add Fill.asm

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.
+
+// addr = SCREEN map start address
+@SCREEN
+D=A
+@addr
+M=D
+
+// reg = SCREEN map register total count
+@8192
+D=A
+@reg
+M=D
+
+// itr = current register count
+@itr
+M=0
+
+// black if pix = -1
+// white if pix = 0
+@pix
+M=-1
+
+// loop declaration
+// D = reg - itr
+// if 0 jump to @END
+// else continue
+    (LOOP)
+@itr
+D=M
+@reg
+D=D-M
+@END
+D;JEQ
+
+// addr = 16384..24575
+// @16384 = pix
+// @16385 = pix
+// .
+// .
+// @24575 = pix
+@pix
+D=M
+@addr
+A=M
+M=D
+
+// itr = itr + 1
+// addr = addr + 1
+@itr
+M=M+1
+@addr
+M=M+1
+
+//back to loop declaration
+@LOOP
+0;JMP
+
+
+// clear the screen if no key pressed:
+// pix = 0
+// itr = 0
+// addr = 16384
+// back to loop declaration
+    (CLEAR)
+@pix
+M=0
+
+@itr
+M=0
+
+@SCREEN
+D=A
+@addr
+M=D
+
+@LOOP
+0;JMP
+
+// paint the screen if no key pressed:
+// pix = -1
+// itr = 0
+// addr = 16384
+// back to loop declaration
+    (PAINT)
+@pix
+M=-1
+
+@itr
+M=0
+
+@SCREEN
+D=A
+@addr
+M=D
+
+@LOOP
+0;JMP
+
+// main feedback loop
+// jump to @CLEAR if no key pressed
+// jump to @PAINT if any key pressed
+    (END)
+@KBD
+D=M
+@CLEAR
+D;JEQ
+
+@KBD
+D=M
+@PAINT
+D;JGT
+
+@END
+0;JMP