;; 
		;; Author: Bill Slough
		;;
		;; Example of a subroutine in the LC-3 assembly language
		;;
		;; Lights one pixel on the video display
		;; 

		.ORIG x3000 

		LD	R0,ROW
		LD	R1,COLUMN
		LD	R2,RED
		JSR 	DRAW_PIXEL			; draw_pixel(2, 8, RED)
STOP   		HALT

ROW		.FILL	2				; which row?
COLUMN		.FILL	8				; which column?
RED		.FILL	x7C00				; desired color

ROWS		.FILL	124				; Number of rows of the video display
COLS		.FILL	128				; Number of columns of the video display


DRAW_PIXEL
		;; Draw a pixel on the graphics display unit
		;; 	R0 - row
		;; 	R1 - column
		;; 	R2 - color

		ST	R3,SAVE_R3 		; Save the registers modified within this routine
		ST	R4,SAVE_R4
	
		ADD	R3,R0,#0	  	;
		LD	R4,PIXELS_PER_ROW 	;
		MUL	R3,R3,R4		;
		ADD	R3,R3,R1		; R3 = PIXELS_PER_ROW * row + column

		LD	R4,VIDEO_ADDR
		ADD	R4,R4,R3 		; R4 = address of desired pixel

		STR	R2,R4,#0 		; Video[row,column] = color

		LD	R3,SAVE_R3 		; Restore the registers and exit
		LD	R4,SAVE_R4
		RET	
SAVE_R3		.BLKW	1			; Save area for registers
SAVE_R4		.BLKW	1
VIDEO_ADDR	.FILL	xC000			; base address of the video display
PIXELS_PER_ROW	.FILL	128			; number of pixels per row
		.END