pokecrystal-board/home/vblank.asm

; VBlank is the interrupt responsible for updating VRAM.

; In Pokemon Crystal, VBlank has been hijacked to act as the
; main loop. After time-sensitive graphics operations have been
; performed, joypad input and sound functions are executed.

; This prevents the display and audio output from lagging.

VBlank::
	push af
	push bc
	push de
	push hl

	ldh a, [hVBlank]
	and 7

	ld e, a
	ld d, 0
	ld hl, .VBlanks
	add hl, de
	add hl, de
	ld a, [hli]
	ld h, [hl]
	ld l, a

	call _hl_

	call GameTimer

	pop hl
	pop de
	pop bc
	pop af
	reti

.VBlanks:
	dw VBlank0
	dw VBlank1
	dw VBlank2
	dw VBlank3
	dw VBlank4
	dw VBlank5
	dw VBlank6
	dw VBlank0 ; just in case

VBlank0::
; normal operation

; rng
; scx, scy, wy, wx
; bg map buffer
; palettes
; dma transfer
; bg map
; tiles
; oam
; joypad
; sound

	; inc frame counter
	ld hl, hVBlankCounter
	inc [hl]

	ldh a, [hROMBank]
	ldh [hROMBankBackup], a

	; enable window back in case LCD interrupt disabled it mid-frame due to hWindowHUDLY
	ldh a, [rLCDC]
	set rLCDC_WINDOW_ENABLE, a
	ldh [rLCDC], a

	ld a, [hWindowHUDLY]
	and a
	jr z, .next

	xor a
	ldh [hWY], a

.next
	ldh a, [hSCX]
	ldh [rSCX], a
	ldh a, [hSCY]
	ldh [rSCY], a
	ldh a, [hWY]
	ldh [rWY], a
	ldh a, [hWX]
	ldh [rWX], a

	; There's only time to call one of these in one vblank.
	; Calls are in order of priority.

	call UpdateBGMapBuffer
	jr c, .done
	call UpdatePalsIfCGB
	jr c, .done
	call DMATransfer
	jr c, .done
	call UpdateBGMap

	; These have their own timing checks.

	call Serve2bppRequest
	call Serve1bppRequest
	call AnimateTileset

.done

	ldh a, [hOAMUpdate]
	and a
	jr nz, .done_oam
	call hTransferShadowOAM
.done_oam

	; vblank-sensitive operations are done

	xor a
	ldh [hVBlankOccurred], a

	; if hWindowHUDLY is active, enable interrupts so the LCD interrupt can trigger
	ldh a, [hWindowHUDLY]
	and a
	jr z, .next2

	; enable lcd stat
	ld a, 1 << LCD_STAT
	ldh [rIE], a
	ei

.next2
	; advance random variables
	ldh a, [rDIV]
	ld b, a
	ldh a, [hRandomAdd]
	adc b
	ldh [hRandomAdd], a

	ldh a, [rDIV]
	ld b, a
	ldh a, [hRandomSub]
	sbc b
	ldh [hRandomSub], a

	ldh a, [hOverworldDelay]
	and a
	jr z, .ok
	dec a
	ldh [hOverworldDelay], a
.ok

	ldh a, [hTextDelayFrames]
	and a
	jr z, .ok2
	dec a
	ldh [hTextDelayFrames], a
.ok2

	call UpdateJoypad

	ld a, BANK(_UpdateSound)
	rst Bankswitch
	call _UpdateSound
	ldh a, [hROMBankBackup]
	rst Bankswitch

	; if hWindowHUDLY is not active, we're done
	ldh a, [hWindowHUDLY]
	and a
	ret z

	; interrupts must be enabled in the cycle that rLY becomes [hWindowHUDLY] to prevent flickering
	; wait until [hWindowHUDLY] - [rLY] is NOT between 0 and 2 before disabling interrupts
.wait_loop
	ldh a, [rLY]
	ld b, a
	ldh a, [hWindowHUDLY]
	sub b
	cp 2 + 1
	jr c, .wait_loop

	; restore normal interrupts: enable ints besides joypad and let vblank finish
	di
	ld a, IE_DEFAULT
	ldh [rIE], a

	ret

VBlank2::
; sound only

	ldh a, [hROMBank]
	ldh [hROMBankBackup], a

	ld a, BANK(_UpdateSound)
	rst Bankswitch
	call _UpdateSound

	ldh a, [hROMBankBackup]
	rst Bankswitch

	xor a
	ldh [hVBlankOccurred], a
	ret

VBlank1::
; scx, scy
; palettes
; bg map
; tiles
; oam
; sound / lcd stat

	ldh a, [hROMBank]
	ldh [hROMBankBackup], a

	ldh a, [hSCX]
	ldh [rSCX], a
	ldh a, [hSCY]
	ldh [rSCY], a

	call UpdateCGBPals
	jr c, .done

	call UpdateBGMap
	call Serve2bppRequest_VBlank

	call hTransferShadowOAM

.done
	xor a
	ldh [hVBlankOccurred], a

	; get requested ints
	ldh a, [rIF]
	ld b, a
	; discard requested ints
	xor a
	ldh [rIF], a
	; enable lcd stat
	ld a, 1 << LCD_STAT
	ldh [rIE], a
	; rerequest serial int if applicable (still disabled)
	; request lcd stat
	ld a, b
	and 1 << SERIAL
	or 1 << LCD_STAT
	ldh [rIF], a

	ei
	ld a, BANK(_UpdateSound)
	rst Bankswitch
	call _UpdateSound
	ldh a, [hROMBankBackup]
	rst Bankswitch
	di

	; get requested ints
	ldh a, [rIF]
	ld b, a
	; discard requested ints
	xor a
	ldh [rIF], a
	; enable ints besides joypad
	ld a, IE_DEFAULT
	ldh [rIE], a
	; rerequest ints
	ld a, b
	ldh [rIF], a
	ret

VBlank3::
; scx, scy
; palettes
; bg map
; tiles
; oam
; sound / lcd stat

	ldh a, [hROMBank]
	ldh [hROMBankBackup], a

	ldh a, [hSCX]
	ldh [rSCX], a
	ldh a, [hSCY]
	ldh [rSCY], a

	ldh a, [hCGBPalUpdate]
	and a
	call nz, ForceUpdateCGBPals
	jr c, .done

	call UpdateBGMap
	call Serve2bppRequest_VBlank

	call hTransferShadowOAM
.done

	xor a
	ldh [hVBlankOccurred], a

	ldh a, [rIF]
	push af
	xor a
	ldh [rIF], a
	ld a, 1 << LCD_STAT
	ldh [rIE], a
	ldh [rIF], a

	ei
	ld a, BANK(_UpdateSound)
	rst Bankswitch
	call _UpdateSound
	ldh a, [hROMBankBackup]
	rst Bankswitch
	di

	; request lcdstat
	ldh a, [rIF]
	ld b, a
	; and any other ints
	pop af
	or b
	ld b, a
	; reset ints
	xor a
	ldh [rIF], a
	; enable ints besides joypad
	ld a, IE_DEFAULT
	ldh [rIE], a
	; request ints
	ld a, b
	ldh [rIF], a
	ret

VBlank4::
; bg map
; tiles
; oam
; joypad
; serial
; sound

	ldh a, [hROMBank]
	ldh [hROMBankBackup], a

	call UpdateBGMap
	call Serve2bppRequest

	call hTransferShadowOAM

	call UpdateJoypad

	xor a
	ldh [hVBlankOccurred], a

	call AskSerial

	ld a, BANK(_UpdateSound)
	rst Bankswitch
	call _UpdateSound

	ldh a, [hROMBankBackup]
	rst Bankswitch
	ret

VBlank5::
; scx
; palettes
; bg map
; tiles
; joypad
;

	ldh a, [hROMBank]
	ldh [hROMBankBackup], a

	ldh a, [hSCX]
	ldh [rSCX], a

	call UpdatePalsIfCGB
	jr c, .done

	call UpdateBGMap
	call Serve2bppRequest
.done

	xor a
	ldh [hVBlankOccurred], a

	call UpdateJoypad

	xor a
	ldh [rIF], a
	ld a, 1 << LCD_STAT
	ldh [rIE], a
	; request lcd stat
	ldh [rIF], a

	ei
	ld a, BANK(_UpdateSound)
	rst Bankswitch
	call _UpdateSound
	ldh a, [hROMBankBackup]
	rst Bankswitch
	di

	xor a
	ldh [rIF], a
	; enable ints besides joypad
	ld a, IE_DEFAULT
	ldh [rIE], a
	ret

VBlank6::
; palettes
; tiles
; dma transfer
; sound

	ldh a, [hROMBank]
	ldh [hROMBankBackup], a

	; inc frame counter
	ld hl, hVBlankCounter
	inc [hl]

	call UpdateCGBPals
	jr c, .done

	call Serve2bppRequest
	call Serve1bppRequest
	call DMATransfer
.done

	xor a
	ldh [hVBlankOccurred], a

	ld a, BANK(_UpdateSound)
	rst Bankswitch
	call _UpdateSound

	ldh a, [hROMBankBackup]
	rst Bankswitch
	ret