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.

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