Do you have another 2MBit (256kB) flash chips ? It does not necessarily have to be AE29F2008. Looks like your BIOS chip is defective 🙁

The funny thing about this is that programming the bios backup is successful.

zyga64 wrote on 2023-07-28, 18:07:

Do you have another 2MBit (256kB) flash chips ? It does not necessarily have to be AE29F2008. Looks like your BIOS chip is defective 🙁

Maybe I could buy one but besides being a 2MBit DIP-32, what other aspects should I consider when choosing it?

It should be TTL compatible (5V) - not low voltage version.
In my box of 2MBit BIOS chips from old/broken motherboards I have: Winbond W49F002A, w49F002U, SyncMos F29C51002T, SST 39SF020, EON EN29F002 and PMC PM29F002T. Sometimes also location of boot-block is important (top or bottom of flash), but anyway - it is worth to try.
Try to find datasheets of few of those, and check if pinout is the same (to be 100% sure). Even if your TL866 doesn't support some of them there is big chance that with Uniflash 2.0 from here https://soggi.org/motherboards/bios-update-fl … h-utilities.htm you'll be able to hotflash it. I have done it many times successfully.

Check and see if this BIOS is the same one on the Retro Web: https://web.archive.org/web/20061020193316fw_ … d-bios.asp#M748

vico wrote on 2023-07-28, 17:56:

I have this motherboard M748MR Rev:1.3 that is giving me some problems, the boot is very slow and apparently it is because the d […]
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I have this motherboard M748MR Rev:1.3 that is giving me some problems, the boot is very slow and apparently it is because the disk and the cdrom take a long time to be recognized and lately the disk is not recognized and gives an error: "sec master hdd error", sometimes also the serial mouse is not recognized. Apparently these issues are caused by a faulty bios.

I have found the following BIOS but an error is produced when trying to program it into the chip.
https://www.elhvb.com/mboards/pcchips/686_M7x … 702S_2M_AMI.zip

Please help me find a working bios update or give me hints on how to program it correctly.

I am attaching images and a backup of the apparently defective BIOS.

The EEPROM is protected/locked from erasure and re-programming unfortunately. The reason why programming the original backup ROM images works is because nothing has actually changed in the EEPROM from any reprogramming attempts, so the verify matches. You'll notice that programming a different ROM image probably fails at the first point a byte is different between the image and what it's the EEPROM. It uses a normal pinout so most 32pin DIP 2MBit EEPROMs will work as a replacement.