Extend Music Slots in Sonic 1
From Sonic Retro
This guide is now outdated because if you want to have more music tracks go here
Important note: This guide prefers label names of the Sonic 1 GitHub disassembly, but labels of the 2005 disassembly are also mentioned.
You may notice that in the Sound Test stops on $CF, and can't go any further than $CF. So here are instructions on how to extend the Music Slots to $FF in Sonic 1.
Step 1: editing the sound driver
To start go to Sound_ChkValue, if you see this line:
cmpi.b #$E4,d7 bls.s Sound_E0toE4
Add this under Sound_E0toE4:
cmpi.b #$FF,d7 bls.w Sound_PlayMoreBGM
Then, go to Sound_PlayBGM (Sound_81to9F in the 2005 disassembly), and add this right above Sound_PlayBGM:
Sound_PlayMoreBGM: ; Sound_E5toFF subi.b #$45, d7 ; map E5-FF range to A0-BA ; fall through to Sound_PlayBGM
Then, go to MusicIndex. At first make sure that Music81 to Music9F are listed. If the list contains only Music81 to Music93, add Music94 to Music9F (and don't forget to define/include those songs).
Then add MusicE5 to MusicFF to the end of the list, after Music9F:
dc.l MusicE5, MusicE6, MusicE7, MusicE8 dc.l MusicE9, MusicEA, MusicEB, MusicEC dc.l MusicED, MusicEE, MusicEF, MusicF0 dc.l MusicF1, MusicF2, MusicF3, MusicF4 dc.l MusicF5, MusicF6, MusicF7, MusicF8 dc.l MusicF9, MusicFA, MusicFB, MusicFC dc.l MusicFD, MusicFE, MusicFF
Then search for Music9F:
Music9F: incbin sound\music9F.bin even
and add these lines:
MusicE5: incbin sound\musicE5.bin even MusicE6: incbin sound\musicE6.bin even MusicE7: incbin sound\musicE7.bin even MusicE8: incbin sound\musicE8.bin even MusicE9: incbin sound\musicE9.bin even MusicEA: incbin sound\musicEA.bin even MusicEB: incbin sound\musicEB.bin even MusicEC: incbin sound\musicEC.bin even MusicED: incbin sound\musicED.bin even MusicEE: incbin sound\musicEE.bin even MusicEF: incbin sound\musicEF.bin even MusicF0: incbin sound\musicF0.bin even MusicF1: incbin sound\musicF1.bin even MusicF2: incbin sound\musicF2.bin even MusicF3: incbin sound\musicF3.bin even MusicF4: incbin sound\musicF4.bin even MusicF5: incbin sound\musicF5.bin even MusicF6: incbin sound\musicF6.bin even MusicF7: incbin sound\musicF7.bin even MusicF8: incbin sound\musicF8.bin even MusicF9: incbin sound\musicF9.bin even MusicFA: incbin sound\musicFA.bin even MusicFB: incbin sound\musicFB.bin even MusicFC: incbin sound\musicFC.bin even MusicFD: incbin sound\musicFD.bin even MusicFE: incbin sound\musicFE.bin even MusicFF: incbin sound\musicFF.bin even
If you would try to play songs in the range E5-FF now, they might not always play. The reason for this is, that the sound priority list has values for sound IDs 81-E4 only. So go to SoundPriorities (SoundTypes in the 2005 disassembly). The last line of the array is:
dc.b $90,$90,$90,$90,$90 ; $E0
Now add the additional values for sound IDs E5-FF by replacing the line with:
dc.b $90,$90,$90,$90,$90,$90,$90,$90,$90,$90,$90,$90,$90,$90,$90,$90 ; $E0 dc.b $90,$90,$90,$90,$90,$90,$90,$90,$90,$90,$90,$90,$90,$90,$90,$90 ; $F0
Step 2: extending the sound test
Next, we are going to extend the slots in the Sound Test. First go to LevSel_SndTest, and change:
moveq #$4F,d0 ; if sound test moves below 0, set to $4F
moveq #$7F,d0 ; if sound test moves below 0, set to $7F
Then, go to LevSel_Right, and change:
Last but not least, we are going to allow the Sound Test to read slots $E5 to $FF, so go to LevSel_NoCheat, and comment all lines until LevSel_PlaySnd out. (It should be 4 instructions.)
Then, build the rom and check out your newly added tunes.