BOBs: Blitter Objects

Overview

The Amiga has only 8 hardware sprites, each 16 pixels wide. For games needing more objects—or larger, more colourful characters—programmers used BOBs (Blitter OBjects). The Blitter copied sprite graphics into the playfield bitplanes, with masking to preserve the background. More flexible than hardware sprites, but requiring careful buffer management.

BOBs vs hardware sprites

Feature	Hardware sprites	BOBs
Count	8 (or 4 attached)	Unlimited*
Width	16 pixels fixed	Any size
Colours	3 or 15 (attached)	Playfield depth
Speed	Zero CPU	Blitter time
Background	Automatic	Requires save/restore

*Limited by Blitter time and frame budget.

The BOB rendering process

1. Save background

Before drawing, save what’s underneath:

save_background:
    ; Copy screen area to save buffer
    move.l  bob_x,d0
    move.l  bob_y,d1
    jsr     calc_screen_address

    ; Blit from screen to save buffer
    move.w  #$09f0,BLTCON0(a6)  ; A->D copy
    move.l  screen_addr,BLTAPT(a6)
    move.l  save_buffer,BLTDPT(a6)
    move.w  #(height<<6)|words,BLTSIZE(a6)

2. Draw BOB

Cookie-cut the sprite onto the screen:

draw_bob:
    ; A = mask data
    ; B = sprite data
    ; C = screen (background)
    ; D = screen (destination)

    move.w  #$0fca,BLTCON0(a6)  ; cookie cut minterm
    move.l  mask_ptr,BLTAPT(a6)
    move.l  image_ptr,BLTBPT(a6)
    move.l  screen_addr,BLTCPT(a6)
    move.l  screen_addr,BLTDPT(a6)

    ; Set modulos for screen width
    move.w  screen_mod,BLTCMOD(a6)
    move.w  screen_mod,BLTDMOD(a6)
    move.w  #0,BLTAMOD(a6)
    move.w  #0,BLTBMOD(a6)

    ; Start blit
    move.w  #(height<<6)|words,BLTSIZE(a6)

3. Restore background (next frame)

Before drawing at new position, restore old background:

restore_background:
    ; Copy save buffer back to screen
    move.l  last_screen_addr,BLTDPT(a6)
    move.l  save_buffer,BLTAPT(a6)
    move.w  #$09f0,BLTCON0(a6)
    move.w  #(height<<6)|words,BLTSIZE(a6)

Double buffering with BOBs

Essential for flicker-free BOBs:

; Frame N:
; - Display buffer A
; - Clear BOBs from buffer B (restore backgrounds)
; - Draw BOBs to buffer B
; - Swap: B becomes display, A becomes draw

frame_update:
    ; Wait for VBlank
    jsr     wait_vblank

    ; Swap display pointers
    move.l  draw_buffer,d0
    move.l  display_buffer,draw_buffer
    move.l  d0,display_buffer

    ; Update copper to show new display buffer
    jsr     update_copper

    ; Now safe to modify draw_buffer
    jsr     restore_all_bobs
    jsr     update_bob_positions
    jsr     draw_all_bobs

Pixel-accurate positioning

The Blitter works on word boundaries. For sub-word positioning:

draw_bob_shifted:
    ; Calculate shift amount (0-15)
    move.w  bob_x,d0
    and.w   #$000f,d0       ; pixel offset within word

    ; Put shift value in BLTCON0/1
    lsl.w   #4,d0           ; shift to position
    lsl.w   #8,d0
    or.w    #$0fca,d0       ; add minterm
    move.w  d0,BLTCON0(a6)

When shifting, you need an extra word:

32-pixel BOB shifted = 3 words instead of 2

Sorting for draw order

BOBs must be drawn back-to-front (painter’s algorithm):

sort_bobs_by_y:
    ; Simple bubble sort (fine for small counts)
    ; Or use insertion sort as positions change incrementally
    ...

draw_sorted_bobs:
    lea     sorted_list,a0
.loop:
    move.w  (a0)+,d0        ; bob index
    bmi     .done           ; -1 = end of list
    jsr     draw_single_bob
    bra     .loop
.done:
    rts

Interleaved blitting

Overlap Blitter operations with CPU work:

; Don't wait after every blit
; Queue multiple operations

draw_multiple:
    jsr     start_bob1_blit  ; start blitter
    jsr     do_game_logic    ; CPU works while blitter runs
    jsr     wait_blitter     ; ensure done before next
    jsr     start_bob2_blit
    ; ...

Memory requirements

Component	Size
BOB image	width × height × planes
BOB mask	width × height
Save buffer	Same as BOB (per BOB)
Second screen	Full screen size

Example: 32×32 BOB, 5 planes

Image: 32×32×5 = 640 bytes
Mask: 32×32 = 128 bytes
Save: 32×32×5 = 640 bytes per instance

Performance considerations

Factor	Impact
BOB size	Larger = slower
BOB count	More = slower
Bitplane depth	More planes = slower
Shifting	Adds ~50% time
Screen width	Affects modulo

Typical budget: 10-20 medium BOBs at 50 fps.

Software sprites on the Amiga