Hack Machine Language I/O

Fill.tst

// This is a program of the 4th project of Nand2Tetris 
// https://www.nand2tetris.org/project04

// The program 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; the screen should remain fully black as 
// long as the key is pressed. 

// When no key is pressed, the program clears the screen, i.e. writes "white"
// in every pixel; the screen should remain fully clear as long as no key is pressed.

// Save the first addres in RAM[0] and RAM[2]
@16384 // First Address
D=A
@0 
M=D
@2
M=D

// Set max address in RAM[1]
@24575 // Max Adress
D=A
@1
M=D

//Test if key is being presses
@24576
D=M
@10
D;JEQ

//Loop to make screen black
@2
A=M // Read current address
M=!M // invert bits on the current address
@2 
M=M+1 //increment the current address
D=M // save current address on D
@1
D=D-M // current address - max address
@14
D;JLE

//Holds black screen
@24576
D=M
@24
D;JNE

@2
M=M-1 //decrease current address by one

//Loop to make screen white
@2
A=M // Read current address
M=!M // invert bits on the current address
@2 
M=M-1 //decrement the current address
D=M // save current address on D
@0
D=D-M // current address - min address
@30
D;JGE

@2
M=M+1 //increse current address
@10 
M;JMP

The JUMP instructions refer to the operation number, not the line. It is done this way because the CPU emulator just loads the operation lines