project04: fillstatic

04/fill/FillStatic.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/FillStatic.asm
+
+// Blackens the screen, i.e. writes "black" in every pixel.
+// Key presses are ignored.
+// This is an intermediate step added to help you out.
+
+(LOOP) // main loop
+    @FILL_BLACK // sel fill black
+    0;JMP // jump unconditionally
+
+(FILL_BLACK) // fill screen with black
+    // init count and pointer
+    // screen = 512x256 pixels = 131072 pixels total
+    // each word holds 16 pixels, so 131072/16 = 8192 words
+    @8192 // sel value for count (8192)
+    D=A // load 8192 to data
+    @count // sel count var
+    M=D // set count = 8192
+    @SCREEN // sel screen start
+    D=A // load screen address to data
+    @pointer // sel pointer var
+    M=D // set pointer = screen start
+
+(BLACK_LOOP) // loop for filling black
+    // check if count = 0
+    @count // sel count
+    D=M // load count to data
+    @LOOP // sel main loop
+    D;JEQ // jump to main if done
+
+    // set word to -1 (black)
+    @pointer // sel pointer
+    A=M // set address to pointer value
+    M=-1 // set screen word to black
+
+    // inc pointer
+    @pointer // sel pointer
+    M=M+1 // inc pointer
+
+    // dec count
+    @count // sel count
+    M=M-1 // dec count
+
+    // jump back to black loop
+    @BLACK_LOOP // sel black loop
+    0;JMP // jump back to loop