Moving sprites

Sprites can be moved individually if they're not stuck together. If they are, the sprite block can be moved by only setting the driving sprite's position. The chained sprites will move accordingly.

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

Horizontal position

The 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

The 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 (right out of the visible area).

Example: writing $1E2 (496-14) 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

Example

Animates ~100 individual and grouped sprites.

Source and romset: [nyanmvs.zip]