ChangeBlockColor

.proc ChangeBlockColor ; A-New color (0 to 3), Y-Block index
                       ; http://wiki.nesdev.com/w/index.php/Attribute_table
  AttrIndexTemp = 4
  OnValue = 5
  SaveColor = 6
  SaveX = 7

  stx SaveX       ; level renderer uses X, so save it
  sta SaveColor
  and #3
  tax         ; save the particular color being used
  lda ValueMasks,x  ; start with it unshifted
  sta OnValue
  ; now determine how much to shift
  ldx #0      ; start at no shift
  tya
  lsr         ; odd X? move up one
  bcc :+
    inx
: tya
  and #$10 ;odd Y? move up two
  beq :+
    inx
    inx
  :

  lda OnValue
  and ShiftOnMasks,x
  sta OnValue

  ; Take the level buffer index and make it into an AttribMap index
  tya
  alr #$e0 ; Y half (discard last bit)
  lsr
  sta AttrIndexTemp
  tya
  alr #$e     ; X half (discard last bit)
  ora AttrIndexTemp
  tay

  lda AttribMap,y
  and ShiftOffMasks,x
  ora OnValue
  sta AttribMap,y
  sta OnValue
  ldx SaveX

  bit SaveColor
  bpl :+
    rts
  :
; Now locate a slot to queue the actual change
  ldx #0
: lda TileUpdateA1,x
  beq Found
  inx
  cpx #MaxNumTileUpdates
  bne :-
  clc ; failed
  rts
Found:

  lda OnValue
  sta TileUpdateT,x
  lda #$23
  sta TileUpdateA1,x
  tya
  add #$c0
  sta TileUpdateA2,x
  sec
  rts

ValueMasks:
  .byt %00000000
  .byt %01010101
  .byt %10101010
  .byt %11111111
ShiftOnMasks:
  .byt %00000011
  .byt %00001100
  .byt %00110000
  .byt %11000000
ShiftOffMasks:
  .byt %11111100
  .byt %11110011
  .byt %11001111
  .byt %00111111
.endproc