1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
|
; SameBoy SGB bootstrap ROM
include "sameboot.inc"
SECTION "BootCode", ROM0[$0000]
Start:
; Init stack pointer
ld sp, $FFFE
; Clear memory VRAM
ld hl, _VRAM
xor a
.clearVRAMLoop
ldi [hl], a
bit 5, h
jr z, .clearVRAMLoop
; Init Audio
ld a, AUDENA_ON
ldh [rNR52], a
assert AUDENA_ON == AUDLEN_DUTY_50
ldh [rNR11], a
ld a, $F3
ldh [rNR12], a ; Envelope $F, decreasing, sweep $3
ldh [rNR51], a ; Channels 1+2+3+4 left, channels 1+2 right
ld a, $77
ldh [rNR50], a ; Volume $7, left and right
; Init BG palette to white
ld a, %00_00_00_00
ldh [rBGP], 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, NintendoLogo
ld hl, _VRAM + $10 ; 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 LOW(NintendoLogoEnd)
jr nz, .loadLogoLoop
; Load trademark symbol
ld de, TrademarkSymbol
ld c, TrademarkSymbolEnd - TrademarkSymbol
.loadTrademarkSymbolLoop:
ld a, [de]
inc de
ldi [hl], a
inc hl
dec c
jr nz, .loadTrademarkSymbolLoop
; Set up tilemap
ld a, $19 ; Trademark symbol tile ID
ld [_SCRN0 + 8 * SCRN_VX_B + 16], a ; ... put in the superscript position
ld hl, _SCRN0 + 9 * SCRN_VX_B + 15 ; Bottom right corner of the logo
ld c, 12 ; 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, LCDCF_ON | LCDCF_BLK01 | LCDCF_BGON
ldh [rLCDC], a
ld a, $F1 ; Packet magic, increases by 2 for every packet
ldh [hCommand], a
ld hl, NintendoLogo ; Header start
xor a
ld c, a ; JOYP
.sendCommand
xor a
ldh [c], a
ld a, $30
ldh [c], a
ldh a, [hCommand]
call SendByte
push hl
ld b, 14
ld d, 0
.checksumLoop
call ReadHeaderByte
add d
ld d, a
dec b
jr nz, .checksumLoop
; Send checksum
call SendByte
pop hl
ld b, 14
.sendLoop
call ReadHeaderByte
call SendByte
dec b
jr nz, .sendLoop
; Done bit
ld a, $20
ldh [c], a
ld a, $30
ldh [c], a
; Wait 4 frames
ld e, 4
ld a, 1
ldh [rIE], a
xor a
.waitLoop
ldh [rIF], a
halt
nop
dec e
jr nz, .waitLoop
ldh [rIE], a
; Update command
ldh a, [hCommand]
add 2
ldh [hCommand], a
ld a, $58
cp l
jr nz, .sendCommand
; Write to sound registers for DMG compatibility
ld c, LOW(rNR13)
ld a, $C1
ldh [c], a
inc c
ld a, $7
ldh [c], a
; Init BG palette
ld a, %11_11_11_00
ldh [rBGP], a
; Set registers to match the original SGB boot
IF DEF(SGB2)
ld a, BOOTUP_A_MGB
ELSE
ld a, BOOTUP_A_DMG
ENDC
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
TrademarkSymbol:
pusho
opt b.X
db %..XXXX..
db %.X....X.
db %X.XXX..X
db %X.X..X.X
db %X.XXX..X
db %X.X..X.X
db %.X....X.
db %..XXXX..
popo
TrademarkSymbolEnd:
SECTION "BootGame", ROM0[$00FE]
BootGame:
ldh [rBANK], a
SECTION "HRAM", HRAM[_HRAM]
hCommand:
ds 1
|
