VOGONS

How to Fix a Rom Checksum Error?

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First post, by douglar

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I'm trying to validate some old VLB IDE roms that are on the Vogondrivers website

I'm looking at this one:
https://www.vogonsdrivers.com/getfile.php?fileid=2059

The file is 32KB, but only has info in the first 8KB

The motherboard BIOS doesn't acknowledge any option rom when I put it on an ST M27C256B. I tried on an IDE controller and on an XTide Card.

I burned the image on an Amtel AT28C256. Still no response when on the IDE controller, but when on the XTide card, I get a rom checksum error.

Is there a tool for setting the ROM checksum?

What should I worry about if I try to cut it down and stick it on an 8KB rom ?

Reply 1 of 4, by jakethompson1

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It's 16K, not 8K, and as a 16K option ROM the checksum is correct

Reply 2 of 4, by douglar

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jakethompson1 wrote on 2024-11-20, 02:06:

It's 16K, not 8K, and as a 16K option ROM the checksum is correct

Burning the image to an M27128A made it work correctly, thanks!

Reply 3 of 4, by douglar

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jakethompson1 wrote on 2024-11-20, 02:06:

It's 16K, not 8K, and as a 16K option ROM the checksum is correct

How were you able to tell that it was a 16K option rom?

Reply 4 of 4, by jakethompson1

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The option ROM header is: 55 aa, followed by one byte representing the size of the option ROM in sectors (512 byte units), followed by a jmp short (2 bytes) to the entry point of the ROM. That's five bytes, which is why for XT disk controllers, there's conventionally another jmp to the low level format routine at offset 5. In this case the option ROM size was 32.

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