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Combining System Bios and XT-IDE into one chip?

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First post, by kingcake

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So my system's BIOS image is only 64K but it uses a 128K chip.

I want to burn a new chip with XT-IDE on board the system's BIOS rom image.

Does anyone have experience with this?

I've started to do some research about how option ROMs work:

IMG-3169.jpg

So normally the BIOS scans C8000-F4000 for option ROMS. So I need to modify my BIOS to scan much lower, or jump straight to the address of the on board XT-IDE ROM, right?

Reply 1 of 6, by jakethompson1

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You've read the BIOS off the chip with an external reader and are sure that it only occupies the second 64K?
If so, I think you could just put the XT-IDE at the beginning of the chip.
When they say F4000 that is equivalent to F400:0000. Your BIOS would occupy F000:0000 and the first 64K of the chip should occupy E000:0000 which would be in that range. Unless the first 64K isn't wired to go to your bios chip somehow.
Is it flash? No reason not to try then right?

Reply 2 of 6, by kingcake

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jakethompson1 wrote on 2020-09-28, 03:31:
You've read the BIOS off the chip with an external reader and are sure that it only occupies the second 64K? If so, I think you […]
Show full quote

You've read the BIOS off the chip with an external reader and are sure that it only occupies the second 64K?
If so, I think you could just put the XT-IDE at the beginning of the chip.
When they say F4000 that is equivalent to F400:0000. Your BIOS would occupy F000:0000 and the first 64K of the chip should occupy E000:0000 which would be in that range. Unless the first 64K isn't wired to go to your bios chip somehow.
Is it flash? No reason not to try then right?

Yes read it with a eeprom reader/burner.

I tried that with PCem and it doesn't post.

Reply 3 of 6, by jakethompson1

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kingcake wrote on 2020-09-29, 00:03:
jakethompson1 wrote on 2020-09-28, 03:31:
You've read the BIOS off the chip with an external reader and are sure that it only occupies the second 64K? If so, I think you […]
Show full quote

You've read the BIOS off the chip with an external reader and are sure that it only occupies the second 64K?
If so, I think you could just put the XT-IDE at the beginning of the chip.
When they say F4000 that is equivalent to F400:0000. Your BIOS would occupy F000:0000 and the first 64K of the chip should occupy E000:0000 which would be in that range. Unless the first 64K isn't wired to go to your bios chip somehow.
Is it flash? No reason not to try then right?

Yes read it with a eeprom reader/burner.

I tried that with PCem and it doesn't post.

It might work on real hardware, PCem may not be expecting the BIOS to be a different size

Reply 4 of 6, by kingcake

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jakethompson1 wrote on 2020-09-29, 01:18:
kingcake wrote on 2020-09-29, 00:03:
jakethompson1 wrote on 2020-09-28, 03:31:
You've read the BIOS off the chip with an external reader and are sure that it only occupies the second 64K? If so, I think you […]
Show full quote

You've read the BIOS off the chip with an external reader and are sure that it only occupies the second 64K?
If so, I think you could just put the XT-IDE at the beginning of the chip.
When they say F4000 that is equivalent to F400:0000. Your BIOS would occupy F000:0000 and the first 64K of the chip should occupy E000:0000 which would be in that range. Unless the first 64K isn't wired to go to your bios chip somehow.
Is it flash? No reason not to try then right?

Yes read it with a eeprom reader/burner.

I tried that with PCem and it doesn't post.

It might work on real hardware, PCem may not be expecting the BIOS to be a different size

something weird is going on here...I get the phoenix bios at both locations even though I hand made the file with both bioses in the correct positions.

Capture.jpg
subaru forester 0 60

Reply 5 of 6, by digistorm

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Maybe your mainboard connects both address lines to the same pin of the ROM chip? Or the address decoding on your mainboard works in a way that both addresses end up reading the same location of your ROM chip?

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