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projects/04/fill/Fill.asm

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// 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;
// 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.

// Put your code here.

// Fill: for length of screen, checks keypress. If keypress is nonzero, set
// current 16 pixels to black, otherwise set them to white. Point to next 16
// pixels and repeat to enf of screen, then restart at start of screen.
//
// R0 - current screen pointer (to-be incremented until end of screen is hit)

(SET_SCREEN_PTR)
    // R0 = SCREEN pointer
    @SCREEN
    D=A
    @R0
    M=D

(IS_KEYPRESSED)
    // end of screen: 0x5fff (dec 24575)
    // if R0 == 24575; goto SET_SCREEN_PTR
    @R0
    D=M
    @24575
    D=D-A
    @SET_SCREEN_PTR
    D;JEQ

    // D=KBD, goto WHITE_SCREEN if D == 0
    @KBD
    D=M
    @WHITE_SCREEN
    D;JEQ

(BLACK_SCREEN)
    // *R0 = -1
    @R0
    A=M
    M=-1

    @INC_SCR_PTR_AND_LOOP
    0;JMP

(WHITE_SCREEN)
    // *R0 = 0
    @R0
    A=M
    M=0

(INC_SCR_PTR_AND_LOOP)
    // ++R0
    @R0
    M=M+1

    @IS_KEYPRESSED
    0;JMP

// should never reach here, but just in case...
(END)
    @END
    0;JMP