Description of the headers used in the SNES Classic ROM files (.sfrom)

snescheaders.md

Header

Start	Size	Description	Notes
0x00	0x4	0x00000100
0x04	0x4	File size	Includes the header and footer
0x08	0x4	Location of ROM	Always 0x00000030 in official files
0x0C	0x4	Location of PCM samples	Comes after end of ROM in official files
0x10	0x4	Location of PCM footer	Equals file size if PCM data missing
0x14	0x4	Location of footer (below)	Same as New 3DS's SNES VC header starting from 0x30
0x18	0x4	Location of S-DD1 data offset	Footer location + 0x2C
0x1C	0x4	0x00000000
0x20	0x4	Unknown	Footer location + 0x1B. Some kind of a flag?
0x24	0x8	Wii U Virtual Console Game ID	WUP-J**X, X — title region (E = USA, J = Japan, P = EUR)
0x2C	0x4	0x00000000

Footer

Start	Size	Description	Notes
0x00	0x1	Emulation speed in FPS	0x3C == 60
0x01	0x4	ROM Size
0x05	0x4	Size of PCM samples	0 if missing
0x09	0x4	Size of PCM footer	0 if missing
0x0D	0x2	Game preset ID	Varies between games
0x0F	0x1	# of players?	Usually 0x02. Setting to 0x0 disables the second controller
0x10	0x1	Sound volume
0x11	0x1	ROM Type	0x15 for HiROM and 0x14 for LoROM
0x12	0x1	Super-FX flag?	0x0C if the game uses SFX Chip, 0x00 otherwise
0x13	0x7	0x00000000000000
0x1A	0x4?	Unknown	Usually 0x00000100
0x1E	0x4?	Unknown	Always 0x00000100

Taken from this pastebin from @Cluster_M, cleaned up with my own findings added in.

The pastebin was originally written by pcm720 on the GBAtemp forums.

By the way, I think that you got header offsets 0x20 and 0x18 mixed up.
Header offset 0x20 in SFROM always points to footer offset 0x1B, not 0x18.
If there is some pointer for S-DD1 data, it's probably header offset 0x18 since it always points to the end of file.

That's entirely possible. I kept getting two of them mixed up in my own notes and it was probably those two. I'll double check when I get the chance.

Okay, can confirm.

Take a look at Street Fighter Alpha 2 from Wii U VC (it uses the S-DD1 chip):
Header:
0x000000: 00 01 00 00 B2 6F A7 00 30 00 00 00 30 00 40 00
0x000010: 10 EE 5B 00 50 F0 5B 00 70 F0 5B 00 00 00 00 00
0x000020: 63 F0 5B 00 57 55 50 2D 4A 43 47 45 00 00 00 00

Footer:
0x5BF050: 3C 00 00 40 00 E0 ED 1B 00 40 02 00 00 DF 10 02
0x5BF060: A5 00 00 01 00 00 00 01 00 00 00 00 00 00 00 00
0x5BF070: 80 F0 5B 00 00 00 00 00 00 00 00 00 00 00 00 00

Header offset 0x20 points to footer offset 0x13, and 0x18 points to footer offset 0x20.
At 0x5BF080, there's some kind of ROM data, 0x4B7F40 bytes in size. I'm pretty sure that this is data for the S-DD1.
So, it looks like 0x20 points to footer offset 0x20 that contains the location of S-DD1 data.

By the way, this ROM doesn't work on SNES Mini.

You're totally correct. I've updated the table. It's interesting that it has the first 0x01 byte at 0x13 in the footer, whereas every SNEC-C ROM I've examined so far has it at 0x1B. I wonder what they're even for.

Something interesting to note is that Super Mario RPG has this:

Header

          00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
0x000000: 00 01 00 00 C0 81 4D 00 30 00 00 00 30 00 40 00
0x000010: 70 74 4D 00 84 81 4D 00 B0 81 4D 00 00 00 00 00
0x000020: 9F 81 4D 00 57 55 50 2D 4A 41 42 45 00 00 00 00

Footer

          00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
0x4D8180: XX XX XX XX 3C 00 00 40 00 40 74 0D 00 14 0D 00
0x4D8190: 00 9C 10 02 32 14 00 00 00 00 00 00 00 00 00 01
0x4D81A0: 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00
0x4D81B0: C0 81 4D 00 00 00 00 00 00 00 00 00 00 00 00 00

The file is 0x4D81C0 bytes long, so that address in 0x4D81B0 is just the EOF. Also, this offset is located at footer + 0x2C rather than footer + 0x20 like in your example.

Who knows...
The footer structure is very inconsistent. That value at 0x4D81B0 does nothing, the game will boot anyway.
You can even set 0x18 to C0 81 4D and 0x4D81B0 to 00 00 00, and there will be no difference.

I wish we'd know more about their preset IDs. I had no luck with S-DD1, and SA1 is really weird.
Some games work with one preset, some games work with other, some display garbled graphics, and some refuse to boot at all...

Another thing I just noticed is that the ROM type at 0x11 in your SF2A footer is 0x00

I set 0x18 and 0x20 to 0x0 in the header and everything but ROM size and type in the footer to 0x0 for Super Mario All-Stars and it still works. It doesn't seem like the emulator cares about a lot of these values.

It looks like using this header with a combined footer (like the 3DS VC) works just fine for Super Mario All-Stars. If this format works for all ROMs, it would greatly simplify my conversion script.

      00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
0x00: 00 01 00 00 60 00 28 00 50 00 00 00 00 00 00 00
0x10: 00 00 00 00 30 00 00 00 00 00 00 00 00 00 00 00
0x20: 00 00 00 00 57 55 50 2D 4A 44 49 45 00 00 00 00
0x30: 00 00 00 28 00 00 00 00 00 00 00 00 00 00 00 00
0x40: 00 14 00 00 00 00 00 00 00 00 00 00 00 00 00 00

EDIT: Confirmed working with Final Fantasy Mystic Quest as well.
Mega Man X2 works with this header and a preset ID of 0x113D, except there's graphical corruption whenever the Cx4 chip is used.
The preset ID 0x1117 (MMX2 on Wii U VC) works fine.

That's interesting. I can't see any reason why it won't work since we directly point emulator to a footer.

So, preset IDs are incompatible? That's very disappointing. It looks like they rely on emulation profiles instead of emulating the SNES properly...
At least DSP-1 and SuperFX games work fine with a single preset ID (or do they?)

i think you should have a look at this:
https://www.reddit.com/r/miniSNES/comments/74z4oo/street_fighter_alpha_2_running_on_canoe/?sort=new