M projects/04/fill/Fill.asm → projects/04/fill/Fill.asm

@@ -1,74 +1,72 @@ 
-// 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.
-
-// TODO: optimize better in the future
-
-(SET_SCREEN_PTR)
-    // R0 = SCREEN pointer
-    @SCREEN
-    D=A
-    @R0
-    M=D
-
-// TODO: fix infinitely incrementing screen pointer bug
-// end of screen: 0x5fff (dec 24575)
-
-(IS_KEYPRESSED)
-    // 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
-
-    // ++R0
-    @R0
-    M=M+1
-
-    @IS_KEYPRESSED
-    0;JMP
-    // TODO: replace with D;JLT or something
-
-(WHITE_SCREEN)
-    // *R0 = 0
-    @R0
-    A=M
-    M=0
-
-    // ++R0
-    @R0
-    M=M+1
-
-    @IS_KEYPRESSED
-    0;JMP
-    // TODO: replace with D;JLT or something
-
-(END)
-    @END
-    0;JMP
-
+// 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.
+
+// TODO: optimize better in the future
+
+// have separate branch _only_ for drawing black/white pixels
+
+(SET_SCREEN_PTR)
+    // R0 = SCREEN pointer
+    @SCREEN
+    D=A
+    @R0
+    M=D
+
+// TODO: fix infinitely incrementing screen pointer bug
+// end of screen: 0x5fff (dec 24575)
+
+(IS_KEYPRESSED)
+    // 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
+