The "FIXED BITS" message is not at all mandatory. It's just a helpful diagnostic produced by MAME's romcmp tool, which can only catch a few types of common dumping errors. Depending on the hardware, some romcmp errors might not even be errors: oversize ROMs are common and graphics ROMs and bipolar decoding PROMs might not need all the data outputs that the part has.
A few types of dumping errors romcmp can't detect but have occurred:
- One or two bad bits (as in Sky Army).
- Fixed bits that extend only through certain areas of the dump. (I've patched a few cases of this by hand.)
- Bridges between address lines that cause 3/4 mirroring (as happened with the Eeekk! color PROM).
- Faulty internal latches on 8048, 8041 or related MCUs.
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Re: skyarmy and rom FIXED BITS curiosity
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