You might need to check on the write capabilities. There was another case where someone tried using a 39 PROM and had problems with the computer hanging when it tried to update the ESCD: Biostar MB-8433UUD-A and 39SF010A - boot issue (hang during Updating ESCD)

The only thing I can find that seems to really differ is how the IC performs the writes (byte-program v page-write), product ID, and some timings with how long it takes the IC to perform an operation; otherwise the OP mode selection sequences, software command sequences, etc. seem to be 1:1

But maybe it's the fact the IC writes in a completely different manner (and that I cant find any blocks in the data sheet for Software Data Protect despite the new chip saying it supports that stuff per JEDEC spec) that would throw things off... Well, it'd be ~$1.8 USD and it's going to get bundled in with some caps I need so I don't think it'd cost me too much to find out if it'll work or not.

Failing that, there's apparently a bit of work I can do via

I used UNIFLASH to save the BIOS from a working system (using the SST PH29EE010-150-3CF flash chip) into a file. Then I flashed this saved BIOS (and not the original UUD2014.BIN file) onto the 39SF010A flash chip and it worked like a charm. No "Updating ESCD ... " message anymore and therefore also no hang. Apparently, when UNIFLASH saves a BIOS to file, it also saves the ESCD data alongside the actual BIOS.

with the caveat that I can't be swapping cards around lest I make the BIOS want to reconfigure itself, but that shouldn't be too much a problem.

For what it's worth, not long ago I got an MB-8433UUD-A v3.1 that had an EPROM BIOS instead of EEPROM. It would print "Updating ESCD..." message with the EPROM, not hang, and continue booting without printing "Success". With the SST39SF020A, it would hang. Not sure what all the ramifications of having nowhere to store the ESCD would be. Guessing it might cause issues with PnP cards in Windows 95.

if the new eeproms are cheap, it is worth trying. Looking on ebay, 29EE020 eeproms are plentiful and very cheap, less than 2 USD per piece.

I replaced 3 defective bios chips with other 3 random bios chips (different brands) bought from ebay and new chips all worked.

jakethompson1 wrote on 2021-09-15, 00:29:

For what it's worth, not long ago I got an MB-8433UUD-A v3.1 that had an EPROM BIOS instead of EEPROM. It would print "Updating ESCD..." message with the EPROM, not hang, and continue booting without printing "Success". With the SST39SF020A, it would hang. Not sure what all the ramifications of having nowhere to store the ESCD would be. Guessing it might cause issues with PnP cards in Windows 95.

I have an ASUS mobo wwith a beta bios that doesn't say ESCD success being beta 🤣