Super NES Programming/Animated Sprites
This article is for slightly more advanced ASM programmers than the rest of this website.
By default, the SNES is limited to 16kB of sprite patterns, which is the same as: 512 8x8 sprites 128 16x16 sprites 32 32x32 sprites 8 64x64 sprites
As you can see, animated sprites can eat away the available memory pretty quickly. This is where DMA comes in. The only problem is DMA isn't all that fast. You only have time to DMA up to 6kB during v-blank, and even less if you take into account scrolling and OAM updating. For the purpose of this article, let's stick to 4kB.
How do we prevent DMAing over 4kB, regardless of what we throw at the screen? We need to make the system know when to stop, and continue the DMA loading into the next frame. How? My solution is to divide the v-ram into 1kB slots. I chose 1kB, because 1kB happens to be a row of 8 16x16 sprites.
!apple = "$0000" !banana = "$0001" !table = "$0080" !update_flag = "$0100" !vram = "$0101" !source = "$0102" ;================================================================ macro change_vram(source, vram) php rep #$20 sep #$10 lda <syntaxhighlight> ldy <vram> jsr change_slot plp endmacro ;================================================================ change_slot: cmp !table,y beq redundant_vram_load sta !table,y ldx !apple ;; apple is just a stupid name I chose for a temporary register sta !source,x phy ldy #$01 phy pla sta !update_flag,x inx #4 stx !apple redundant_vram_load: rts ;============================================================= update_vram_slots: php sep #$30 ldx !banana ;; banana is just another stupid name I chose ldy #$04 ;; to name another temporary register lda #$80 sta $2115 lda #$01 sta $4300 lda #$18 sta $4301 vram_loop: lda !update_flag,x beq finish stz $2116 lda !vram,x sta $2117 stz $4302 lda !source,x sta $4303 lda !source+1,x sta $4304 stz $4305 lda #$04 sta $4306 lda #$01 sta $420b lda #$00 sta !update_flag,x inx #4 dey bne vram_loop finish: stx !banana sep #$30 lda !apple sec sbc !banana cmp #$10 bcc lag jmp end_of_game_code lag: plp rts
All you have to do is jump to the "update_vram_slot" subroutine during vblank, and use the "change_vram" macro whenever you need to update an animation slot.
For example if you want update slot $0400 in vram from address $98a300 use this macro:
This code isn't limited to just sprite animation, it also can be used for background animation and scrolling.