Use VBlank interrupt for Unbricked frame timing

src/part2/objects.md

@@ -96,18 +96,27 @@ There are actually two object palettes, but we're only going to use one.
 
 Now that you have an object on the screen, let's move it around.
 Previously, the `Done` loop did nothing; let's rename it to `Main` and use it to move our object.
-We're going to wait for VBlank before changing OAM, just like we did before turning off the screen.
+We're going to use the VBlank interrupt to wait for the next frame before changing OAM.
 
-```rgbasm,linenos,start={{#line_no_of "^Main:" ../../unbricked/objects/main.asm}}
+First, add an interrupt handler at the start of the file:
+
+```rgbasm,linenos,start={{#line_no_of "" ../../unbricked/objects/main.asm:vblank-interrupt}}
+{{#include ../../unbricked/objects/main.asm:vblank-interrupt}}
+```
+
+Next, add this helper function before the graphics data:
+
+```rgbasm,linenos,start={{#line_no_of "" ../../unbricked/objects/main.asm:wait-for-vblank}}
+{{#include ../../unbricked/objects/main.asm:wait-for-vblank}}
+```
+
+For now, the VBlank interrupt handler just returns. This is enough for `halt` to stop the CPU until the next VBlank interrupt wakes it back up.
+
+Now rename the `Done` loop to `Main` and call that helper before moving the paddle:
+
+```rgbasm
 Main:
-    ; Wait until it's *not* VBlank
-    ld a, [rLY]
-    cp 144
-    jp nc, Main
-WaitVBlank2:
-	ld a, [rLY]
-	cp 144
-	jp c, WaitVBlank2
+	call WaitForVBlank
 
 	; Move the paddle one pixel to the right.
 	ld a, [STARTOF(OAM) + 1]

unbricked/audio/main.asm

@@ -6,6 +6,10 @@ DEF BRICK_RIGHT EQU $06
 DEF BLANK_TILE EQU $08
 ; ANCHOR_END: constants
 
+SECTION "VBlank Interrupt", ROM0[$40]
+VBlankInterrupt:
+	reti
+
 SECTION "Header", ROM0[$100]
 
 	jp EntryPoint
@@ -93,14 +97,15 @@ ClearOam:
 	ld a, 0
 	ld [wFrameCounter], a
 
+	; Enable the VBlank interrupt
+	xor a, a
+	ld [rIF], a
+	ld a, IE_VBLANK
+	ld [rIE], a
+	ei
+
 Main:
-	ld a, [rLY]
-	cp 144
-	jp nc, Main
-WaitVBlank2:
-	ld a, [rLY]
-	cp 144
-	jp c, WaitVBlank2
+	call WaitForVBlank
 
 	; Add the ball's momentum to its position in OAM.
 	ld a, [wBallMomentumX]
@@ -347,6 +352,10 @@ Input:
 .knownret
   ret
 
+WaitForVBlank:
+	halt
+	ret
+
 ; Copy bytes from one area to another.
 ; @param de: Source
 ; @param hl: Destination

unbricked/bcd/main.asm

@@ -10,6 +10,10 @@ DEF SCORE_TENS   EQU $9870
 DEF SCORE_ONES   EQU $9871
 ; ANCHOR_END: score-tile-location
 
+SECTION "VBlank Interrupt", ROM0[$40]
+VBlankInterrupt:
+	reti
+
 SECTION "Header", ROM0[$100]
 
 	jp EntryPoint
@@ -102,14 +106,15 @@ ClearOam:
 	ld [wScore], a
 	; ANCHOR_END: init-variables
 
+	; Enable the VBlank interrupt
+	xor a, a
+	ld [rIF], a
+	ld a, IE_VBLANK
+	ld [rIE], a
+	ei
+
 Main:
-	ld a, [rLY]
-	cp 144
-	jp nc, Main
-WaitVBlank2:
-	ld a, [rLY]
-	cp 144
-	jp c, WaitVBlank2
+	call WaitForVBlank
 
 	; Add the ball's momentum to its position in OAM.
 	ld a, [wBallMomentumX]
@@ -378,6 +383,10 @@ Input:
 .knownret
   ret
 
+WaitForVBlank:
+	halt
+	ret
+
 ; Copy bytes from one area to another.
 ; @param de: Source
 ; @param hl: Destination

unbricked/bricks/main.asm

@@ -6,6 +6,10 @@ DEF BRICK_RIGHT EQU $06
 DEF BLANK_TILE EQU $08
 ; ANCHOR_END: constants
 
+SECTION "VBlank Interrupt", ROM0[$40]
+VBlankInterrupt:
+	reti
+
 SECTION "Header", ROM0[$100]
 
 	jp EntryPoint
@@ -96,14 +100,15 @@ ClearOam:
 	ld [wCurKeys], a
 	ld [wNewKeys], a
 
+	; Enable the VBlank interrupt
+	xor a, a
+	ld [rIF], a
+	ld a, IE_VBLANK
+	ld [rIE], a
+	ei
+
 Main:
-	ld a, [rLY]
-	cp 144
-	jp nc, Main
-WaitVBlank2:
-	ld a, [rLY]
-	cp 144
-	jp c, WaitVBlank2
+	call WaitForVBlank
 
 	; Add the ball's momentum to its position in OAM.
 	ld a, [wBallMomentumX]
@@ -343,6 +348,10 @@ Input:
 .knownret
   ret
 
+WaitForVBlank:
+	halt
+	ret
+
 ; Copy bytes from one area to another.
 ; @param de: Source
 ; @param hl: Destination

unbricked/collision/main.asm

@@ -1,5 +1,9 @@
 INCLUDE "hardware.inc"
 
+SECTION "VBlank Interrupt", ROM0[$40]
+VBlankInterrupt:
+	reti
+
 SECTION "Header", ROM0[$100]
 
 	jp EntryPoint
@@ -96,15 +100,16 @@ ClearOam:
 	ld [wCurKeys], a
 	ld [wNewKeys], a
 
+	; Enable the VBlank interrupt
+	xor a, a
+	ld [rIF], a
+	ld a, IE_VBLANK
+	ld [rIE], a
+	ei
+
 ; ANCHOR: momentum
 Main:
-	ld a, [rLY]
-	cp 144
-	jp nc, Main
-WaitVBlank2:
-	ld a, [rLY]
-	cp 144
-	jp c, WaitVBlank2
+	call WaitForVBlank
 
 	; Add the ball's momentum to its position in OAM.
 	ld a, [wBallMomentumX]
@@ -330,6 +335,10 @@ UpdateKeys:
 .knownret
   ret
 
+WaitForVBlank:
+	halt
+	ret
+
 ; Copy bytes from one area to another.
 ; @param de: Source
 ; @param hl: Destination

unbricked/functions/main.asm

@@ -1,5 +1,9 @@
 INCLUDE "hardware.inc"
 
+SECTION "VBlank Interrupt", ROM0[$40]
+VBlankInterrupt:
+	reti
+
 SECTION "Header", ROM0[$100]
 
 	jp EntryPoint
@@ -72,15 +76,16 @@ ClearOam:
 	ld a, 0
 	ld [wFrameCounter], a
 
+	; Enable the VBlank interrupt
+	xor a, a
+	ld [rIF], a
+	ld a, IE_VBLANK
+	ld [rIE], a
+	ei
+
 ; ANCHOR: main
 Main:
-	ld a, [rLY]
-	cp 144
-	jp nc, Main
-WaitVBlank2:
-	ld a, [rLY]
-	cp 144
-	jp c, WaitVBlank2
+	call WaitForVBlank
 
 	ld a, [wFrameCounter]
 	inc a
@@ -99,6 +104,10 @@ WaitVBlank2:
 	jp Main
 ; ANCHOR_END: main
 
+WaitForVBlank:
+	halt
+	ret
+
 ; ANCHOR: memcpy
 ; Copy bytes from one area to another.
 ; @param de: Source

unbricked/input/main.asm

@@ -1,5 +1,9 @@
 INCLUDE "hardware.inc"
 
+SECTION "VBlank Interrupt", ROM0[$40]
+VBlankInterrupt:
+	reti
+
 SECTION "Header", ROM0[$100]
 
 	jp EntryPoint
@@ -76,15 +80,16 @@ ClearOam:
 	ld [wNewKeys], a
 	; ANCHOR_END: initialize-vars
 
+	; Enable the VBlank interrupt
+	xor a, a
+	ld [rIF], a
+	ld a, IE_VBLANK
+	ld [rIE], a
+	ei
+
 ; ANCHOR: main
 Main:
-	ld a, [rLY]
-	cp 144
-	jp nc, Main
-WaitVBlank2:
-	ld a, [rLY]
-	cp 144
-	jp c, WaitVBlank2
+	call WaitForVBlank
 
 	; Check the current keys every frame and move left or right.
 	call UpdateKeys
@@ -120,6 +125,10 @@ Right:
 	jp Main
 ; ANCHOR_END: main
 
+WaitForVBlank:
+	halt
+	ret
+
 ; ANCHOR: input-routine
 UpdateKeys:
   ; Poll half the controller

unbricked/objects/main.asm

@@ -2,6 +2,12 @@
 ; ANCHOR_END: dummy  Note that lines matching /^; ANCHOR/ are stripped from the online version
 INCLUDE "hardware.inc"
 
+; ANCHOR: vblank-interrupt
+SECTION "VBlank Interrupt", ROM0[$40]
+VBlankInterrupt:
+	reti
+; ANCHOR_END: vblank-interrupt
+
 SECTION "Header", ROM0[$100]
 
 	jp EntryPoint
@@ -98,14 +104,15 @@ ClearOam:
 	ld a, 0
 	ld [wFrameCounter], a
 
+	; Enable the VBlank interrupt
+	xor a, a
+	ld [rIF], a
+	ld a, IE_VBLANK
+	ld [rIE], a
+	ei
+
 Main:
-	ld a, [rLY]
-	cp 144
-	jp nc, Main
-WaitVBlank2:
-	ld a, [rLY]
-	cp 144
-	jp c, WaitVBlank2
+	call WaitForVBlank
 
 	ld a, [wFrameCounter]
 	inc a
@@ -124,6 +131,12 @@ WaitVBlank2:
 	jp Main
 ; ANCHOR_END: main-loop
 
+; ANCHOR: wait-for-vblank
+WaitForVBlank:
+	halt
+	ret
+; ANCHOR_END: wait-for-vblank
+
 Tiles:
 	dw `33333333
 	dw `33333333