This is an extension of an answer I posted years ago to the Retrocomputing question:

How did Apple fail to tap the business and scientific markets?

Most (all?) IBM PC systems required parity memory, but most early Macintosh models did not. The Macintosh IIci was the first to finally meet this specification.

After further research on the matter, I have found that Apple used to produce a document called the "Memory Guide" for early Macintosh products, updated every two months. The November 2000 version of the guide can be found here:


On page 14 it says:

There are two logic boards for the Macintosh IIci, one with and one without parity. You can identify the Parity logic board by the large parity IC installed to the left of the SIMM slots. If you have a parity logic board (identified by the presence of the parity chip at the location noted), be sure to install parity SIMMs for proper operation.

However the actual motherboard / logic board part numbers are not listed.

  • I don’t remember PC motherboards requiring parity RAM (especially since RAM was expensive, and each parity bit increased memory costs by 12.5%, or more).
    – RonJohn
    Commented Jul 4, 2023 at 4:52
  • Server and workstation mobos usually speced parity RAM, but bog standard PCs and clones certainly weren’t servers or workstations.
    – RonJohn
    Commented Jul 4, 2023 at 4:54

1 Answer 1

  • 661-0532 Logic Board Mac IIci
  • 661-0583 Logic Board Mac IIci Parity

from myoldmac.net - Apple Computer Model Part Numbers, confirmed in Apple's own Apple Module Identification, June 1994, pages xx and xxi.

I can't find any pictures of the 0583 Parity board, but the image at Macintosh IIci Logic Board – Apple Rescue of Denver has a large unpopulated position for a surface-mount chip just above the memory slots.

Parity was a special order option on the IIci, but it is not clear what the special application for parity RAM was intended to be (see Macintosh® IIci Special Options and Technical Information manual). It may have been intended for server operations, as the IIci could also be ordered with a locking power switch to prevent accidental power-down.

The IIfx was the first 68k Mac to have parity RAM as standard.

  • Were the IIci memory boards interchangeable on the motherboard, i.e. whether or not you had the parity board the machine would function the same? Did it require support from the OS to detect parity errors or was it self-contained in the memory board?
    – bjb
    Commented Jul 3, 2023 at 17:01
  • (as far as I can tell, I'm only working from pictures and old internet references) i) No, they used completely different SIMMs (the motherboards look to be physically compatible with the same IIci enclosure); ii) The (large) parity chip did all the work, though what it did (debug mode/halted? reset?) with bad parity, I don't know. Apparently (from Special Options booklet) Macs without parity controller produced mark parity (that is, always set) on the bus
    – scruss
    Commented Jul 3, 2023 at 20:18

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