Sprite Control Words

Overview

Amiga sprites have two control words at the start of their data that define vertical start/stop positions and horizontal position. Update these each frame to move sprites around the screen. The encoding packs multiple values into 16-bit words.

Code

; =============================================================================
; SPRITE CONTROL WORDS - AMIGA
; Calculate sprite position from X/Y coordinates
; Taught: Game 1 (Signal), Unit 2
; CPU: ~40 cycles | Memory: ~30 bytes
; =============================================================================

SPRITE_HEIGHT   equ 16          ; Sprite height in pixels

; Update sprite control words
; Input: A0 = sprite data address
;        frog_pixel_x, frog_pixel_y = coordinates
update_sprite:
            move.w  frog_pixel_y,d0     ; D0 = Y position (VSTART)
            move.w  frog_pixel_x,d1     ; D1 = X position (HSTART)

            ; === Control word 1: VSTART and HSTART ===
            ; Format: VSTART[7:0] in high byte, HSTART[8:1] in low byte
            move.w  d0,d2               ; D2 = VSTART
            lsl.w   #8,d2               ; Shift to high byte
            lsr.w   #1,d1               ; HSTART / 2 (lose bit 0)
            or.b    d1,d2               ; Combine
            move.w  d2,(a0)             ; Write control word 1

            ; === Control word 2: VSTOP and control bits ===
            ; Format: VSTOP[7:0] in high byte, control bits in low byte
            add.w   #SPRITE_HEIGHT,d0   ; VSTOP = VSTART + height
            lsl.w   #8,d0               ; Shift to high byte
            ; Low byte contains: ATTACH, unused, VSTOP[8], VSTART[8], HSTART[0]
            ; For simple sprites, low byte is usually 0
            move.w  d0,2(a0)            ; Write control word 2

            rts

frog_pixel_x:   dc.w    160         ; Screen X coordinate
frog_pixel_y:   dc.w    100         ; Screen Y coordinate

Extended version with all control bits:

; Full control word calculation with all bits
; Handles sprites in lower screen area (Y > 255)
update_sprite_full:
            move.w  sprite_y,d0         ; VSTART
            move.w  sprite_x,d1         ; HSTART
            move.w  d0,d2
            add.w   #SPRITE_HEIGHT,d2   ; VSTOP

            ; Control word 1
            move.w  d0,d3               ; VSTART[7:0]
            lsl.w   #8,d3
            move.w  d1,d4
            lsr.w   #1,d4               ; HSTART[8:1]
            or.b    d4,d3
            move.w  d3,(a0)

            ; Control word 2
            move.w  d2,d3               ; VSTOP[7:0]
            lsl.w   #8,d3

            ; Build low byte: bit 7=ATTACH, bit 2=VSTOP[8], bit 1=VSTART[8], bit 0=HSTART[0]
            moveq   #0,d4
            btst    #8,d2               ; VSTOP bit 8?
            beq.s   .no_vstop8
            bset    #2,d4
.no_vstop8:
            btst    #8,d0               ; VSTART bit 8?
            beq.s   .no_vstart8
            bset    #1,d4
.no_vstart8:
            btst    #0,d1               ; HSTART bit 0?
            beq.s   .no_hstart0
            bset    #0,d4
.no_hstart0:
            or.b    d4,d3
            move.w  d3,2(a0)

            rts

Trade-offs

Aspect	Cost
CPU	~40 cycles
Memory	~30 bytes
Limitation	Must update every frame for moving sprites

When to use: Any program using hardware sprites.

When to avoid: Never - sprites always need valid control words.

Control Word Format

Word 1 (offset 0):

Bit 15-8: VSTART[7:0] - Vertical start position (low 8 bits)
Bit 7-0:  HSTART[8:1] - Horizontal position (bits 8-1, divided by 2)

Word 2 (offset 2):

Bit 15-8: VSTOP[7:0] - Vertical stop position (low 8 bits)
Bit 7:    ATTACH     - Attach to previous sprite (for 15-colour sprites)
Bit 6-3:  Unused
Bit 2:    VSTOP[8]   - Vertical stop bit 8
Bit 1:    VSTART[8]  - Vertical start bit 8
Bit 0:    HSTART[0]  - Horizontal position bit 0

Position Ranges

Axis	Range	Notes
X	0-447	Visible area ~64-383
Y	0-312 (PAL)	Visible area ~44-300

Related

Patterns: Copper List Basics

Vault: Commodore Amiga