If the board boots with CPU at correct speed, you can't have messed the jumpers up too badly.

This sounds like a BIOS detection thing. There is definitely a newer version available, 4.27gn2a
http://cwcyrix.duckdns.org/ftp-archives/ftp.f … 486/486-vip-io/

Very interesting note in the .txt:

Please find attached file of BETA BIOS for 486 GIO VT, 486, 486 GAC-V 486 GAC-2, 486 VIP IO, 486 GIO VP and 486 GVT2 which […]
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Please find attached file of BETA BIOS for 486 GIO VT, 486, 486 GAC-V
486 GAC-2, 486 VIP IO, 486 GIO VP and 486 GVT2 which will support
Year 2000.

Those Motherboards can use the same BIOS.

They all use similar Via chipsets, so this is plausible. Means you can look at BIOS from those other boards too - although only try that if you have recovery options (EEPROM flasher)

I can confirm BIOS version 4.26GN2 does not detect DX4-type CPUs correctly. But version 4.27xxxs does.
I would recommend to flash to an EEPROM chip with the "hotswap method". The old EPROM remains untouched as a fallback and you avoid the UV-erasing process (plus no external programmer is needed).

And you only need the second 64K from that ROM if you don't have integrated VGA on your motherboard.

Can you tell me which type of EEPROM I will need for flashing?

michaeljk wrote on 2021-03-25, 14:17:

Can you tell me which type of EEPROM I will need for flashing?

If you use the entire Bios VGA & Motherboard, you need a DIP32 128KByte eeprom (aka 1024kbit) such as a Winbond W27C010

If you only need the motherboard 1/2 of the bios (starting at 0x10000) because you have a VGA card with a BIOS already , you can get by with a DIP28 64Kb eeprom (aka 512kbit) such as a Winbond W27C512, and align the non-divot side of the eeprom with the non-divot side of the socket, leaving 4 unpopulated pins at the divot side.

If I only do 1/2 the BIOS, does that free up more upper memory for TSRs, or would the VGA bios on my card over lap the address space with the VGA bios on the motherboard?

douglar wrote on 2021-03-25, 15:03:

If you use the entire Bios VGA & Motherboard, you need a DIP32 128KByte eeprom (aka 1024kbit) such as a Winbond W27C010 […]
Show full quote

michaeljk wrote on 2021-03-25, 14:17:

Can you tell me which type of EEPROM I will need for flashing?

If you use the entire Bios VGA & Motherboard, you need a DIP32 128KByte eeprom (aka 1024kbit) such as a Winbond W27C010

If you only need the motherboard 1/2 of the bios (starting at 0x10000) because you have a VGA card with a BIOS already , you can get by with a DIP28 64Kb eeprom (aka 512kbit) such as a Winbond W27C512, and align the non-divot side of the eeprom with the non-divot side of the socket, leaving 4 unpopulated pins at the divot side.

If I only do 1/2 the BIOS, does that free up more upper memory for TSRs, or would the VGA bios on my card over lap the address space with the VGA bios on the motherboard?

The 27C010 or 27C512 are EPROM chips that need UV light for erasing and are programmed with carefully timed programming pulses. Programming is typically done using an EPROM programmer, but not implemented on mainboards. For "flashing" on a mainboard, you need an EEPROM or flash EPROM. The most widely used BIOS flash chip from that era is the 29EE011.

These seemed to work OK for me without using UV erase--

My board is an older FIC-486-VIP and it doesn't seem to support flashing, so I use an eeprom burner.

That second chip is W27C512-45Z

Huh, interesting. Just seen an ST datasheet that has 27c101 in either UV-erasable, or as one-time-programmable. Are those ones reprogrammable?

snufkin wrote on 2021-03-25, 19:11:

Huh, interesting. Just seen an ST datasheet that has 27c101 in either UV-erasable, or as one-time-programmable. Are those ones reprogrammable?

My chip is a 27c010, not a 27c101, yes it is rewritable

douglar wrote on 2021-03-25, 19:03:

These seemed to work OK for me without using UV erase--

My board is an older FIC-486-VIP and it doesn't seem to support flashing, so I use an eeprom burner.

They do work without UV erasing, because they come erased from the factory. But once you wrote something into them, you can not reprogram them without UV erasing. My comment about the EPROMs being the wrong type was in context of the suggestion to do a hot-swap flash.

snufkin wrote on 2021-03-25, 19:11:

Huh, interesting. Just seen an ST datasheet that has 27c101 in either UV-erasable, or as one-time-programmable. Are those ones reprogrammable?

And as you see, the chips you posted have a plastic case, not a ceramic case with a quartz window, so you will have a very hard time getting UV light to the die. The chips you posted a photo of are the "one-time programmable" chips. There are stories about erasing these chips with a dentist's x-ray gun. While the x-rays are able to penetrate the plastic case, and are also able to erase the EPROM die inside, the x-rays also do a lot of colateral damage to the die, and the chip does not survice more than a couple of program / erase cycles.

mkarcher wrote on 2021-03-25, 19:59:

There are stories about erasing these chips with a dentist's x-ray gun. While the x-rays are able to penetrate the plastic case, and are also able to erase the EPROM die inside, the x-rays also do a lot of colateral damage to the die,

...and probably your body too, if you don't know what you're doing...

I thought I wrote to those eproms more than once, but maybe I didn't. I'll have to check it out.