Easy 6502 Labyrinth by tklein23

Introduction

This little game is the a-maze-ing result of a Coding Dojo on 6502 assembly. We wrote a little labyrinth walk using the Easy 6502 simulator and this excellent 6502 tutorial.

The labyrinth has been generated by this (animated :)) maze generator. The generator first creates a maze while animating the current state. The result has been converted to assembler code (simple load/store instructions) and statically pasted into the labyrinth shown below.

You can walk this labyrinth using the W, A, S and D. Enjoy.

a-maze-ing Labyrinth Game


;                                   _
;  __ _   _ __ ___   __ _ _______  (_)_ __   __ _
; / _` | | '_ ` _ \ / _` |_  / _ \ | | '_ \ / _` |
;| (_| | | | | | | | (_| |/ /  __/ | | | | | (_| |
; \__,_| |_| |_| |_|\__,_/___\___| |_|_| |_|\__, |
;                       __  ____   ___ ____ |___/
;  ___  __ _ ___ _   _ / /_| ___| / _ \___ \
; / _ \/ _` / __| | | | '_ \___ \| | | |__) |
;|  __/ (_| \__ \ |_| | (_) |__) | |_| / __/
; \___|\__,_|___/\__, |\___/____/ \___/_____|
;                |___/
;
; Change direction: W A S D
;
; $00    => color of robot
; $02    => direction (1 => up, 2 => right, 4 => down, 8 => left)
; $0e-0f => counter for key presses
; $10-11 => the current robot position
; $12-13 => the robots target position

jsr drawLabyrinth
jsr gameInit
jsr gameLoop
jmp gameOver

gameLoop:
  jsr copyPosition
  jsr readKeys
  jsr increaseKeyPressCounter
  jsr updatePosition

  ;TODO: resetting pressed keys and stored directions
  LDA #$00 ; neutral value
  STA $ff ; pressed keys
  STA $02 ; stored direction

  jsr checkWallCollisions

  jsr drawNewPosition
  jsr restorePosition

  jsr spinWheels
  jmp gameLoop

gameInit:
  LDA #$03
  STA $00

  ; robot start position: use $0200 for (0,0) and $05ff for (31,31)
  ; data is stored as little endian, so don't forget to swap the bytes
  LDA #$00
  STA $10
  LDA #$02
  STA $11

  ;TODO: resetting pressed keys and stored directions
  LDA #$00 ; neutral value
  STA $ff ; pressed keys
  STA $02 ; stored direction

  jsr copyPosition
  jsr drawNewPosition
  RTS

checkWallCollisions:
  LDY #$00
  LDA ($12),Y
  
  CMP #$01
  BEQ collision

  RTS

drawNewPosition:
  ; mark previous field black:$00 or yellow:$07
  ; green:$05, light green:$0d
  LDA #$0d
  LDY #$00
  STA ($10),Y

  ; draw new position
  LDA $00
  LDY #$00
  STA ($12),Y

  LDA #$03
  STA $00

  RTS

copyPosition:
  lda $10
  sta $12
  lda $11
  sta $13
  rts

restorePosition:
  lda $12
  sta $10
  lda $13
  sta $11
  rts

updatePosition:

  ;now determine where to move the robot.
  ;lsr: Logical Shift Right. Shift all bits in register A one bit to the right
  ;the bit that "falls off" is stored in the carry flag
  lda $02

  lsr
  bcs up    ;if a 1 "fell off", we started with bin 0001, so the robot needs to go up
  lsr
  bcs right ;if a 1 "fell off", we started with bin 0010, so the robot needs to go right
  lsr
  bcs down  ;if a 1 "fell off", we started with bin 0100, so the robot needs to go down
  lsr
  bcs left  ;if a 1 "fell off", we started with bin 1000, so the robot needs to go left
  rts

up:
  lda $12   ;load the least significant byte into register A
  sec       ;set carry flag
  sbc #$20  ;Subtract with Carry: If overflow occurs the carry bit is clear.
  sta $12
  bcc upup
  rts

upup:
  dec $13   ;decrement the most significant byte
  lda #$1   ;load hex value $1 into register A
  cmp $13   ;if in strip 1, we overflowed.
  beq collision
  rts

right:
  inc $12   ;increment least significant byte
  lda #$1f  ;load value hex $1f into register A
  bit $12   ;AND value with hex $1f (bin 11111)
  beq collision ;branch to collision if zero flag is set
  rts       ;return

down: 
  lda $12   ;load the least significant byte in register A
  clc       ;clear carry flag
  adc #$20  ;add hex $20 to register A and set the carry flag on overflow
  sta $12
  bcs downdown ;if the carry flag is set, we ran out of current strip
  rts

downdown:
  inc $13   ;increment most significant byte
  lda #$6   ;load the value hex $6 into the A register
  cmp $13   ;if the most significant byte equals to 6, we're off
  beq collision ;collision with the bottom of the screen
  rts
 
left:
  dec $12   ;decrement least significant byte
  lda $12
  and #$1f  ;AND the value hex $1f (binary 11111) with the value in register A
  cmp #$1f  ;compare the ANDed value above with binary 11111.
  beq collision ;collision with left border of the screen
  rts

collision:
  LDA #$0e
  STA $00

  jsr copyPosition
  rts

;functions to read pressed keys and write them to $02
readKeys:
  ; address $ff in the zero page contains the key pressed
  lda $ff

  cmp #$77
  beq upKey
  cmp #$64
  beq rightKey
  cmp #$73
  beq downKey
  cmp #$61
  beq leftKey

  rts
 
upKey:
  lda #1
  sta $02
  rts

rightKey:
  lda #2
  sta $02
  rts
 
downKey:
  lda #4
  sta $02
  rts
 
leftKey:
  lda #8
  sta $02
  rts
 
illegalMove:
  rts

increaseKeyPressCounter:
  LDA $02
  CMP #$00
  BEQ increaseKeyPressCounterNop
  INC $0e
  LDA $0e
  CMP #$00
  BEQ increaseKeyPressCounterOverflow
  RTS

increaseKeyPressCounterOverflow:
  INC $0f
  RTS

increaseKeyPressCounterNop:
  RTS

;slow the game down by wasting cycles
spinWheels:
  ldx #0
spinloop:
  nop
  nop
  dex
  bne spinloop
  rts

gameOver:
  brk

drawLabyrinth:

LDX #$01 ; wall
LDA #$00 ; floor

STA $0200
STX $0201
STX $0202
STA $0203
STX $0204
STA $0205
STA $0206
STA $0207
STA $0208
STA $0209
STA $020a
STA $020b
STA $020c
STX $020d
STA $020e
STA $020f
STA $0210
STA $0211
STA $0212
STA $0213
STA $0214
STA $0215
STA $0216
STA $0217
STA $0218
STA $0219
STX $021a
STA $021b
STA $021c
STA $021d
STX $021e
STA $021f
STA $0220
STA $0221
STA $0222
STA $0223
STA $0224
STA $0225
STX $0226
STX $0227
STX $0228
STX $0229
STX $022a
STX $022b
STA $022c
STX $022d
STX $022e
STX $022f
STA $0230
STX $0231
STX $0232
STX $0233
STX $0234
STX $0235
STX $0236
STA $0237
STX $0238
STA $0239
STA $023a
STA $023b
STX $023c
STA $023d
STA $023e
STA $023f
STX $0240
STX $0241
STX $0242
STX $0243
STX $0244
STX $0245
STX $0246
STA $0247
STA $0248
STA $0249
STA $024a
STX $024b
STA $024c
STA $024d
STA $024e
STX $024f
STX $0250
STX $0251
STA $0252
STA $0253
STA $0254
STA $0255
STX $0256
STX $0257
STX $0258
STX $0259
STA $025a
STX $025b
STX $025c
STX $025d
STX $025e
STA $025f
STA $0260
STX $0261
STA $0262
STA $0263
STA $0264
STA $0265
STA $0266
STA $0267
STX $0268
STX $0269
STX $026a
STX $026b
STX $026c
STX $026d
STA $026e
STA $026f
STX $0270
STX $0271
STA $0272
STX $0273
STX $0274
STA $0275
STA $0276
STX $0277
STA $0278
STX $0279
STA $027a
STX $027b
STX $027c
STA $027d
STA $027e
STA $027f
STA $0280
STA $0281
STA $0282
STX $0283
STX $0284
STX $0285
STA $0286
STX $0287
STX $0288
STX $0289
STA $028a
STA $028b
STA $028c
STX $028d
STX $028e
STA $028f
STA $0290
STA $0291
STA $0292
STX $0293
STX $0294
STX $0295
STA $0296
STX $0297
STA $0298
STA $0299
STA $029a
STX $029b
STA $029c
STA $029d
STX $029e
STA $029f
STA $02a0
STX $02a1
STA $02a2
STA $02a3
STX $02a4
STX $02a5
STX $02a6
STX $02a7
STA $02a8
STA $02a9
STA $02aa
STX $02ab
STA $02ac
STA $02ad
STX $02ae
STX $02af
STX $02b0
STX $02b1
STX $02b2
STX $02b3
STA $02b4
STX $02b5
STA $02b6
STX $02b7
STA $02b8
STX $02b9
STA $02ba
STX $02bb
STA $02bc
STX $02bd
STX $02be
STA $02bf
STA $02c0
STX $02c1
STX $02c2
STA $02c3
STA $02c4
STX $02c5
STX $02c6
STA $02c7
STA $02c8
STX $02c9
STX $02ca
STX $02cb
STX $02cc
STA $02cd
STA $02ce
STA $02cf
STA $02d0
STA $02d1
STA $02d2
STA $02d3
STA $02d4
STX $02d5
STA $02d6
STX $02d7
STA $02d8
STX $02d9
STX $02da
STX $02db
STX $02dc
STX $02dd
STA $02de
STA $02df
STA $02e0
STA $02e1
STX $02e2
STX $02e3
STA $02e4
STX $02e5
STA $02e6
STA $02e7
STX $02e8
STX $02e9
STA $02ea
STX $02eb
STX $02ec
STX $02ed
STA $02ee
STX $02ef
STX $02f0
STX $02f1
STX $02f2
STA $02f3
STX $02f4
STX $02f5
STA $02f6
STX $02f7
STA $02f8
STA $02f9
STA $02fa
STA $02fb
STA $02fc
STX $02fd
STX $02fe
STA $02ff
STA $0300
STX $0301
STX $0302
STX $0303
STX $0304
STX $0305
STA $0306
STX $0307
STX $0308
STX $0309
STA $030a
STX $030b
STA $030c
STX $030d
STA $030e
STA $030f
STA $0310
STX $0311
STA $0312
STA $0313
STX $0314
STA $0315
STA $0316
STX $0317
STA $0318
STX $0319
STX $031a
STX $031b
STA $031c
STA $031d
STX $031e
STA $031f
STA $0320
STX $0321
STA $0322
STA $0323
STA $0324
STA $0325
STA $0326
STX $0327
STA $0328
STA $0329
STA $032a
STA $032b
STA $032c
STX $032d
STX $032e
STX $032f
STA $0330
STX $0331
STX $0332
STX $0333
STX $0334
STA $0335
STX $0336
STX $0337
STA $0338
STA $0339
STX $033a
STX $033b
STX $033c
STA $033d
STX $033e
STX $033f
STA $0340
STA $0341
STA $0342
STX $0343
STX $0344
STX $0345
STX $0346
STX $0347
STA $0348
STX $0349
STA $034a
STX $034b
STA $034c
STX $034d
STA $034e
STA $034f
STA $0350
STX $0351
STX $0352
STA $0353
STA $0354
STA $0355
STX $0356
STA $0357
STA $0358
STX $0359
STX $035a
STA $035b
STX $035c
STA $035d
STA $035e
STX $035f
STA $0360
STX $0361
STX $0362
STX $0363
STA $0364
STX $0365
STA $0366
STA $0367
STA $0368
STX $0369
STX $036a
STX $036b
STX $036c
STX $036d
STA $036e
STX $036f
STA $0370
STX $0371
STA $0372
STA $0373
STX $0374
STX $0375
STX $0376
STX $0377
STA $0378
STA $0379
STA $037a
STA $037b
STX $037c
STX $037d
STA $037e
STA $037f
STA $0380
STA $0381
STA $0382
STA $0383
STA $0384
STA $0385
STA $0386
STX $0387
STA $0388
STX $0389
STX $038a
STA $038b
STA $038c
STA $038d
STA $038e
STX $038f
STA $0390
STX $0391
STA $0392
STX $0393
STX $0394
STX $0395
STA $0396
STA $0397
STA $0398
STX $0399
STX $039a
STX $039b
STX $039c
STX $039d
STX $039e
STA $039f
STX $03a0
STX $03a1
STX $03a2
STX $03a3
STX $03a4
STX $03a5
STX $03a6
STX $03a7
STA $03a8
STA $03a9
STX $03aa
STA $03ab
STX $03ac
STX $03ad
STX $03ae
STX $03af
STA $03b0
STX $03b1
STA $03b2
STA $03b3
STA $03b4
STX $03b5
STX $03b6
STA $03b7
STX $03b8
STX $03b9
STA $03ba
STA $03bb
STA $03bc
STX $03bd
STA $03be
STA $03bf
STA $03c0
STX $03c1
STX $03c2
STA $03c3
STA $03c4
STA $03c5
STA $03c6
STX $03c7
STX $03c8
STA $03c9
STX $03ca
STA $03cb
STA $03cc
STX $03cd
STX $03ce
STA $03cf
STA $03d0
STX $03d1
STA $03d2
STX $03d3
STA $03d4
STA $03d5
STX $03d6
STA $03d7
STX $03d8
STA $03d9
STA $03da
STX $03db
STA $03dc
STA $03dd
STA $03de
STX $03df
STA $03e0
STA $03e1
STA $03e2
STA $03e3
STX $03e4
STX $03e5
STA $03e6
STA $03e7
STX $03e8
STA $03e9
STX $03ea
STX $03eb
STA $03ec
STA $03ed
STX $03ee
STX $03ef
STX $03f0
STX $03f1
STX $03f2
STX $03f3
STX $03f4
STA $03f5
STX $03f6
STA $03f7
STX $03f8
STA $03f9
STX $03fa
STX $03fb
STX $03fc
STX $03fd
STA $03fe
STA $03ff
STA $0400
STX $0401
STX $0402
STX $0403
STX $0404
STX $0405
STX $0406
STA $0407
STA $0408
STA $0409
STX $040a
STX $040b
STX $040c
STA $040d
STA $040e
STA $040f
STA $0410
STX $0411
STX $0412
STA $0413
STX $0414
STA $0415
STX $0416
STX $0417
STX $0418
STA $0419
STX $041a
STA $041b
STA $041c
STX $041d
STX $041e
STA $041f
STA $0420
STX $0421
STA $0422
STA $0423
STA $0424
STA $0425
STX $0426
STX $0427
STX $0428
STX $0429
STX $042a
STA $042b
STX $042c
STX $042d
STX $042e
STA $042f
STX $0430
STX $0431
STA $0432
STA $0433
STX $0434
STA $0435
STA $0436
STX $0437
STA $0438
STA $0439
STX $043a
STX $043b
STA $043c
STA $043d
STA $043e
STA $043f
STA $0440
STX $0441
STA $0442
STX $0443
STX $0444
STA $0445
STA $0446
STA $0447
STX $0448
STA $0449
STA $044a
STA $044b
STA $044c
STA $044d
STX $044e
STA $044f
STA $0450
STA $0451
STA $0452
STX $0453
STX $0454
STX $0455
STA $0456
STX $0457
STA $0458
STX $0459
STX $045a
STX $045b
STX $045c
STX $045d
STX $045e
STA $045f
STA $0460
STX $0461
STX $0462
STX $0463
STX $0464
STX $0465
STX $0466
STA $0467
STA $0468
STA $0469
STX $046a
STX $046b
STA $046c
STX $046d
STX $046e
STX $046f
STA $0470
STX $0471
STX $0472
STX $0473
STA $0474
STX $0475
STA $0476
STX $0477
STA $0478
STX $0479
STA $047a
STA $047b
STA $047c
STA $047d
STX $047e
STA $047f
STA $0480
STA $0481
STA $0482
STA $0483
STA $0484
STA $0485
STX $0486
STX $0487
STX $0488
STX $0489
STX $048a
STX $048b
STA $048c
STA $048d
STA $048e
STX $048f
STA $0490
STA $0491
STA $0492
STA $0493
STA $0494
STA $0495
STA $0496
STX $0497
STA $0498
STX $0499
STA $049a
STX $049b
STX $049c
STA $049d
STX $049e
STA $049f
STX $04a0
STX $04a1
STX $04a2
STX $04a3
STX $04a4
STA $04a5
STA $04a6
STA $04a7
STA $04a8
STA $04a9
STA $04aa
STX $04ab
STA $04ac
STX $04ad
STX $04ae
STX $04af
STX $04b0
STX $04b1
STX $04b2
STX $04b3
STX $04b4
STX $04b5
STX $04b6
STX $04b7
STA $04b8
STX $04b9
STA $04ba
STX $04bb
STA $04bc
STA $04bd
STX $04be
STA $04bf
STA $04c0
STX $04c1
STA $04c2
STX $04c3
STX $04c4
STX $04c5
STX $04c6
STX $04c7
STX $04c8
STX $04c9
STA $04ca
STX $04cb
STA $04cc
STA $04cd
STA $04ce
STA $04cf
STA $04d0
STA $04d1
STA $04d2
STX $04d3
STA $04d4
STA $04d5
STA $04d6
STX $04d7
STA $04d8
STA $04d9
STA $04da
STX $04db
STA $04dc
STX $04dd
STX $04de
STX $04df
STA $04e0
STA $04e1
STA $04e2
STA $04e3
STX $04e4
STA $04e5
STA $04e6
STA $04e7
STX $04e8
STX $04e9
STA $04ea
STX $04eb
STA $04ec
STX $04ed
STX $04ee
STA $04ef
STX $04f0
STX $04f1
STA $04f2
STA $04f3
STA $04f4
STX $04f5
STX $04f6
STX $04f7
STA $04f8
STX $04f9
STX $04fa
STX $04fb
STA $04fc
STA $04fd
STA $04fe
STA $04ff
STA $0500
STX $0501
STX $0502
STA $0503
STA $0504
STA $0505
STX $0506
STA $0507
STA $0508
STX $0509
STA $050a
STX $050b
STX $050c
STX $050d
STA $050e
STA $050f
STA $0510
STX $0511
STX $0512
STX $0513
STX $0514
STX $0515
STA $0516
STA $0517
STA $0518
STX $0519
STA $051a
STX $051b
STX $051c
STX $051d
STA $051e
STX $051f
STA $0520
STA $0521
STX $0522
STX $0523
STA $0524
STX $0525
STX $0526
STX $0527
STA $0528
STX $0529
STA $052a
STA $052b
STA $052c
STX $052d
STA $052e
STX $052f
STA $0530
STA $0531
STA $0532
STA $0533
STA $0534
STA $0535
STA $0536
STX $0537
STA $0538
STA $0539
STA $053a
STX $053b
STX $053c
STA $053d
STA $053e
STA $053f
STA $0540
STX $0541
STX $0542
STA $0543
STA $0544
STX $0545
STA $0546
STA $0547
STA $0548
STX $0549
STX $054a
STX $054b
STA $054c
STX $054d
STX $054e
STX $054f
STX $0550
STX $0551
STX $0552
STA $0553
STX $0554
STA $0555
STX $0556
STX $0557
STA $0558
STX $0559
STX $055a
STX $055b
STA $055c
STA $055d
STX $055e
STA $055f
STA $0560
STA $0561
STX $0562
STA $0563
STX $0564
STX $0565
STX $0566
STX $0567
STX $0568
STX $0569
STA $056a
STX $056b
STA $056c
STA $056d
STA $056e
STA $056f
STA $0570
STA $0571
STX $0572
STX $0573
STX $0574
STA $0575
STX $0576
STX $0577
STX $0578
STX $0579
STA $057a
STA $057b
STA $057c
STX $057d
STX $057e
STA $057f
STX $0580
STA $0581
STX $0582
STA $0583
STA $0584
STX $0585
STX $0586
STA $0587
STA $0588
STA $0589
STA $058a
STX $058b
STX $058c
STX $058d
STX $058e
STX $058f
STX $0590
STA $0591
STA $0592
STX $0593
STX $0594
STA $0595
STX $0596
STA $0597
STA $0598
STA $0599
STA $059a
STX $059b
STX $059c
STX $059d
STA $059e
STA $059f
STX $05a0
STA $05a1
STX $05a2
STX $05a3
STA $05a4
STA $05a5
STA $05a6
STA $05a7
STX $05a8
STA $05a9
STX $05aa
STX $05ab
STA $05ac
STA $05ad
STA $05ae
STA $05af
STX $05b0
STX $05b1
STA $05b2
STA $05b3
STX $05b4
STX $05b5
STX $05b6
STA $05b7
STX $05b8
STX $05b9
STX $05ba
STX $05bb
STA $05bc
STX $05bd
STX $05be
STA $05bf
STA $05c0
STA $05c1
STA $05c2
STX $05c3
STX $05c4
STX $05c5
STX $05c6
STX $05c7
STX $05c8
STA $05c9
STA $05ca
STX $05cb
STA $05cc
STX $05cd
STA $05ce
STX $05cf
STX $05d0
STX $05d1
STX $05d2
STA $05d3
STX $05d4
STX $05d5
STA $05d6
STA $05d7
STX $05d8
STX $05d9
STA $05da
STA $05db
STA $05dc
STA $05dd
STX $05de
STA $05df
STA $05e0
STX $05e1
STA $05e2
STA $05e3
STA $05e4
STA $05e5
STA $05e6
STA $05e7
STX $05e8
STX $05e9
STA $05ea
STA $05eb
STA $05ec
STX $05ed
STA $05ee
STA $05ef
STA $05f0
STA $05f1
STA $05f2
STA $05f3
STA $05f4
STA $05f5
STA $05f6
STX $05f7
STX $05f8
STA $05f9
STA $05fa
STX $05fb
STX $05fc
STA $05fd
STA $05fe
STA $05ff

; red pixel as target
LDA #$0a
STA $05ff

RTS

Debugger

Monitor Start: $ Length: $