ROM type signature

This documentation is a proposal for ROM type signature to help Flash tools and emulators correctly detect ROM formats and, in particular, the type of mapper used.

This proposal is aimed above all at new MSX productions, for which there are still detection problems, particularly with mapped ROMs (Lilly's Saga, Phenix Corrupta, etc.).

Note: All the strings of this page are defined without zero-terminator.

ROM type signature

The ROM type signature is 8 bytes long and placed just after the 16 bytes ROM header starting with "AB".

Type	Signature
ASCII 8 KB	"ROM_ASC8"
ASCII 16 KB	"ROM_AS16"
Konami 8 KB (K4)	"ROM_KON4"
Konami 8 KB with SCC (K5)	"ROM_KON5"
NEO 8 KB	"ROM_NEO8"
NEO 16 KB	"ROM_NE16"
Plain 16 KB (page 0)	"ROM_16P0"
Plain 16 KB (page 1)	"ROM_16P1"
Plain 16 KB (page 2)	"ROM_16P2"
Plain 16 KB (page 3)	"ROM_16P3"
Plain 32 KB (page 0-1)	"ROM_32P0"
Plain 32 KB (page 1-2)	"ROM_32P1"
Plain 32 KB (page 2-3)	"ROM_32P2"
Plain 48 KB (page 0-2)	"ROM_48P0"
Plain 48 KB (page 1-3)	"ROM_48P1"
Plain 64 KB (page 0-3)	"ROM_64P0"
OR
Plain (no mapper)	"ROM_PL", X, 0 (see bellow)

Note: These are just a few examples to show the intent, but if this signature format is of interest to developers and tool creators (flash and emulators) then we could draw up a more exhaustive list of existing ROM types.

Plain ROM

For plain ROM, "ROM_PL" may be followed by a byte describing the ROM format instead of 0:

7	6	5	4	3	2	1	0
1	x	Mirror	Head	Last page		First page

Where:

• First page: First page number of the ROM (0-3)
• Last page: Last page number of the ROM (0-3)
• Head: Page number of the ROM header (0: 0x4000, 1: 0x8000)
• Mirror: Is ROM mirrored in other pages (0: No, 1: Yes)

For example:

Type	Signature
Plain 16 KB (page #1)	"ROM_PL", 0b10000101, 0
Plain 16 KB (page #2)	"ROM_PL", 0b10011010, 0
Plain 32 KB	"ROM_PL", 0b10001001, 0
Plain 48 KB	"ROM_PL", 0b10001000, 0
Plain 64 KB	"ROM_PL", 0b10001100, 0

Note: This format don't support 8 KB plain ROM but this ROM size are no longer of any interest nowadays.

Detection

Most of the time, the ROM header is at the beginning of the .ROM file, so the signature is at offset 16 of this file. However, there are formats where this is not the case, such as a 32 KB ROM with its header visible at address 0x8000 of the memory space and therefore placed in the .ROM file at offset 0x4000.

For the ROM mappers, the header can be in the segment selected by default in the bank that cover addresses from 0x4000 or in segment selected by default in the bank that cover addresses from 0x8000. In ASCII mappers, segment 0 (the first segment) is selected in all banks, so the header is necessarily at the beginning of the .ROM file. However, with the Konami with SCC mapper, if bank 0 (which covers addresses from 0x4000) is initialized on segment 0 (the start of the ROM file), bank 2 (which covers addresses from 0x8000) is initialized on segment 2 at offset 0x4000 of the ROM file. Both locations should be checked for ROM header and signature.

Appendix

Signature as code

The 4 first bytes of the ROM type signature "ROM_", have been choose because the Z80 instructions it generates are incoherent enough so it's extremely unlikely that a program would start with these instructions and therefore unintentionally contain a valid signature. The next 4 bytes further increase the security of this signature.

Z80 instructions generated by the first 4 bytes of the signature:

ld    d, d ; 'R'
ld    c, a ; 'O'
ld    c, l ; 'M'
ld    e, a ; '_'

Known ROM signature

Device	Signature	Size
MSX-Audio	"AUDIO"	5 bytes
Internal MSX-Music (and FM-PAC clone)	"APRLOPLL"	8 bytes
FM-PAC	"OPLL"	4 bytes