157 lines
2.6 KiB
NASM
157 lines
2.6 KiB
NASM
; Sameboy CGB bootstrap ROM
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; Todo: use friendly names for HW registers instead of magic numbers
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SECTION "BootCode", ROM0[$0]
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Start:
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; Init stack pointer
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ld sp, $fffe
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; Clear memory VRAM
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ld hl, $8000
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.clearVRAMLoop
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ldi [hl], a
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bit 5, h
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jr z, .clearVRAMLoop
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; Init Audio
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ld a, $80
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ldh [$26], a
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ldh [$11], a
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ld a, $f3
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ldh [$12], a
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ldh [$25], a
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ld a, $77
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ldh [$24], a
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; Init BG palette
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ld a, $fc
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ldh [$47], a
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; Load logo from ROM.
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; A nibble represents a 4-pixels line, 2 bytes represent a 4x4 tile, scaled to 8x8.
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; Tiles are ordered left to right, top to bottom.
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ld de, $104 ; Logo start
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ld hl, $8010 ; This is where we load the tiles in VRAM
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.loadLogoLoop
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ld a, [de] ; Read 2 rows
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ld b, a
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call DoubleBitsAndWriteRow
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call DoubleBitsAndWriteRow
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inc de
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ld a, e
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xor $34 ; End of logo
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jr nz, .loadLogoLoop
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; Load trademark symbol
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ld de, TrademarkSymbol
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ld c,$08
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.loadTrademarkSymbolLoop:
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ld a,[de]
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inc de
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ldi [hl],a
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inc hl
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dec c
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jr nz, .loadTrademarkSymbolLoop
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; Set up tilemap
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ld a,$19 ; Trademark symbol
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ld [$9910], a ; ... put in the superscript position
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ld hl,$992f ; Bottom right corner of the logo
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ld c,$c ; Tiles in a logo row
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.tilemapLoop
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dec a
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jr z, .tilemapDone
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ldd [hl], a
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dec c
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jr nz, .tilemapLoop
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ld l,$0f ; Jump to top row
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jr .tilemapLoop
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.tilemapDone
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; Turn on LCD
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ld a, $91
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ldh [$40], a
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; Wait ~0.75 seconds
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ld b, 45
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call WaitBFrames
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; Play first sound
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ld a, $83
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call PlaySound
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ld b, 5
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call WaitBFrames
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; Play second sound
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ld a, $c1
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call PlaySound
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; Wait ~1.15 seconds
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ld b, 70
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call WaitBFrames
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; Set registers to match the original DMG boot
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IF DEF(MGB)
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ld hl, $01B0
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ELSE
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ld hl, $FFB0
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ENDC
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push hl
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pop af
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ld hl, $014D
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ld bc, $0013
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ld de, $00D8
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; Boot the game
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jp BootGame
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DoubleBitsAndWriteRow:
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; Double the most significant 4 bits, b is shifted by 4
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ld a, 4
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ld c, 0
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.doubleCurrentBit
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sla b
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push af
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rl c
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pop af
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rl c
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dec a
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jr nz, .doubleCurrentBit
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ld a, c
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; Write as two rows
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ldi [hl], a
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inc hl
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ldi [hl], a
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inc hl
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ret
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WaitFrame:
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push hl
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ld hl, $FF0F
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res 0, [hl]
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.wait
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bit 0, [hl]
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jr z, .wait
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pop hl
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ret
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WaitBFrames:
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call WaitFrame
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dec b
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jr nz, WaitBFrames
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ret
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PlaySound:
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ldh [$13], a
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ld a, $87
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ldh [$14], a
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ret
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TrademarkSymbol:
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db $3c,$42,$b9,$a5,$b9,$a5,$42,$3c
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SECTION "BootGame", ROM0[$fe]
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BootGame:
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ldh [$50], a |