NES ASM program

  .inesprg 1   ; 1x 16KB PRG code
  .ineschr 1   ; 1x  8KB CHR data
  .inesmap 0   ; mapper 3 = CNROM, 8KB CHR swapping
  .inesmir 0   ; background mirroring

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;---Variables---

character_number	.rs 1	;For all the characters
enemy_number		.rs 1	;Just for enemies
enemy_direction		.rs 4
enemy_animation		.rs 4
RandomLow		.rs 1
RandomHigh		.rs 1
RandomTemp		.rs 1
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;---Constants---

HERO_Y			= $0200
HERO_X			= $0203
ENEMY_Y			= $0220
ENEMY_X			= $0223

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

  .bank 0
  .org $8000

RESET:
  SEI	;Fuck IRQs
  CLD	;Decimals too! Fuck 'em
  LDX #$40
  STX $4017	;APU frame IRQ! I said FUCK IRQs!!!
  LDX #$FF
  TXS	;Set up stack
  INX	;X register overflow, X=0
  STX $2000	;no NMI for now
  STX $2001	;No rendering for now
  STX $4010	;Dang! DMC IRQs! y so many of 'em?

vblankwait1:       ; First wait for vblank to make sure PPU is ready
  BIT $2002
  BPL vblankwait1

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    ;move all sprites off screen
  INX
  BNE clrmem

vblankwait2:      ; Second wait for vblank, PPU is ready after this
  BIT $2002
  BPL vblankwait2

;NES is ready now! LET'S DO THIS! (LEEEEEEEEEEEEEROOOOOOOOOOOY JJJJJJJJJJJENKINS!)

PalLoad:
  LDA $2002	;PPU? r u there? We need to reset the HiLo latch!
  LDA #$3F	;Hi part of $3F00
  STA $2006
  LDA #$00	;Lo part of it
  STA $2006
  LDX #$00
PalLoop:
  LDA pal, x
  STA $2007	;Load 'em palettes to PPU
  INX
  CPX #$20
  BNE PalLoop	;Advance only when all 32 palettes are copied!

LoadSprites:
  LDX #$00
LoadSpritesLoop:
  LDA sprite, x         ;load palette byte
  STA $0200, x          ;write to PPU
  INX                   ;set index to next byte
  CPX #$A0
  BNE LoadSpritesLoop  ;if x = $20, 32 bytes copied, all done

LoadBackground:
  LDA $2002             ; read PPU status to reset the high/low latch
  LDA #$20
  STA $2006             ; write the high byte of $2000 address
  LDA #$00
  STA $2006             ; write the low byte of $2000 address
  LDX #$00              ; start out at 0
LoadBackgroundLoop:
  LDA #$56      	; load data from address (background + the value in x)
  STA $2007             ; write to PPU
  INX                   ; X = X + 1
  CPX #$00              ; Compare X to hex $80, decimal 128 - copying 128 bytes
  BNE LoadBackgroundLoop  ; Branch to LoadBackgroundLoop if compare was Not Equal to zero
                        ; if compare was equal to 128, keep going down

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;           ________________      _____________________      _________________            ;
;          |                |    |          |          |             |                    ;
;          |                |    |          |          |             |                    ;
;          |                |    |                     |     ________|________            ;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

NMI:
  LDA #$00
  STA $2003  ; set the low byte (00) of the RAM address
  LDA #$02
  STA $4014  ; set the high byte (02) of the RAM address, start the transfer

PPUCleanUp:
  LDA #%10010000   ; enable NMI, sprites from Pattern Table 0, background from Pattern Table 1
  STA $2000
  LDA #%00011110   ; Enable sprites, enable background, no clipping on left side
  STA $2001
  LDA #$00         ; no scrolling
  STA $2005
  STA $2005

EnginePlaying:
  INC RandomLow
  BNE .SeedComplete
  INC RandomHigh
.SeedComplete
LatchController: 			;I know there are better controller reads, but for now I'm lazy
  LDA #$01
  STA $4016
  LDA #$00
  STA $4016 ; tell both the controllers to latch buttons
  LDA $4016
  LDA $4016
  LDA $4016
  LDA $4016
  LDA $4016
  AND #%00000001
  BEQ MoveUpDone
  LDA HERO_Y
  SEC
  SBC #$01
  STA HERO_Y
MoveUpDone:
  LDA $4016
  AND #%00000001
  BEQ MoveDownDone
  LDA HERO_Y
  CLC
  ADC #$01
  STA HERO_Y
MoveDownDone:
  LDA $4016
  AND #%00000001
  BEQ MoveLeftDone
  LDA HERO_X
  SEC
  SBC #$01
  STA HERO_X
MoveLeftDone:
  LDA $4016
  AND #%00000001
  BEQ MoveRightDone
  LDA HERO_X
  CLC
  ADC #$01
  STA HERO_X
MoveRightDone:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  LDA #$00
  STA enemy_number
MoveSprites:
  LDX enemy_number
  INC enemy_animation,x
  JSR Random
  CMP #$03
  BCS NoChange
  STA enemy_direction,x
NoChange:
  LDA enemy_direction,x
  BNE .skip1
  JSR enemy_up
  JMP endskip
.skip1
  CMP #$01
  BNE .skip2
  JSR enemy_down
  JMP endskip
.skip2
  CMP #$02
  BNE .skip3
  JSR enemy_left
  JMP endskip
.skip3
  JSR enemy_right
endskip:
  INC enemy_number
  LDA enemy_number
  CMP #$04
  BNE MoveSprites
  LDA #$00
  STA enemy_number
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

UpdatePlayerPos:

  LDA $0200			;Update 16x32 metsprite vertically
  STA $0204
  CLC
  ADC #$08
  STA $0208
  STA $020C
  CLC
  ADC #$08
  STA $0210
  STA $0214
  CLC
  ADC #$08
  STA $0218
  STA $021C

  LDA $0203			;Update 16x32 metasprite horizontally
  STA $020B
  STA $0213
  STA $021B
  CLC
  ADC #$08
  STA $0207
  STA $020F
  STA $0217
  STA $021F

UpdatePos:

  LDX enemy_number
  LDA updateconstants,x
  TAX

  LDA $0220,x			;Update 16x32 metsprite vertically
  STA $0224,x
  CLC
  ADC #$08
  STA $0228,x
  STA $022C,x
  CLC
  ADC #$08
  STA $0230,x
  STA $0234,x
  CLC
  ADC #$08
  STA $0238,x
  STA $023C,x

  LDA $0223,x			;Update 16x32 metasprite horizontally
  STA $022B,x
  STA $0233,x
  STA $023B,x
  CLC
  ADC #$08
  STA $0227,x
  STA $022F,x
  STA $0237,x
  STA $023F,x

  INC enemy_number
  LDA enemy_number
  CMP #$04
  BEQ CharMovementDone		;We got only 5 characters to move
  JMP UpdatePos
CharMovementDone:
  LDA #$00
  STA enemy_number
  RTI

Random:
      ; See "linear-congruential random number generator" for more.
      ; rand = (rand * 5 + 0x3611) & 0xffff;
      ; return (rand >> 8) & 0xff;

  LDA RandomHigh    ; multiply by 5
  STA RandomTemp
  LDA RandomLow
  ASL a             ; rand = rand * 4 + rand
  ROL RandomTemp
  ASL a
  ROL RandomTemp
  CLC
  ADC RandomLow
  PHA
  LDA RandomTemp
  ADC RandomHigh
  STA RandomHigh
  PLA               ; rand = rand + 0x3611
  CLC
  ADC #$11
  STA RandomLow
  LDA RandomHigh
  ADC #$36
  STA RandomHigh
  RTS


;------------------------------

enemy_up:

  LDX enemy_number                ;move the sprite up
  LDY updateconstants,x
  LDA ENEMY_Y,y
  SEC
  SBC #$01
  STA ENEMY_Y,y

  RTS

;------------------------------

enemy_down:

  LDX enemy_number                ;move the sprite down
  LDY updateconstants,x
  LDA ENEMY_Y,y
  CLC
  ADC #$01
  stA ENEMY_Y,y

  RTS

;------------------------------

enemy_right:

  LDX enemy_number                ;move the sprite right
  LDY updateconstants,x
  LDA ENEMY_X,y
  CLC
  ADC #$01
  STA ENEMY_X,y

  RTS

;------------------------------

enemy_left:

  LDX enemy_number                ;move the sprite left
  LDY updateconstants,x
  LDA ENEMY_X,y
  SEC
  SBC #$01
  STA ENEMY_X,y

  RTS

;------------------------------

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

  .bank 1
  .org $E000

pal:
  .db $30,$00,$10,$16,$1A,$38,$30,$31,$1A,$06,$36,$20,$1A,$28,$38,$16
  .db $30,$0F,$17,$36,$1A,$0F,$21,$36,$1A,$1F,$11,$37,$1A,$0F,$38,$30

sprite:
  .db $80,$00,$00,$80,$80,$01,$00,$88,$88,$10,$00,$80,$88,$11,$00,$88  ;;  $0200
  .db $90,$20,$00,$80,$90,$21,$00,$88,$98,$30,$00,$80,$98,$31,$00,$88  ;;  $0210
  .db $A0,$02,$01,$80,$A0,$03,$01,$88,$A8,$12,$01,$80,$A8,$13,$01,$88  ;;  $0220
  .db $B0,$22,$01,$80,$B0,$23,$01,$88,$B8,$32,$01,$80,$B8,$33,$01,$88  ;;  $0230
  .db $60,$02,$01,$80,$60,$03,$01,$88,$68,$12,$01,$80,$68,$13,$01,$88  ;;  $0240
  .db $70,$22,$01,$80,$70,$23,$01,$88,$78,$32,$01,$80,$78,$33,$01,$88  ;;  $0250
  .db $40,$02,$01,$80,$40,$03,$01,$88,$48,$12,$01,$80,$48,$13,$01,$88  ;;  $0260
  .db $50,$22,$01,$80,$50,$23,$01,$88,$58,$32,$01,$80,$58,$33,$01,$88  ;;  $0270
  .db $20,$04,$01,$80,$20,$05,$01,$88,$28,$14,$01,$80,$28,$15,$01,$88  ;;  $0280
  .db $30,$22,$01,$80,$30,$23,$01,$88,$38,$32,$01,$80,$38,$33,$01,$88  ;;  $0290

updateconstants:
  .db $00,$20,$40,$60

randomenemydirection:            ;these are random numbers used with the random_direction2 to make enemies switch direction
  .db $57,$CD,$AF,$05,$BC


  .org $FFFA     ;first of the three vectors starts here
  .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

;;;;;;;;;;;;;;


  .bank 2
  .org $0000
  .incbin "blaster.chr"   ;includes 8KB graphics file


;  .bank 3
;  .org $0000
;  .incbin "blaster1.chr"   ;includes another 8KB graphics file 'cuz I got teh B4NK5WITCHUZ!!!