Editing IBM PS/2 Model 25 (section)

== Memory ==

The Model 25 (8086) ships with 512 KB on-board and accepts a single '''128 KB IBM memory module''' (IBM P/N 78X8955) at a proprietary 30-pin RAM socket to reach the architectural ceiling of 640 KB. The on-board DRAM is DIP 41464-class and OKI M3764-class chips '''with parity'''.

The Model 25 286 has two proprietary 30-pin MMK '''parity SIMM sockets''' (J9/J10). The MMK pinout is similar to the standard JEDEC 30-pin SIMM but '''not interchangeable''': the IBM modules have different chip-select routing and require modules with the IBM-style edge connector. The largest supported configuration is 4 MB (2 × 2 MB).

The Model 25 SX has '''one 72-pin SIMM socket''' on the planar (in addition to soldered DRAM at U34 and optional sockets at U32/U33/U48/U49). The SIMM must be of an '''ECC parity''' type and must have the '''four presence-detect pins (67&ndash;70) configured for IBM size and speed encoding'''. Standard PC-style 72-pin parity SIMMs will not be recognised unless the presence-detect pins are tied to GND in the correct configuration.