
    int tmp_value;

    do {
        tmp_value = value;
        value /= base;
        *ptr++ = "zyxwvutsrqponmlkjihgfedcba9876543210123456789abcdefghijklmnopqrstuvwxyz" [35 + (tmp_value - value * base)];
    } while ( value );

    // Apply negative sign
    if (tmp_value < 0) *ptr++ = '-';
    *ptr-- = '\0';
    while(ptr1 < ptr) {
        tmp_char = *ptr;
        *ptr--= *ptr1;
        *ptr1++ = tmp_char;
    }
    return result;
}
    
 
#if defined(__cplusplus)
extern "C" /* Use C linkage for kernel_main. */
#endif
void kernel_main() {
    /* Initialize terminal interface */
    terminal_initialize();
 
    /* Since there is no support for newlines in terminal_putchar
         * yet, '\n' will produce some VGA specific character instead.
         * This is normal.
         */

    char buffer[8];

    for (int i = 1; i <= 100; i++) {
        if (i % 3 == 0 && i % 5 == 0) {
            terminal_writestring("FizzBuzz");
        } else if (i % 3 == 0) {
            terminal_writestring("Fizz");
        } else if (i % 5 == 0) {
            terminal_writestring("Buzz");
        } else {
            terminal_writestring(itoa(i, buffer, 10));
        }

        if (i < 100) {
            terminal_writestring(", ");
        }
    }
}