CPU Type Encoding

CPU Type Encoding
What About Zero?
Further Reading

List compiled by David Beem. Sources: PS/2 Assistant, empirical evidence.

CPU Type Encoding

The middle digit of the sub-model identification indicates type and clock speed of the installed CPU, except for the early PS/2 systems (all models 25, 30, 50, 60, 70, P70, P75, and 80) where it has a different meaning.

3386SX-16 (duplicate of '1'), Theory: working FLOAT pin?
6486SLC2-40 (CPU daughtercard on 8556/8557)
7Theory: 486SLC3-60? (CPU daughtercard on 8556/8557)
B486SLC2-50 (9533/9553/9556/9557 planar or CPU daughtercard on 9556/9557)
E486SLC3-75 (9556/9557 planar or CPU daughtercard on 9556/9557)
FTheory: 486SX-16?
ITheory: 486DX-16?
PPentium 60
QPentium 66
U, X486SX-33 (Theory: one PGA, the other PQFP?)
YPentium 90

Note: We are making a few assumptions here (marked as "Theory"), to see if it can be fully pinned down...

Possible missing encodings: POD63/POD83, IBM 486DLC2, 486SL.

IBM even had little stickers included with the CPU daughtercards to put over the sub-model label.

What About Zero?

From Tim:
   What about 0 for the Type 0 and 0- 386DX-20 complexes in the Model 90s?

David replies:
   The 8590-402 was a '4' "Special Bid" unit that makes use of the sub-model middle digit CPU encoding for other uses - There is no '0' CPU as far as I know.

Another late night thought is the Intel 486DX3-75 and classic Pentium 100 MHz CPUs not on any factory PS/2 configuration. I believe IBM attempted to cover every Intel and IBM CPU from the 386SX to classic Pentium at all offered clock speeds. IBM was anal enough to have a separate encoding of whether the same IBM 486SLC2 CPU daughtercard was running at 40 MHz (from an 8556/8557 planar host) or 50 MHz (from a 9556/9557 planar host or 9533/9553 planar) internally.

I also thought about checking the CPU encoding for the PS/2 N45 ("PS/2 Note") sub-models since it uses the Intel 486SL-25 CPU (there is also an Intel 486SL-33)...

Further Reading

IBM PS/2 Assistant - Understanding PS/2 Model Numbers
"Numerological" - CSIPH Thread

Content created and/or collected by:
Louis F. Ohland, Peter H. Wendt, David L. Beem, William R. Walsh, Tatsuo Sunagawa, Tomáš Slavotínek, Jim Shorney, Tim N. Clarke, Kevin Bowling, and many others.

Ardent Tool of Capitalism is maintained by Tomáš Slavotínek.
Last update: 20 Jan 2024 - Changelog | About | Legal & Contact