[HELP] Streets of Rage 2 Title Screen Hacking Questions
 

Hi. I'm currently experimenting with hacking title screens for Genesis, specifically Streets of Rage 2, so I'm looking for someone with Genesis assembly experience to provide a little guidance. I don't have much Genesis hacking experience, as I've spent the last few summers working with the SNES, but I have some idea of where I need to go.

1) I figure I'd first export the title screen via Pancake 2, but I'm not sure of the offset for the screen. Would anyone be able to help locate and provide this?

2) I'm next going to import that result into SonMapEd and try to figure out the tile mapping. Would anyone be able to help locate and provide the offset for the title screen's palette?

3) Having done this, I'm going to overlay my title screen onto the mapping and try to apply my title screen's palette, then export to .bin and drop that data into expanded rom. How many colors is the title screen limited to, 16 or 256?

4) I may need to add commands to expanded rom to display the title screen, plus re-route original code to that title screen. From portrait editing, I'm wondering if placing this code before the new title screen data will do the trick, [XX][YY][ZZ] being a relative pointer to the new title screen:

33 FC 81 3C 00 C0 00 04 23 FC 94 0E 93 A0 00 C0
00 04 23 FC 96 [YY] 95 [ZZ] 00 C0 00 04 33 FC 97 [XX]
00 C0 00 04 23 FC 60 00 00 81 00 C0 00 04 4E 75

5) With the new title screen in expanded rom, I need to point to it in the original code. Will the original pointer use code 41 F9 00 [AA][BB][CC], where [AA][BB][CC] is that offset I'm looking for in #1?

6) If #5 is yes, do I then change that code to be 4E B9 00 [DD][EE][FF], where [DD][EE][FF] is my offset in expanded rom that has the 33 FC... code per #4 above?

7) I assume the code I need for the pointer is around where the decompression algorithm is, similar to the portraits on the player select screen. Not positive of this though. And I just 4E 71 / NOP that decompression code?

Any questions you can help answer would be a huge help!

I genuinely feel you would have a lot better luck asking for help at a genesis specific forum like maybe sprites mind, or the sonic forums, or of course RHDN, but I think I'll touch on a few points here:


1) if it supports it, sure I suppose. Are you asking about the offset for where the title screen mappings are located in the ROM? If so what I would probably do is find an emulator that can spit out the VDP registers, figure out which layer the title screen is on, what address it's set to in VRAM, dump the ram, select a section of the data in the vram and do a search for those values in ROM. If you're lucky it'll be stored uncompressed in the ROM, maybe.


2) I wouldn't recommend that. Unless it has some sort of "universal genesis game" feature that I'm unaware of, those tools are tailored to the games they are designed for only. As for the palette, if you are able to dump the palette ram, you may have luck looking for that in the ROM too.


3) Genesis tiles are limited to 4bbp so each tile itself is limited to a 16 color palette. The VDP can only store 4 palettes at one time, which limits you to 64 colors at once. Of course there are ways around this, but that would require extensive assembly hacking to pull off.


The rest honestly I can't really answer due to me not really knowing anything about SoR2 to offer any helpful input, but if you're trying to paste machine code, you may want to just post the disassembly instead so we have a better understanding of what you're talking about.

Thanks for the tips! The hex code comes from a tutorial on how to hack portraits, that tutorial using Pancake 2 to decompress the portrait titles once the offset is entered, and then SonMapEd used to map the tiles so the replacement can mimic it. I was trying to "reverse engineer" it to place a title screen in ROM. I don't have a disassembly, so I was going to do searches in the hex for matching bytes and verify I had the right routine based on the title screen offset being nearby, once I located a title screen offset.

For reference, the tutorial is here:
https://www.romhacking.net/documents/791/

To my knowledge, the title screen's compressed. Most of the sprites are, as are the portraits.

I used the r57 shell mod for Gens (debugger) and was able to see the title screen in VDP RAM.

Snippit of VDP RAM: https://files.catbox.moe/wsuiox.PNG

I dumped the RAM and attempted to do a search on a snippit of it in the ROM using HxD. No luck. Tried another. Same. I'm thinking it likely is compressed.

I was able to dump the palette in VDP RAM and match that in the ROM, so I think the title screen palette is at $5D74.

I figured I'd try to use a disassembly with a breakpoint on the $5D74 value to perhaps find the decompression scheme or a table with the title screen offset. The game breaks at $18E6, right before the title screen comes up, so I feel like I'm in the right place. The command at $1836 is move.w (A6)+,D4, where A6 appears to be represented by $5D74. I'm not familiar with 68k assembly, so I'm not sure yet how to interpret it.

Snippit is here, for reference: https://files.catbox.moe/uekax1.PNG

In the meantime, I'm trying to set up my hack to put shadows on sprites. It's quite a pain, so I figured in the meantime, if I could, I'd try to see if a new title screen were viable, hence the thread.

Oh, so you're trying to find the the actual location of the tiles themselves.


Well, if they are being uncompressed, what I have found is that usually what most games will do is decompress into a buffer in RAM and then DMA the tiles over to vram, so usually that buffer data is still there in RAM, so what I would do is dump the ram right at the title screen and view the ram dump in a tile editor, and if you find something (like say tile data @ $8000 for example) I would set a write breakpoint at that location and you should, hopefully, find the decompression routine.

Indeed! So I ran a tracelog from the point at which the demo fade out to the point at which the title screen displays. I did some searches on $00:A900 and found this code:

This appears to be the offset for the screen!

I opened the ROM in Pancake 2, decompressed from that offset and - lo and behold - I was able to drop the resulting .bin into SonMapEd and see the title screen.

Next issue is that the title screen code must be mapping the same tiles more than once, particularly for the "E"s and maybe part of the "S" in "Streets". Likely elsewhere too, but I didn't look into it that far.

The above being the case, if I were to drop my title screen into expanded ROM, I'm not sure yet how to
1) repoint the ROM to it (perhaps via that LEA I wrote above?);
2) create code that will assign the tiles in the proper order; and
3) jump back to the proper code, ignoring the decompression routine

Made some great progress today, thanks to a lot of help and guidance. Here's what I have so far:

The offset for the title screen tiles: $39018

The offset for the title screen palette: $5D74

Mapping Values for Plane B for Title Screen: $38B94 to $38FD4 (440 Hex Bytes)
Offset to Plane B Table for Title Screen: $5C2C
Length of Title (hex): $5C39
Height of Title (hex): $5C3B
Y Position of Title (hex): $5C34
X Position of Title (hex): $5C35

Not entirely sure what to do just yet to put all this info together, but good progress so far. I also have a 16 color title screen ready to go once I get a little more guidance re: next steps.

You're welcome. Glad I could help. :P


With genesis plane formats, I don't know too much to really help out, so whoever else is helping you with this might be able to guide you further.

Wanted to say thanks again for the help! If you're curious, the "final" product is here:

https://www.romhacking.net/hacks/4666/

You're welcome, and congrats! :thumb up: