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.

(CHECK)     //check if key is pressed
  @SCREEN   //screen adressing
    D=A
  @addr
    M=D
  @KBD      //keypress check
    D=M
  @CLEAN
    D;JEQ   //clean if not pressed
  @FILL
    D;JGT   //fill if pressed

(FILL)
  @addr
    A=M     //jump to addres
    M=-1    //fill
  @addr
    M=M+1   //increase address
  //END OF SCREEN CHECK
    D=M
  @KBD
    D=D-A
  @FILL     //if addr < KBD
    D;JLT
  @CHECK    //if addr=KBD
    D;JEQ

(CLEAN)
  @addr
    A=M     //jump to addres
    M=0     //clean
  @addr
    M=M+1   //increase address
  //END OF SCREEN CHECK
    D=M
  @KBD
    D=D-A
  @CLEAN    //if addr<KBD loop
    D;JLT
  @CHECK    //if addr=KBD end loop
    D;JEQ