Writing NES games! with assembly!!

Talk given at !!Con 2017.

I’d like to take you on a stroll down memory lane and dig into the internals of the Nintendo Entertainment System (NES) to figure out how it works. While we’re there, we’ll see how to build a game for the NES using 6502 assembly with the help of a few modern tools. We’ll gain a new respect for ’80s developers and an appreciation for the high-level languages we have today!

Transcript

7. PRG ROM CHR ROM
8. chr rom
13. PRG ROM CHR ROM battery RAM
14. Milestone 1
15. NO WAY! OMG omg!! it works!!! is it normal that we are this excited about displaying a ball?
16. NO WAY! OMG omg!! it works!!! is it normal that we are this excited about displaying a ball?
17. NO WAY! OMG omg!! it works!!! is it normal that we are this excited about displaying a ball?
18. NO WAY! OMG omg!! it works!!! is it normal that we are this excited about displaying a ball?
19. NO WAY! OMG omg!! it works!!! is it normal that we are this excited about displaying a ball?
21. 00008000: 20 8e 10 40 2c 02 20 2c 02 20 10 fb 8a 95 00 9d 00008010: 00 01 9d 00 03 9d 00 04 9d 00 05 9d 00 06 9d 00 00008020: 07 e8 d0 e9 a9 ff 9d 00 02 e8 d0 fa 2c 02 20 10 00008030: fb 4c b6 80 a9 01 85 02 a9 01 85 03 a9 70 8d 03 00008040: 02 a9 70 8d 00 02 a9 00 8d 02 02 a9 01 8d 01 02 00008050: 60 e6 00 e6 01 a5 00 8d 03 02 a5 01 8d 00 02 60 00008060: a9 01 8d 16 40 85 05 4a 8d 16 40 ad 16 40 29 03 00008070: c9 01 26 04 ad 17 40 29 03 c9 01 26 05 90 ec 60 00008080: a5 05 85 07 a5 04 85 06 20 70 80 a5 05 85 09 a5 00008090: 04 85 08 20 70 80 a2 01 b5 04 d5 08 f0 04 b5 06 000080a0: 95 04 ca 10 f3 60 a9 3f 8d 06 20 a9 00 8d 06 20 000080b0: a9 0f 8d 07 20 a9 3f 8d 06 20 a9 11 8d 06 20 a9 000080c0: 30 8d 07 20 a9 0f 8d 07 20 8d 07 20 20 44 80 a9 000080d0: 18 8d 01 20 a9 80 8d 00 20 a9 01 85 0e 20 90 80 000080e0: a5 04 f0 03 20 00 81 20 61 80 20 25 81 4c e9 80 000080f0: a5 04 29 0c f0 0c a5 04 29 08 f0 03 e6 03 60 c6 00008100: 03 60 a5 04 29 03 f0 0b a5 04 29 02 f0 03 c6 02 00008110: 60 e6 02 60 60 e6 11 a5 11 d0 fc 60 48 8a 48 98 00008120: 48 a5 0e f0 0a a9 00 8d 03 20 a9 02 8d 14 40 a5 00008130: 0f f0 08 2c 02 20 20 24 81 c6 0f a5 10 f0 17 a5 00008140: 0b 8d 01 20 a5 0a 8d 00 20 2c 02 20 a5 0c 8d 05 00008150: 20 a5 0d 8d 05 20 a9 00 85 11 68 a8 68 aa 68 40
22. cc65 toolchain asm asm asm ca65 ld65 asm asm obj ines
23. 69 70 main: 71 ; Set universal background color ($3F00) to black ($0F) 72 ; Set PPU address to $3F00 73 lda #$3F 74 sta PPUADDR 75 lda #$00 76 sta PPUADDR 77 ; Write $0F to that PPU address 78 lda #$0F 79 sta PPUDATA 80 81 ; Set sprite palette 0 ($3F11-$3F13) to white ($30), black ($0F), black ($0F) 82 ; Set PPU address to $3F11 83 lda #$3F 84 sta PPUADDR 85 lda #$11 86 sta PPUADDR 87 ; Write $30, $0F, $0F to PPU 88 lda #$30 89 sta PPUDATA ; PPUADDR will increase by 1 each time we write to it 90 lda #$0F 91 sta PPUDATA 92 sta PPUDATA 93 94 jsr ballSetup 95 96 ; Enable sprite and background rendering 97 lda #%00011000 98 sta PPUMASK 99 100 ; Enable NMI 101 lda #%10000000 102 sta PPUCTRL 103
24. 6502 assembly LDA STA memory ADC SBC INC DEC arithmetics AND ORA CMP compare BCC BCS BEQ BMI BNE BPL branch
25. 6502 assembly LDA STA memory ADC SBC INC DEC arithmetics AND ORA CMP compare BCC BCS BEQ BMI BNE BPL branch
26. 6502 assembly LDA STA memory ADC SBC INC DEC arithmetics AND ORA CMP compare BCC BCS BEQ BMI BNE BPL branch
27. 6502 assembly LDA STA memory ADC SBC INC DEC arithmetics AND ORA CMP compare BCC BCS BEQ BMI BNE BPL branch
28. 6502 assembly LDA STA memory ADC SBC INC DEC arithmetics AND ORA CMP compare BCC BCS BEQ BMI BNE BPL branch
29. $0600: LDA $05 $0602: CMP #$08 $0604: BEQ $060b $0606: INC $05 $0608: JMP $0600 $060b: RTS PC
30. Milestone 2 !
31. Milestone 2 !
32. picture processing unit
33. 256px 240px
35. 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F 20 21 22 23 24 25 26 27 28 29 2A 2B 2C 2D 2E 2F 30 31 32 33 34 35 36 37 38 39 3A 3B 3C 3D 3E 3F nes palette
39. y tile attrs x 23 01 00 01 64 x
40. PPU registers $2000-$2007
41. ! Milestone 3 " " "
42. OMG! Wait... Is that real? yeah!!! NO WAY!! yeah!!!
43. OMG! Wait... Is that real? yeah!!! NO WAY!! yeah!!!
44. OMG! Wait... Is that real? yeah!!! NO WAY!! yeah!!!
45. OMG! Wait... Is that real? yeah!!! NO WAY!! yeah!!!
46. OMG! Wait... Is that real? yeah!!! NO WAY!! yeah!!!
47. OMG! Wait... Is that real? yeah!!! NO WAY!! yeah!!!
48. video frame vertical blank
49. video frame vertical blank
50. nmi interrupt
51. loop: INC $05 JMP loop nmi: ; Do stuff RTI PC
52. loop: INC $05 JMP loop nmi: ; Do stuff RTI PC
53. loop: INC $05 JMP loop nmi: ; Do stuff RTI PC
54. loop: INC $05 JMP loop nmi: ; Do stuff RTI PC
55. loop: INC $05 JMP loop nmi: ; Do stuff RTI PC
56. loop: INC $05 JMP loop nmi: ; Do stuff RTI PC
57. loop: INC $05 JMP loop nmi: ; Do stuff RTI PC
58. loop: INC $05 JMP loop nmi: ; Do stuff RTI PC
59. loop: INC $05 JMP loop nmi: ; Do stuff RTI PC
60. Milestone 4 # # "
61. controllers $4016-$4017
63. 0 0 0 0 0 0 0 0 a B se st
64. $ 1 0 0 0 0 0 0 0 a B se st
65. $ $ 1 0 0 0 0 0 0 1 a B se st
66. LDA controller1 AND #%10000000 BEQ notPressed
 ; A is pressed notPressed: RTS
67. LDA controller1 AND #%10000000 BEQ notPressed
 ; A is pressed notPressed: RTS 1 0 0 0 0 0 0 0 a B se st
68. Milestone 5 # # " % & "
69. wine + fceux
71. lua scripting
72. emu.frameadvance() memory.readbyte(0x0000) memory.writebyte(0x0001, 0x30)
73. emu.registerafter(function() ballX = memory.readbyte(0x0000) ballY = memory.readbyte(0x0001) gui.text( 10, 30, string.format('Ball: %02x,%02x', ballX, ballY) ) end)
75. '
76. ' ( lua
77. ! )
78. function TestBall:test_ball_moves_down() memory.writebyte(ballYAddress, 0x30) memory.writebyte(ballYDirection, 0x01) emu.frameadvance() ballY = memory.readbyte(ballYAddress) luaunit.assertEquals(ballY, 0x31) end
82. Read the source code when in doubt
84. NExt steps
85. @cjoudrey