Rotate90.asm

;@ Rotate a 10x10 matrix clockwise 90 degrees.
;@ Input: 	R0 = pointer to input buffer.
;@		  	R1 = Cols
;@ 		  	R2 = Rows
;@			
;@ Output: 	R0 = Output Buffer Pointer.

;@ Note: As a challenge, I am to do all processing on the input buffer. This is a destructive algorithm to save space.
;@ NOTE: R3 is being used as a on-chip stack for PUSHes and POPs.

	PUSH	{r0 - r12, lr} 		;@ Save Previous registers
;@					// Start of setup
	CMP	r1, #1		;@ Check if the input Cols is lower than 1.
	BLS 	exit			;@ If the input Cols is equal to or lower than 1, then exit.
	CMP	r2, #1		;@ Check if the input Rows is lower than 1.
	BLS 	exit			;@ If the input Rows is equal to or lower than 1, then exit.
	MUL	r4, r2, r1	;@ R4 is the bottom right of the input buffer.
	MUL	r5, r2, r1	;@ R5 is the bottom left of the input buffer.
	SUB	r5, r5, r1	;@ Subtracts one entire row to get the beginning of the bottom row.
	ADD	r4, r0, r4	;@ Set R4 to the actual location of the bottom right of the buffer
	ADD	r5, r0, r5	;@ Set R5 to the actual location of the bottom left of the buffer.
	ADD	r6, r0, r1	;@ Set R6 to the actual location of the top right of the buffer.
	MOV	r7, [r1, #-1]	;@ Copy over the Cols over to r7 subtracted by one.
	MOV	r8, [r2, #-1]	;@ Copy over the Rows over to r8 subtracted by one. 
	MOV	r9, r8, LSL #1	;@ This is to go through the entire buffer.
;@					   The reason why I subtract by one is because I needed to stop before I hit the end of the number of columns because it was already done.
	MOV	r11, r7
	B 	Cols		;@ Start the process of rotating
;@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
;@ 
;@ The reason I start at columns is because I have to decrement the ending length and increment the beginning length.
;@
;@				---------------------------------->|
;@				0,   1,  2,  3,  4,  5,  6,  7,  8,  9,
;@				    -------------------------->|
;@				10, 11, 12, 13, 14, 15, 16, 17, 18, 19
;@					------------------>|
;@				20, 21, 22, 23, 24, 25, 26, 27, 28, 29
;@					    ---------->|
;@				30, 31, 32, 33, 34, 35, 36, 37, 38, 39
;@						-->|
;@				40, 41, 42, 43, 44, 45, 46, 47, 48, 49
;@				50, 51, 52, 53, 54, 55, 56, 57, 58, 59
;@				60, 61, 62, 63, 64, 65, 66, 67, 68, 69
;@				70, 71, 72, 73, 74, 75, 76, 77, 78, 79
;@				80, 81, 82, 83, 84, 85, 86, 87, 88, 89								
;@				90, 91, 92, 93, 94, 95, 96, 97, 98, 99
;@
;@		I have the increment and decrement at the beginning of each row which makes 
;@			it difficult unless I start with the columns because I have to ensure that 
;@ 			I go through all rows including the top. If I were to start with the Rows, then
;@ 			it would increment the counters before running through the top of the buffer.
;@		Starting at the columns allows me to go through the entire buffer.
;@
;@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@	
	
	
	
Rows:
	SUB	r0, r0, r7
	ADD	r0, [r2, #1]
	MOV	r11, [r7, #-1]!
	SUBS	r9, #1
	BEQ	exit
Cols:	

					;@START Rotating Loop
	LDRB	r10, [r0]		;@ Load the contents of r0 into r10 to move it to the very right. 
	
	LDRB	r3, [r10]		;@ Save the contents of r0 for later.
	MOV	r10, r4			;@ Copy the location of r4 for later.
	LDRB	r12, [r4]		;@ Copy the contents of r4 for later. 
	STRB	r3, [r10]		;@ Take out the contents of r0 and store it into r4.
	
	LDRB 	r3, [r12]		;@ Store the contents of r4 for later.
	MOV	r10, r5			;@ Copy the location of r5 for later.
	LDRB	r12, [r5]		;@ Get the contents of r5 for later.
	STRB 	r3, [r10]		;@ store the contents of r4 into the location of r5.
	
	LDRB 	r3, [r12]		;@ Store the contents of r5 for later.
	MOV	r10, r6			;@ Copy the location of r6 for later.
	LDRB	r12, [r6]		;@ Copy the contents of r6 for later.
	STRB	r3, [r10]		;@ Store the contents of r5 into the location of r6.
	
	LDRB 	r3, [r12]		;@ Store the contents of r6. 
	MOV	r10, r0			;@ Copy over location of r0.
	STRB	r3, [r10]		;@ Store the contents of r6 into the location of r0.
	ADD	r0, r0, #1
	SUBS	r11, #1			;@ Decrement the counter.
	BNE 	Cols			;@ If the counter does not equal zero, keep going through Cols.
	B	Rows

	
	
exit: 
	POP	{r0 - r12, pc}