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SCHG How-to:Add Spin Dash to Sonic 1/Part 3
From Sonic Retro
(Original guide by shobiz)
While the end result of following Lightning's and Puto's guides is a near-perfect Spin Dash in Sonic 1, there are still a few things not right about it, so let's fix those.
Contents |
Converting Sonic's art buffer to a DMA queue
This isn't exactly a bug per se, but the method used to port over the DMA queue to Sonic 1 in Puto's guide is slightly hack-ish. In stock Sonic 1, Sonic's sprites are copied over into a RAM buffer at $FFFFC800 before being DMAed to VRAM. This buffer occupies $300 bytes, so if we convert it to a DMA queue, besides getting a proper structure we also save $200 bytes of RAM (some of which will be used later on in this guide).
Firstly, let's modify the actual queue routines to use this RAM space. So go to DMA_68KtoVRAM and replace everything from there till the end of Process_DMA with:
; --------------------------------------------------------------------------- ; Subroutine for queueing VDP commands (seems to only queue transfers to VRAM), ; to be issued the next time ProcessDMAQueue is called. ; Can be called a maximum of 18 times before the buffer needs to be cleared ; by issuing the commands (this subroutine DOES check for overflow) ; --------------------------------------------------------------------------- ; ||||||||||||||| S U B R O U T I N E ||||||||||||||||||||||||||||||||||||||| ; sub_144E: DMA_68KtoVRAM: QueueCopyToVRAM: QueueVDPCommand: Add_To_DMA_Queue: QueueDMATransfer: movea.l ($FFFFC8FC).w,a1 cmpa.w #$C8FC,a1 beq.s QueueDMATransfer_Done ; return if there's no more room in the buffer ; piece together some VDP commands and store them for later... move.w #$9300,d0 ; command to specify DMA transfer length & $00FF move.b d3,d0 move.w d0,(a1)+ ; store command move.w #$9400,d0 ; command to specify DMA transfer length & $FF00 lsr.w #8,d3 move.b d3,d0 move.w d0,(a1)+ ; store command move.w #$9500,d0 ; command to specify source address & $0001FE lsr.l #1,d1 move.b d1,d0 move.w d0,(a1)+ ; store command move.w #$9600,d0 ; command to specify source address & $01FE00 lsr.l #8,d1 move.b d1,d0 move.w d0,(a1)+ ; store command move.w #$9700,d0 ; command to specify source address & $FE0000 lsr.l #8,d1 move.b d1,d0 move.w d0,(a1)+ ; store command andi.l #$FFFF,d2 ; command to specify destination address and begin DMA lsl.l #2,d2 lsr.w #2,d2 swap d2 ori.l #$40000080,d2 ; set bits to specify VRAM transfer move.l d2,(a1)+ ; store command move.l a1,($FFFFC8FC).w ; set the next free slot address cmpa.w #$C8FC,a1 beq.s QueueDMATransfer_Done ; return if there's no more room in the buffer move.w #0,(a1) ; put a stop token at the end of the used part of the buffer ; return_14AA: QueueDMATransfer_Done: rts ; End of function QueueDMATransfer ; --------------------------------------------------------------------------- ; Subroutine for issuing all VDP commands that were queued ; (by earlier calls to QueueDMATransfer) ; Resets the queue when it's done ; --------------------------------------------------------------------------- ; ||||||||||||||| S U B R O U T I N E ||||||||||||||||||||||||||||||||||||||| ; sub_14AC: CopyToVRAM: IssueVDPCommands: Process_DMA: Process_DMA_Queue: ProcessDMAQueue: lea ($C00004).l,a5 lea ($FFFFC800).w,a1 ; loc_14B6: ProcessDMAQueue_Loop: move.w (a1)+,d0 beq.s ProcessDMAQueue_Done ; branch if we reached a stop token ; issue a set of VDP commands... move.w d0,(a5) ; transfer length move.w (a1)+,(a5) ; transfer length move.w (a1)+,(a5) ; source address move.w (a1)+,(a5) ; source address move.w (a1)+,(a5) ; source address move.w (a1)+,(a5) ; destination move.w (a1)+,(a5) ; destination cmpa.w #$C8FC,a1 bne.s ProcessDMAQueue_Loop ; loop if we haven't reached the end of the buffer ; loc_14CE: ProcessDMAQueue_Done: move.w #0,($FFFFC800).w move.l #$FFFFC800,($FFFFC8FC).w rts ; End of function ProcessDMAQueue
Next, let's modify Sonic's PLC routine to make use of this routine. Go to LoadSonicDynPLC and replace the entire routine with the Sonic 2 equivalent, slightly modified to make use of Sonic 1's PLC format:
LoadSonicDynPLC: ; XREF: Obj01_Control; et al moveq #0,d0 move.b $1A(a0),d0 ; load frame number cmp.b ($FFFFF766).w,d0 beq.s locret_13C96 move.b d0,($FFFFF766).w lea (SonicDynPLC).l,a2 add.w d0,d0 adda.w (a2,d0.w),a2 moveq #0,d5 move.b (a2)+,d5 subq.w #1,d5 bmi.s locret_13C96 move.w #$F000,d4 move.l #Art_Sonic,d6 SPLC_ReadEntry: moveq #0,d1 move.b (a2)+,d1 lsl.w #8,d1 move.b (a2)+,d1 move.w d1,d3 lsr.w #8,d3 andi.w #$F0,d3 addi.w #$10,d3 andi.w #$FFF,d1 lsl.l #5,d1 add.l d6,d1 move.w d4,d2 add.w d3,d4 add.w d3,d4 jsr (QueueDMATransfer).l dbf d5,SPLC_ReadEntry ; repeat for number of entries locret_13C96: rts ; End of function LoadSonicDynPLC
Third, we have to make the dust object use the new name for the DMA routine. Really simple, just go to loc_1DF0A and replace
jsr (DMA_68KtoVRAM).l
with
jsr (QueueDMATransfer).l
Finally, we have to make the game actually use these routines. Firstly, go to loc_CD4 and replace
tst.b ($FFFFF767).w beq.s loc_D50 lea ($C00004).l,a5 move.l #$94019370,(a5) move.l #$96E49500,(a5) move.w #$977F,(a5) move.w #$7000,(a5) move.w #$83,($FFFFF640).w move.w ($FFFFF640).w,(a5) move.b #0,($FFFFF767).w
with
jsr (ProcessDMAQueue).l
Make similar changes at loc_DAE, loc_EEE and loc_FAE. Secondly, go to loc_D50 and remove these two lines:
move #$83,($FFFFF640).w jsr Process_DMA
Thirdly, go to Level_ClrVars3 and after the
move.w #$8ADF,($FFFFF624).w move.w ($FFFFF624).w,(a6)
add
clr.w ($FFFFC800).w move.l #$FFFFC800,($FFFFC8FC).w
Finally, go to loc_47D4, and after
jsr Hud_Base
add
clr.w ($FFFFC800).w move.l #$FFFFC800,($FFFFC8FC).w
After this, the game will use the DMA queue for Sonic's art reloading, and we'll have $200 bytes of free RAM from $FFFFC900 to $FFFFCAFF.
Making the Spin Dash sound rev
This has always annoyed me a lot, but thanks to Xenowhirl's Sonic 2 sound driver disassembly and Tweaker's music guide, we can fix it. We need to use three RAM variables, so we can utilise our newly freed RAM. $FFFFC900 is a flag which is 1 if the Spin Dash sound was the last one played and 0 if it was not. $FFFFC901 is a timer, and $FFFFC902 is the pitch increase value.
First, go to loc_71BC8 and change it from
loc_71BC8: lea $40(a6),a5 tst.b (a5) bpl.s loc_71BD4 jsr sub_71C4E(pc)
to
loc_71BC8: tst.b ($FFFFC901).w beq.s @cont subq.b #1,($FFFFC901).w @cont: lea $40(a6),a5 tst.b (a5) bpl.s loc_71BD4 jsr sub_71C4E(pc)
Then go to Sound_D1toDF and change if from
Sound_D1toDF: tst.b $27(a6) bne.w loc_722C6 tst.b 4(a6) bne.w loc_722C6 tst.b $24(a6) bne.w loc_722C6 movea.l (Go_SoundIndex).l,a0 sub.b #$A1,d7 bra SoundEffects_Common
to
Sound_D1toDF: tst.b $27(a6) bne.w loc_722C6 tst.b 4(a6) bne.w loc_722C6 tst.b $24(a6) bne.w loc_722C6 clr.b ($FFFFC900).w cmp.b #$D1,d7 ; is this the Spin Dash sound? bne.s @cont3 ; if not, branch move.w d0,-(sp) move.b ($FFFFC902).w,d0 ; store extra frequency tst.b ($FFFFC901).w ; is the Spin Dash timer active? bne.s @cont1 ; if it is, branch move.b #-1,d0 ; otherwise, reset frequency (becomes 0 on next line) @cont1: addq.b #1,d0 cmp.b #$C,d0 ; has the limit been reached? bcc.s @cont2 ; if it has, branch move.b d0,($FFFFC902).w ; otherwise, set new frequency @cont2: move.b #1,($FFFFC900).w ; set flag move.b #60,($FFFFC901).w ; set timer move.w (sp)+,d0 @cont3: movea.l (Go_SoundIndex).l,a0 sub.b #$A1,d7 bra SoundEffects_Common
After this, go to Sound_A0toCF and after
tst.b $24(a6) bne.w loc_722C6
add
clr.b ($FFFFC900).w
Then go to loc_7226E and replace
movea.l dword_722EC(pc,d3.w),a5
with
lea dword_722EC(pc),a5 movea.l (a5,d3.w),a5
Finally, go to loc_72276 and change
move.w (a1)+,8(a5) move.b #1,$E(a5)
to
move.w (a1)+,8(a5) tst.b ($FFFFC900).w ; is the Spin Dash sound playing? beq.s @cont ; if not, branch move.w d0,-(sp) move.b ($FFFFC902).w,d0 add.b d0,8(a5) move.w (sp)+,d0 @cont: move.b #1,$E(a5)
and now the Spin Dash sound revs.
Horizontal scroll delay
Sonic 2's Spin Dash implements a horizontal scroll delay, while the ported one doesn't (yet). The basic idea is to check a certain flag, and if it's non-zero, base scrolling on Sonic's X position a certain number of frames ago rather than his current X position. So go to loc2_1ACD0 and change
move.w d0,($FFFFEED0).w ; move to $EED0
to
move.w d0,($FFFFC904).w ; move to $C904
And then go to ScrollHoriz2 and change
ScrollHoriz2: move.w ($FFFFD008).w,d0 sub.w ($FFFFF700).w,d0
to
ScrollHoriz2: ; XREF: ScrollHoriz move.w ($FFFFC904).w,d1 beq.s @cont1 sub.w #$100,d1 move.w d1,($FFFFC904).w moveq #0,d1 move.b ($FFFFC904).w,d1 lsl.b #2,d1 addq.b #4,d1 move.w ($FFFFF7A8).w,d0 sub.b d1,d0 lea ($FFFFCB00).w,a1 move.w (a1,d0.w),d0 and.w #$3FFF,d0 bra.s @cont2 @cont1: move.w ($FFFFD008).w,d0 @cont2: sub.w ($FFFFF700).w,d0 subi.w #$90,d0 blt.s loc_65F6 subi.w #$10,d0 bge.s loc_65CC clr.w ($FFFFF73A).w rts
And then, to fix a bug with backwards horizontal scrolling, go to loc_65F6 and change it from
loc_65F6: ; XREF: ScrollHoriz2 add.w ($FFFFF700).w,d0 cmp.w ($FFFFF728).w,d0 bgt.s loc_65E4 move.w ($FFFFF728).w,d0 bra.s loc_65E4
to
loc_65F6: ; XREF: ScrollHoriz2 cmpi.w #-$10,d0 bgt.s @cont move.w #-$10,d0 @cont: add.w ($FFFFF700).w,d0 cmp.w ($FFFFF728).w,d0 bgt.s loc_65E4 move.w ($FFFFF728).w,d0 bra.s loc_65E4
to limit the maximum number of pixels the camera can scroll back by in a single frame.
Vertical scroll delay
In stock Sonic 1, the camera scrolls vertically as soon as Sonic looks up or ducks. This can be a bit annoying for the Spin Dash, however, so let's implement the delay present in Sonic 2 by using $FFFFC903 as our delay counter. Firstly, to fix a bit of incorrect code, go to loc2_1AD78 and change
loc2_1AD78: addq.l #4,sp ; increase stack ptr cmpi.w #$60,($FFFFEED8).w ; $EED8 only ever seems beq.s loc2_1AD8C ; to be used in Spin Dash bcc.s loc2_1AD88 addq.w #4,($FFFFEED8).w loc2_1AD88: subq.w #2,($FFFFEED8).w loc2_1AD8C: bsr.w Sonic_LevelBound bsr.w Sonic_AnglePos move.w #$60,($FFFFF73E).w ; reset looking up/down rts
to
loc2_1AD78: addq.l #4,sp ; increase stack ptr cmpi.w #$60,($FFFFF73E).w beq.s loc2_1AD8C bcc.s loc2_1AD88 addq.w #4,($FFFFF73E).w loc2_1AD88: subq.w #2,($FFFFF73E).w loc2_1AD8C: bsr.w Sonic_LevelBound bsr.w Sonic_AnglePos ;move.w #$60,($FFFFF73E).w ; reset looking up/down rts
Then go to Sonic_LookUp and change
Sonic_LookUp: btst #0,($FFFFF602).w ; is up being pressed? beq.s Sonic_Duck ; if not, branch move.b #7,$1C(a0) ; use "looking up" animation cmpi.w #$C8,($FFFFF73E).w beq.s loc_12FC2 addq.w #2,($FFFFF73E).w bra.s loc_12FC2 ; =========================================================================== Sonic_Duck: btst #1,($FFFFF602).w ; is down being pressed? beq.s Obj01_ResetScr ; if not, branch move.b #8,$1C(a0) ; use "ducking" animation cmpi.w #8,($FFFFF73E).w beq.s loc_12FC2 subq.w #2,($FFFFF73E).w bra.s loc_12FC2 ; =========================================================================== Obj01_ResetScr: cmpi.w #$60,($FFFFF73E).w ; is screen in its default position? beq.s loc_12FC2 ; if yes, branch bcc.s loc_12FBE addq.w #4,($FFFFF73E).w ; move screen back to default loc_12FBE: subq.w #2,($FFFFF73E).w ; move screen back to default
to
Sonic_LookUp: btst #0,($FFFFF602).w ; is up being pressed? beq.s Sonic_Duck ; if not, branch move.b #7,$1C(a0) ; use "looking up" animation addq.b #1,($FFFFC903).w cmp.b #$78,($FFFFC903).w bcs.s Obj01_ResetScr_Part2 move.b #$78,($FFFFC903).w cmpi.w #$C8,($FFFFF73E).w beq.s loc_12FC2 addq.w #2,($FFFFF73E).w bra.s loc_12FC2 ; =========================================================================== Sonic_Duck: btst #1,($FFFFF602).w ; is down being pressed? beq.s Obj01_ResetScr ; if not, branch move.b #8,$1C(a0) ; use "ducking" animation addq.b #1,($FFFFC903).w cmpi.b #$78,($FFFFC903).w bcs.s Obj01_ResetScr_Part2 move.b #$78,($FFFFC903).w cmpi.w #8,($FFFFF73E).w beq.s loc_12FC2 subq.w #2,($FFFFF73E).w bra.s loc_12FC2 ; =========================================================================== Obj01_ResetScr: move.b #0,($FFFFC903).w Obj01_ResetScr_Part2: cmpi.w #$60,($FFFFF73E).w ; is screen in its default position? beq.s loc_12FC2 ; if yes, branch bcc.s loc_12FBE addq.w #4,($FFFFF73E).w ; move screen back to default loc_12FBE: subq.w #2,($FFFFF73E).w ; move screen back to default
Finally, to make the screen bias gradually reset during rolling, go to loc_131CC and change
loc_131CC: move.b $26(a0),d0 jsr (CalcSine).l
to
loc_131CC: cmp.w #$60,($FFFFF73E).w beq.s @cont2 bcc.s @cont1 addq.w #4,($FFFFF73E).w @cont1: subq.w #2,($FFFFF73E).w @cont2: move.b $26(a0),d0 jsr (CalcSine).l
And that's it! Let me know if I made any mistakes, or if there's anything still not covered. Here's a ROM after all these steps have been done, and here's the source code (with Puto's SEGA sound fix implemented instead of Esrael's for ASM68k compatibility).
Final code
Here is the final Spin Dash code for references:
Sonic_Spin Dash: tst.b $39(a0) ; already Spin Dashing? bne.s loc2_1AC8E ; if set, branch cmpi.b #8,$1C(a0) ; is anim duck bne.s locret2_1AC8C ; if not, return move.b ($FFFFF603).w,d0 ; read controller andi.b #$70,d0 ; pressing A/B/C ? beq.w locret2_1AC8C ; if not, return move.b #$1F,$1C(a0) ; set Spin Dash anim (9 in s2) move.w #$D1,d0 ; spin sound ($E0 in s2) jsr (PlaySound_Special).l ; play spin sound addq.l #4,sp ; increment stack ptr move.b #1,$39(a0) ; set Spin Dash flag move.w #0,$3A(a0) ; set charge count to 0 cmpi.b #$C,$28(a0) ; ??? oxygen remaining? move.b #2,($FFFFD1DC).w ; Set the Spin Dash dust animation to $2 loc2_1AC84: jsr Sonic_LevelBound jsr Sonic_AnglePos locret2_1AC8C: rts ; --------------------------------------------------------------------------- loc2_1AC8E: move.b #$1F,$1C(a0) move.b ($FFFFF602).w,d0 ; read controller btst #1,d0 ; check down button bne.w loc2_1AD30 ; if set, branch move.b #$E,$16(a0) ; $16(a0) is height/2 move.b #7,$17(a0) ; $17(a0) is width/2 move.b #2,$1C(a0) ; set animation to roll addq.w #5,$C(a0) ; $C(a0) is Y coordinate move.b #0,$39(a0) ; clear Spin Dash flag moveq #0,d0 move.b $3A(a0),d0 ; copy charge count add.w d0,d0 ; double it move.w spdsh_norm(pc,d0.w),$14(a0) ; get normal speed tst.b ($FFFFFE19).w ; is sonic super? beq.s loc2_1ACD0 ; if no, branch move.w spdsh_super(pc,d0.w),$14(a0) ; get super speed loc2_1ACD0: ; TODO: figure this out move.w $14(a0),d0 ; get inertia subi.w #$800,d0 ; subtract $800 add.w d0,d0 ; double it andi.w #$1F00,d0 ; mask it against $1F00 neg.w d0 ; negate it addi.w #$2000,d0 ; add $2000 move.w d0,($FFFFC904).w ; move to $EED0 btst #0,$22(a0) ; is sonic facing right? beq.s loc2_1ACF4 ; if not, branch neg.w $14(a0) ; negate inertia loc2_1ACF4: bset #2,$22(a0) ; set unused (in s1) flag move.b #0,($FFFFD1DC).w ; clear Spin Dash dust animation move.w #$BC,d0 ; spin release sound jsr (PlaySound_Special).l ; play it! move.b #8,($FFFFFF5B).w ; set afterimage counter to 8 bra.s loc2_1AD78 ; ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ spdsh_norm: dc.w $800 ; 0 dc.w $880 ; 1 dc.w $900 ; 2 dc.w $980 ; 3 dc.w $A00 ; 4 dc.w $A80 ; 5 dc.w $B00 ; 6 dc.w $B80 ; 7 dc.w $C00 ; 8 spdsh_super: dc.w $B00 ; 0 dc.w $B80 ; 1 dc.w $C00 ; 2 dc.w $C80 ; 3 dc.w $D00 ; 4 dc.w $D80 ; 5 dc.w $E00 ; 6 dc.w $E80 ; 7 dc.w $F00 ; 8 ; ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ loc2_1AD30: ; If still charging the dash... tst.w $3A(a0) ; check charge count beq.s loc2_1AD48 ; if zero, branch move.w $3A(a0),d0 ; otherwise put it in d0 lsr.w #5,d0 ; shift right 5 (divide it by 32) sub.w d0,$3A(a0) ; subtract from charge count bcc.s loc2_1AD48 ; ??? branch if carry clear move.w #0,$3A(a0) ; set charge count to 0 loc2_1AD48: move.b ($FFFFF603).w,d0 ; read controller andi.b #$70,d0 ; pressing A/B/C? beq.w loc2_1AD78 ; if not, branch move.w #$1F00,$1C(a0) ; reset spdsh animation move.w #$D1,d0 ; was $E0 in sonic 2 move.b #2,$FFFFD1DC.w ; Set the Spin Dash dust animation to $2. jsr (PlaySound_Special).l ; play charge sound addi.w #$200,$3A(a0) ; increase charge count cmpi.w #$800,$3A(a0) ; check if it's maxed bcs.s loc2_1AD78 ; if not, then branch move.w #$800,$3A(a0) ; reset it to max loc2_1AD78: addq.l #4,sp ; increase stack ptr cmpi.w #$60,($FFFFF73E).w beq.s loc2_1AD8C bcc.s loc2_1AD88 addq.w #4,($FFFFF73E).w loc2_1AD88: subq.w #2,($FFFFF73E).w loc2_1AD8C: jsr Sonic_LevelBound jsr Sonic_AnglePos ;move.w #$60,($FFFFF73E).w ; reset looking up/down rts ; End of function Sonic_Spin Dash
