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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.
+
+@8192
+D=A
+@words
+M=D	// words = 8192
+
+(KBD_LOOP)
+	@KBD
+	D=M
+	@FILL_WHITE
+	D;JEQ	// if (KBD == 0) goto FILL_WHITE
+	@color
+	M=-1	// color = 0xffff (black)
+	@FILL
+	0;JMP
+
+(FILL_WHITE)
+	@color
+	M=0	// color = 0x0000 (white)
+	@FILL
+	0;JMP
+
+(FILL)
+	@i
+	M=0	// i = 0
+	@SCREEN
+	D=A
+	@pos
+	M=D	// pos = &SCREEN
+	(DRAW)
+		@color
+		D=M
+		@pos
+		A=M
+		M=D	// *pos = color
+		@pos
+		M=M+1	// pos++
+		@i
+		M=M+1	// i++
+		D=M	// D = i
+		@words
+		D=D-M	// D = i - words
+		@KBD_LOOP
+		D;JEQ	// if (i - words == 0) break
+		@DRAW
+		0;JMP