What possibly went wrong with downgrade

A project log for Asus T100TA resurrection

aka how I reflashed the UEFI after I've bricked it

darth_llamahdarth_llamah 01/05/2016 at 12:520 Comments

I finally had some time to take a look at flash dump and to compare it with "untouched" UEFI image.

Best guess- NVRAM was unreadable for the UEFI.

It seems that NVRAM is stored within the UEFI starting at 0x10358- untouched 314 image on the left, backup on the right:

I also guess that default setup values are stored just before NVRAM, starting (possibly) at 0x10060

However data dumped form the flash chip differs from the default image:

- several bytes also in (possibly) default NVRAM data area have different values (eg. 0x10064, 0x1007B and some other bytes after that)

- starting at 0x1010F data seems to be almost the same in both images but written with a growing offset- at the beginning it's just one byte but later (0x10358) offset grows to 10 bytes

(314 image on the left side , dump on the right)

But this is exactly how 227 image looks like (as before- original on the left side, dump on the right):

My guess:

Asus WinFlash utility moved NVRAM data (and default setup values) to the place compatible with provided UEFI image and after the reboot UEFI's built-in flasher was responsible for image update. But in my case it refused to work (although I forced WinFlash not to check image build date) and I'm not surprised that T100 didn't want to boot without access to proper NVRAM data.

If only there was a good old "clear cmos" jumper somewhere......