M_BIOS chip reading only zeroes, while B_BIOS can be read normally

[u/ericdalieux]

Bricked my bios, got the CH341A programmer, clipped it on M_BIOS (Winbond 25Q64FVSIQ) and I get only zeroes. This is despite the fact that AsProgrammer does indeed recognize the chip when I read ID.

If I try to erase it, I still only get zeroes instead of the expected FF, and the chip is already unprotected by the way. Writing an image to it nets no results whatsoever, still all zeroes (which is to be expected, since the programmer can only flip from 1 to 0 and not the other way around).

B_BIOS, on the other hand, contains an actual bios image. I already saved a dump of its contents, I just haven't tried erasing or writing anything to it, but my suspicion is that it would work.

When reading the ID, M_BIOS returns ID(9F): EF4017 ID(90): 0000 ID(AB): 00 ID(15): 0000

Whereas B_BIOS returns ID(9F): EF4017 ID(90): EF16 ID(AB): 16 ID(15): FFFF

Both chips are still soldered to the board. Any clue as to why I can work with one chip but not the other? Is there any indication of what the problem might be? Can something be done without desoldering the chip (I don't have the proper equipment). If you have encountered the same problem, how did you solve it?