Moving sprites

Sprites can be moved individually if they're not stuck together. If they are, the sprite block can be moving by setting the driving sprite's position.

Like every location in VRAM, these values can be updated mid-frame to create scanline effects.

Horizontal position

Their horizontal (X) position is set in VRAM, in SCB4. It's relative to the left screen border.

Example: writing $0010 to $8402, puts sprite #2 at 16 pixels from the left.

ASM routine to set the X position of a given sprite:

;D0: Sprite #
;D1: X position
SetSprX:
    addi.w  #SCB4,d0
    move.w  d0,VRAM_ADDR
    lsl.w   #7,d1
    move.w  d1,VRAM_RW
    rts

Vertical position

Their vertical (Y) position is set in VRAM, in SCB3. It's reversed (496-Y), and relative to the top screen border. A value of $1F0 sets the sprite to Y=0, $110 sets it at 224 (out of the visible area).

Example: writing 496-14 ($1E2) to $8205, puts sprite #5 at 14 pixels from the top.

ASM routine to set the Y position of a given sprite:

;D0: Sprite #
;D1: Y position
SetSprY:
    addi.w  #SCB3,d0
    move.w  d0,VRAM_ADDR
    neg.w   d1
    addi.w  #496,d1
    lsl.w   #7,d1
    move.w  VRAM_RW,d2
    andi.w  #$007F,d2       ; Preserves the sticky bit and size attributes
    or.w    d1,d2
    move.w  d2,VRAM_RW
    rts