Most late PS/2 systems use Flash EEPROM to store the system BIOS. This type of memory can be reprogrammed by the system CPU, making BIOS updates much easier than in the ealier systems that required replacement ROM chips or reprogramming in a standalone programmer.
Flash chips from the same family are compatible as far as read operations are concerned, different members of the family will however have a unique identifier, and may use different programming protocols. The flash routine stored in the system BIOS only recognizes a few selected identifiers and supports only the corresponding programming protocols. Therefore it's important to use one of the supported parts when replacing deffective flash chip(s), if you want to retain the flash update functionality.
Below is a list of supported parts for all flash-based PS/2s. Two identical parts are required. The information provided here is based on the latest available BIOS code (YMMV if you use some older revision).
Note: I'm not certain whether it's possible to directly replace 12 V parts with 5 V parts or vice versa. More investigation is necessary to see if the programming voltage is fixed (set by soldered resistors/jumpers) or software-controlled.
Note: The flash chips come in a surface mount 32-pin PLCC package, and are typically soldered directly to the PCB (planar or complex)! So some skill and ideally a rework station is required to replace these. Be careful out there...
Model 85 "X"
Model 85 "K/N"
Unlike all the other flash-based PS/2s, the "K/N" has 2 x 256 KB of flash memory.
Model 76/77 i/s "Lacuna"
* This one is suspicious. 2 x 64 KB is not enough to store the SurePath BIOS. Both my Lacunas come with the Am29F010 chips. Needs more digging to see what's going on here.
Model 90/95 Type 4 Complex
The older Type 4 processor boards - "N" and "P" have a local DC/DC converter that is used to generate 12 V programming voltage (Vpp) that is required by some flash chips. The board should support all parts listed in the table above, including the 5 V Am29F010 part. (It has the same pinout, but pin 1, that is normally used to supply the programming voltage, is unused here.) It's however unlikely that any of these older boards came with 5 V Vpp flash chips.
The newer boards - "Q" and "Y" don't have a local 12 V generator. Therefore they won't be able to flash parts that requite this higher programming voltage. Only the 5 V Am29F010 (or 100% compatible alternative) is supported by these boards.