Fill.asm

// This file is part of www.nand2tetris.org
// and the book "The Elements of Computing Systems"
// by Nisan and Schocken, MIT Press.
// File name: projects/04/Fill.asm

// Runs an infinite loop that listens to the keyboard input. 
// When a key is pressed (any key), the program blackens the screen,
// i.e. writes "black" in every pixel. When no key is pressed, the
// program clears the screen, i.e. writes "white" in every pixel.

(KEYCHECK)
@24576 		// This RAM block holds the keyboard input
D=M
@BLACK
D;JNE 		// JMP to the black routine if there is any keyboard input
@WHITE
D;JEQ 		// JMP to the white routine if there isn't any keyboard input (so wasteful)


(WHITE)
@0
D=A
@bw
M=D
@FLIP
0;JMP

(BLACK)
@0
D=!A 		
@bw
M=D
@FLIP
0;JMP

(FLIP) 		//Outer routine (includes setup)
@8191
D=A
@i 			// Counter for flipness
M=D

(FLIPLOOP)  // Inner flip loop
@i
D=M
@SCREEN
D=A+D
@pixel
M=D
@bw 		// should be 0 (white) or -1 (black)
D=M
@pixel
A=M
M=D
@i
MD=M-1

@FLIPLOOP
D;JGE 		// Jump back to the top of the inner loop if we're not done yet

@KEYCHECK 
0;JMP