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