mirror of
https://github.com/hsoft/collapseos.git
synced 2024-11-08 16:08:06 +11:00
41 lines
1.3 KiB
Plaintext
41 lines
1.3 KiB
Plaintext
# Sega Master System ROM signatures
|
|
|
|
When loading ROM, the SMS' BIOS checks for a special signature
|
|
at the end of that ROM. If that signature is incorrect, the ROM
|
|
doesn't load.
|
|
|
|
Collapse OS has a program to generate that signature at B165.
|
|
This document describes what it does.
|
|
|
|
At boot, the BIOS checks 0x10 bytes before the 0x8000, then
|
|
0x4000, then 0x2000 mark for a signature. This signature has
|
|
the following structure.
|
|
|
|
0x00-0x07: String constant: "TMR SEGA"
|
|
0x08-0x09: null bytes
|
|
0x0a-0x0b: checksum
|
|
0x0c-0x0e: null bytes
|
|
0x0f : "size" flag
|
|
|
|
The checksum is a simple 16-bit sum of all bytes up to the
|
|
beginning of the signature.
|
|
|
|
The size flag can have 3 values: 0x4a for an 8K ROM, 0x4b for
|
|
16K and 0x4c for 32K. It can have other values for other kinds
|
|
of sizes, but we don't care about them in the context of
|
|
Collapse OS.
|
|
|
|
## Generating the signature
|
|
|
|
Before generating the signature, you need to have the contents
|
|
of your ROM somewhere in memory. Then, you load B165 and you
|
|
call "segasig" which has the signature "addr size". "addr" is
|
|
the adress of the beginning of the ROM and "size" is 0, 1 or 2
|
|
depending on whether your ROM is 8K, 16K or 32K.
|
|
|
|
Calling the word will write the 0x10 bytes signature at the
|
|
end of the ROM.
|
|
|
|
Note that all I/O use the "Addressed device" words (see
|
|
usage.txt), so I/O indirections will work.
|