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
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
|
; configuration {{{
.asciitable
MAP "-" = 1
MAP "|" = 2
MAP ":" = 3
MAP "." = 4
MAP "," = 5
MAP "'" = 6
.enda
.struct point
x db
y db
.endst
; }}}
; memory layout {{{
; rom {{{
.ROMBANKMAP
BANKSTOTAL 2
BANKSIZE $4000
BANKS 1
BANKSIZE $2000
BANKS 1
.ENDRO
; }}}
; ram {{{
.MEMORYMAP
DEFAULTSLOT 0
SLOTSIZE $4000
SLOT 0 $C000
SLOTSIZE $2000
SLOT 1 $0000 ; location doesn't matter, CHR data isn't in main memory
.ENDME
.ENUM $0000
buttons_pressed DB
sleeping DB
game_state DB ; 0: menu, 1: playing, 2: redrawing
head DW
length DB
direction DB ; 0: up, 1: down, 2: left, 3: right
frame_skip DB
frame_count DB
rand_state DB
apple INSTANCEOF point
.ENDE
; }}}
; }}}
; prg {{{
.bank 0
.org $0000
; main codepath {{{
; initialization {{{
; the ppu takes two frames to initialize, so we have some time to do whatever
; initialization of our own that we want to while we wait. we choose here to
; set up cpu flags in the first frame and clear out system ram in the second
; frame (clearing out ram isn't at all necessary, but we can't do anything
; useful at this point anyway, so we may as well in order to make things more
; predictable).
RESET:
SEI ; disable IRQs
CLD ; disable decimal mode
LDX #$40
STX $4017.w ; disable APU frame IRQ
LDX #$FF
TXS ; Set up stack (grows down from $FF to $00, at $0100-$01FF)
INX ; now X = 0
STX $2000.w ; disable NMI (we'll enable it later once the ppu is ready)
STX $2001.w ; disable rendering (same)
STX $4010.w ; disable DMC IRQs
; First wait for vblank to make sure PPU is ready
- BIT $2002 ; bit 7 of $2002 is reset once vblank ends
BPL - ; and bit 7 is what is checked by BPL
; set everything in ram ($0000-$07FF) to $00, except for $0200-$02FF which
; is conventionally used to hold sprite attribute data. we set that range
; to $FE, since that value as a position moves the sprites offscreen, and
; when the sprites are offscreen, it doesn't matter which sprites are
; selected or what their attributes are
clrmem:
LDA #$00
STA $0000, x
STA $0100, x
STA $0300, x
STA $0400, x
STA $0500, x
STA $0600, x
STA $0700, x
LDA #$FE
STA $0200, x
INX
BNE clrmem
; initialize variables in ram
LDA #$03
LDX #$01
STA head, x
LDA #30
STA frame_skip
; Second wait for vblank, PPU is ready after this
- BIT $2002
BPL -
; now that the ppu is ready, we can start initializing it
LoadPalettes:
LDA $2002 ; read PPU status to reset the high/low latch
LDA #$3F
STA $2006 ; write the high byte of $3F00 address
LDA #$00
STA $2006 ; write the low byte of $3F00 address
LDX #$00
LoadPalettesLoop:
LDA palette.w, x ;load palette byte
STA $2007 ;write to PPU
INX ;set index to next byte
CPX #$20
BNE LoadPalettesLoop ;if x = $20, 32 bytes copied, all done
LDA #%10000000 ; enable NMI interrupts
STA $2000
JSR end_game
; }}}
; main loop {{{
loop:
INC sleeping
- LDA sleeping
BNE -
JSR read_controller1
LDA game_state
BNE +
JSR start_screen_loop
JMP loop
+ JSR game_loop
JMP loop
; }}}
; }}}
; nmi interrupt {{{
NMI:
PHA
TXA
PHA
TYA
PHA
LDA game_state
BEQ reset_sprites
CMP #$01
BEQ draw_game
CMP #$02
BEQ end_nmi
draw_game:
LDY #$01
LDA (head), y
STA $0200
LDA #$00
STA $0201
LDA #$00
STA $0202
LDY #$00
LDA (head), y
STA $0203
LDA apple.y
STA $0204
LDA #$07
STA $0205
LDA #$00
STA $0206
LDA apple.x
STA $0207
JMP do_dmi
reset_sprites:
LDA #$FE
STA $0200
STA $0203
STA $0204
STA $0207
do_dmi:
LDA #$00
STA $2003
LDA #$02
STA $4014
end_nmi:
LDA #$00
STA sleeping
PLA
TAY
PLA
TAX
PLA
RTI
; }}}
; subroutines {{{
start_screen_loop: ; {{{
LDX rand_state
- INX
BEQ - ; lfsr prngs have 0 as a fixed point
STX rand_state
handle_start:
LDA buttons_pressed
AND #%00010000
CMP #$00
BEQ end_start_screen_loop
JSR start_game
end_start_screen_loop:
RTS ; }}}
game_loop: ; {{{
handle_up:
LDA buttons_pressed
AND #%00001000
CMP #$00
BEQ handle_down
LDA #$00
STA direction
handle_down:
LDA buttons_pressed
AND #%00000100
CMP #$00
BEQ handle_left
LDA #$01
STA direction
handle_left:
LDA buttons_pressed
AND #%00000010
CMP #$00
BEQ handle_right
LDA #$02
STA direction
handle_right:
LDA buttons_pressed
AND #%00000001
CMP #$00
BEQ handle_frame
LDA #$03
STA direction
handle_frame:
LDX frame_count
INX
STX frame_count
CPX frame_skip
BMI end_game_loop
LDA #$00
STA frame_count
set_offset:
LDX #$F8 ; i.e., -8
LDA direction
AND #%00000001 ; low bit determines negative or positive
BEQ set_axis
LDX #$08
set_axis:
LDY #$01
LDA direction
AND #%00000010 ; high bit determines which axis to change
BEQ apply_direction
LDY #$00
apply_direction:
TXA
CLC
ADC (head), y ; head.x offset by 1 is head.y
INC head
INC head
STA (head), y
DEC head
DEC head
TYA
EOR #$01
TAY
LDA (head), y
INC head
INC head
STA (head), y
check_collisions
LDY #$00
LDA (head), y
TAX
LDY #$01
LDA (head), y
TAY
CPX #$40
BCC collision
CPX #$C0
BCS collision
CPY #$3D
BCC collision
CPY #$BD
BCS collision
CPX apple.x
BEQ maybe_eat_apple
JMP end_game_loop
collision:
JSR end_game
JMP end_game_loop
maybe_eat_apple:
CPY apple.y
BEQ eat_apple
JMP end_game_loop
eat_apple:
LDX length
INX
TXA
BEQ collision ; for now - this is the win condition
STX length
JSR new_apple
end_game_loop:
RTS ; }}}
read_controller1: ; {{{
; latch
LDA #$01
STA $4016
LDA #$00
STA $4016
; clock
LDX #$00
read_controller1_values:
CPX #$08
BPL end_read_controller1
LDA $4016
AND #%00000001
ASL buttons_pressed
ORA buttons_pressed
STA buttons_pressed
INX
JMP read_controller1_values
end_read_controller1:
RTS ; }}}
start_game: ; {{{
LDA #$02
STA game_state
LDA #$80
LDY #$00
STA (head), y
LDA #$7D
LDY #$01
STA (head), y
JSR new_apple
- BIT $2002
BPL -
LDA #%00000000
STA $2001 ; disable rendering (since this will take longer than vblank)
LDA #$20
STA $2006
LDA #$00
STA $2006
LDA #$20
LDY #$07
-- LDX #$00
- STA $2007
INX
CPX #$20
BNE -
DEY
BNE --
LDX #$00
- LDA game_background_top.w, x
STA $2007
INX
CPX #$20
BNE -
LDY #$10
-- LDX #$00
- LDA game_background_middle.w, x
STA $2007
INX
CPX #$20
BNE -
DEY
BNE --
LDX #$00
- LDA game_background_bottom.w, x
STA $2007
INX
CPX #$20
BNE -
LDA #$20
LDX #$00
- STA $2007
INX
CPX #$20
BNE -
LDA #%00011000
STA $2001 ; reenable rendering
LDA #$01
STA game_state
RTS ; }}}
end_game: ; {{{
LDA #$02
STA game_state
- BIT $2002
BPL -
LDA #%00000000
STA $2001 ; disable rendering (since this will take longer than vblank)
LDA #$20
STA $2006
LDA #$00
STA $2006
LDA #$20
LDX #$0F
-- LDY #$00
- STA $2007
INY
CPY #$20
BNE -
DEX
BNE --
LDY #$00
- LDA intro_screen, y
STA $2007
INY
CPY #$20
BNE -
LDA #$20
LDX #$0E
-- LDY #$00
- STA $2007
INY
CPY #$20
BNE -
DEX
BNE --
LDA #%00011000
STA $2001 ; reenable rendering
LDA #$00
STA game_state
RTS ; }}}
new_apple: ; {{{
JSR rand
LDA rand_state
AND #%01111000
CLC
ADC #$40
STA apple.x
JSR rand
LDA rand_state
AND #%01111000
CLC
ADC #$3D
STA apple.y
RTS ; }}}
rand: ; {{{
; linear feedback shift register with taps at 8, 6, 5, and 4
LDY rand_state
TYA
AND #%10000000
STA rand_state
TYA
AND #%00100000
ASL
ASL
EOR rand_state
STA rand_state
TYA
AND #%00010000
ASL
ASL
ASL
EOR rand_state
STA rand_state
TYA
AND #%00001000
ASL
ASL
ASL
ASL
EOR rand_state
ROL
STA rand_state
TYA
ASL
ORA rand_state
STA rand_state
RTS ; }}}
; }}}
; data {{{
palette: ; {{{
.db $0F,$31,$32,$33,$0F,$35,$36,$37,$0F,$39,$3A,$3B,$0F,$3D,$3E,$0F
.db $0F,$1C,$15,$14,$0F,$02,$38,$3C,$0F,$1C,$15,$14,$0F,$02,$38,$3C
; }}}
intro_screen: ; {{{
.asc " SNAKE "
; }}}
game_background_top: ; {{{
.asc " ,----------------. "
; }}}
game_background_middle: ; {{{
.asc " | | "
; }}}
game_background_bottom: ; {{{
.asc " '----------------: "
; }}}
; }}}
.orga $FFFA ;first of the three vectors starts here
; interrupt vectors {{{
.dw NMI ;when an NMI happens (once per frame if enabled) the
;processor will jump to the label NMI:
.dw RESET ;when the processor first turns on or is reset, it will jump
;to the label RESET:
.dw 0 ;external interrupt IRQ is not used in this tutorial
; }}}
; }}}
; chr {{{
.bank 1 slot 1
.org $0000
.incbin "sprites.chr"
; }}}
|