6502 machine code snippet for stiching and rendering a 4 part sprite on the 8-bit Nintendo (NES)

6502-nes-stitch-n-draw.asm

.define OAM_TABLE_START             $0200

; .zeropage
; param_1:                            .res 1
; param_2:                            .res 1
; param_3:                            .res 1
; temp_var_1:                         .res 1
; temp_var_2:                         .res 1
; temp_var_3:                         .res 1

; Controls the sprite y/x offsets required to stitch
; and render the four sub sprites 
sub_sprite_offsets:
  .byte $00, $00, $00, $08, $08, $00, $08, $08

; param_1: Sprite OAM position offset. Subroutine expects
;          exclusive use of the subsequent 4 OAM slots 
;          (16 bytes)
;                    
; param_2: Sprite Y position
; param_3: Sprite X position
.proc DrawMagpie
  ldx param_1

  ; Sprite sheet offset
  lda #$08  ; The Magpie sprite starts at sprite sheet 1
  sta temp_var_1

  ; X iterates the OAM
  ldx #$00
  ; Y iterates the sub sprite offset table (sub_sprite_offsets)
  ldy #$00

  ; byte 0,1,2,3 = [y, pattern number, attributes, x]
LOAD_SUB_SPRITES:
  ; Y
  lda sub_sprite_offsets, Y
  adc param_2
  sta OAM_TABLE_START, X
  iny
  inx
  ; Pattern
  lda temp_var_1
  sta OAM_TABLE_START, X
  inc temp_var_1
  inx
  ; Attr
  lda #%00010000
  sta OAM_TABLE_START, X
  inx
  ; X
  lda sub_sprite_offsets, Y
  adc param_3
  sta OAM_TABLE_START, X
  iny
  inx
  tya
  cmp #$08
  bcc LOAD_SUB_SPRITES 

  rts
.endproc