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If an ISA card was configured to respond to the address range of the system's BIOS (and the BIOS chips removed), could you boot from it in the same way as the system's default BIOS?

Specifically, I'm wondering if something like this could be modified to load the contents at 0xF0000 (the address that I understand the IBM AT and IBM XT 286 puts the system BIOS). I'm interested in replacing the ROM chips with EEPROMs or flash, since they're easier to program and still being made (making a ROM socket adapter of some kind being plan B).

  • What precise model are you intending to work with? The answer to the question you've asked is, I believe, no (as the original BIOS ROM would still be functioning and attempt to drive the data bus at the same time as your flash chips, most likely resulting in your replacement code being ANDed with the original), but for a specific model there may be a simple fix to the issue. – Jules Dec 25 '17 at 12:40
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    The IBM 5162 is the computer. As for the ISA card, I would make my own if needed. – Sydius Dec 25 '17 at 20:08
  • Wouldn't it be more platform-friendly to use a BIOS Extension ROM on the ISA card in order to control the system boot? Then, you can coexist with the regular PC BIOS, but also run whatever extra code you want from your EEPROM. – Brian H Dec 28 '17 at 16:32
  • If that's possible, then sure. My motivation is to use a ROM that is compatible with XT-IDE cards (the IBM one I have doesn't seem to be) or at least to have custom hard drive parameters. – Sydius Dec 28 '17 at 19:44
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I don't see why not, as long as you are actually able to remove the original BIOS chips (or ensure they are never selected / never drive the data bus).

But keep in mind that there is a difference between a 16-bit memory access and an 8-bit memory access, and that in any single 128K region you cannot mix the two. So if you want to install an 8-bit BIOS card at address F000, you must ensure that all accesses in the E000-EFFF range are also 8-bit.

If you can get a 16-bit ROM card I think that would work better for you.

  • Note that removing the original BIOS chips might prevent them from driving the data bus, but it might not be enough to allow the use of 8-bit ROMs in the E000-FFFF space. – Ken Gober Dec 27 '17 at 17:22

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