SameBoy/BootROMs/sgb_boot.asm

209 lines
3.3 KiB
NASM

; Sameboy CGB bootstrap ROM
; Todo: use friendly names for HW registers instead of magic numbers
SECTION "BootCode", ROM0[$0]
Start:
; Init stack pointer
ld sp, $fffe
; Clear memory VRAM
ld hl, $8000
.clearVRAMLoop
ldi [hl], a
bit 5, h
jr z, .clearVRAMLoop
; Init Audio
ld a, $80
ldh [$26], a
ldh [$11], a
ld a, $f3
ldh [$12], a
ldh [$25], a
ld a, $77
ldh [$24], a
; Init BG palette to white
ld a, $0
ldh [$47], a
; Load logo from ROM.
; A nibble represents a 4-pixels line, 2 bytes represent a 4x4 tile, scaled to 8x8.
; Tiles are ordered left to right, top to bottom.
ld de, $104 ; Logo start
ld hl, $8010 ; This is where we load the tiles in VRAM
.loadLogoLoop
ld a, [de] ; Read 2 rows
ld b, a
call DoubleBitsAndWriteRow
call DoubleBitsAndWriteRow
inc de
ld a, e
xor $34 ; End of logo
jr nz, .loadLogoLoop
; Load trademark symbol
ld de, TrademarkSymbol
ld c,$08
.loadTrademarkSymbolLoop:
ld a,[de]
inc de
ldi [hl],a
inc hl
dec c
jr nz, .loadTrademarkSymbolLoop
; Set up tilemap
ld a,$19 ; Trademark symbol
ld [$9910], a ; ... put in the superscript position
ld hl,$992f ; Bottom right corner of the logo
ld c,$c ; Tiles in a logo row
.tilemapLoop
dec a
jr z, .tilemapDone
ldd [hl], a
dec c
jr nz, .tilemapLoop
ld l,$0f ; Jump to top row
jr .tilemapLoop
.tilemapDone
; Turn on LCD
ld a, $91
ldh [$40], a
ld a, $f1 ; Packet magic, increases by 2 for every packet
ldh [$80], a
ld hl, $104 ; Header start
xor a
ld c, a ; JOYP
.sendCommand
xor a
ldh [c], a
ld a, $30
ldh [c], a
ldh a, [$80]
call SendByte
push hl
ld b, $e
ld d, 0
.checksumLoop
call ReadHeaderByte
add d
ld d, a
dec b
jr nz, .checksumLoop
; Send checksum
call SendByte
pop hl
ld b, $e
.sendLoop
call ReadHeaderByte
call SendByte
dec b
jr nz, .sendLoop
; Done bit
ld a, $20
ldh [c], a
ld a, $30
ldh [c], a
; Update command
ldh a, [$80]
add 2
ldh [$80], a
ld a, $58
cp l
jr nz, .sendCommand
; Write to sound registers for DMG compatibility
ld c, $13
ld a, $c1
ldh [c], a
inc c
ld a, 7
ldh [c], a
; Init BG palette
ld a, $fc
ldh [$47], a
; Set registers to match the original SGB boot
ld a, 1
ld hl, $c060
; Boot the game
jp BootGame
ReadHeaderByte:
ld a, $4F
cp l
jr c, .zero
ld a, [hli]
ret
.zero:
inc hl
xor a
ret
SendByte:
ld e, a
ld d, 8
.loop
ld a, $10
rr e
jr c, .zeroBit
add a ; 10 -> 20
.zeroBit
ldh [c], a
ld a, $30
ldh [c], a
dec d
ret z
jr .loop
DoubleBitsAndWriteRow:
; Double the most significant 4 bits, b is shifted by 4
ld a, 4
ld c, 0
.doubleCurrentBit
sla b
push af
rl c
pop af
rl c
dec a
jr nz, .doubleCurrentBit
ld a, c
; Write as two rows
ldi [hl], a
inc hl
ldi [hl], a
inc hl
ret
WaitFrame:
push hl
ld hl, $FF0F
res 0, [hl]
.wait
bit 0, [hl]
jr z, .wait
pop hl
ret
TrademarkSymbol:
db $3c,$42,$b9,$a5,$b9,$a5,$42,$3c
SECTION "BootGame", ROM0[$fe]
BootGame:
ldh [$50], a