I have a whole bag of 30pin SIMMS so I thought I would see about upgrading the 1 MB of RAM in my Mac SE FDHD to 4 MB.

The original RAM had four sticks of OKI M41256A. These SIMMs have 8 chips each.

From my stash of RAM, I found a bunch of Panasonic MN4A091MOS80 - 1MX9 SIMMs.

These have 9 chips each. Which I THINK that are parity RAM SIMMs.

Also, I switched the jumper on the motherboard to "2/4MB" when previously it was on "1 MB".

When I boot up the system, the Finder shows 2048K. So, only half of what I was suspecting.

My system OS is 6.0.8 and the Finder is 6.1.8 FWIW.

I'm thinking one or both SIMMs are bad in either bank A or bank B.

It's a real pain pulling that board in/out to swap out RAM.

So, before I keep randomly trying many combinations, does anyone know if the OS even supports more than 2 MB?

Do you think the parity RAM could be causing an issue?


  • 2
    System 6 should support up to 8MB RAM. When more than that is plugged in, the Mac will not be able to use it, but still show it in the "About Finder" apple menu.
    – tofro
    Dec 8, 2016 at 23:00
  • 1
    Thanks. I just figure out what it was. I thought there were only two states "1 MB" or "2/4 MB". But there are three states with that jumper. "1 MB", "2 MB" and "4 MB".
    – cbmeeks
    Dec 8, 2016 at 23:01
  • 1
    Other machines (SE/30) took the same System 6.0.8 and supported 32 MB of installed Apple memory . I've seen 128 MB (with the rare 16 MB SIMMs) although there was never Apple branded RAM for that upgrade.
    – Whit3rd
    Dec 20, 2016 at 6:25

1 Answer 1


AH! I found out what it was. I'm stupid.

Apparently, I'm supposed to remove the jumper for the 4 MB config.


I hope this question helps someone else out.

  • The resistor configuration applies to the Mac Plus as well. I had to clip the resistor to take the computer from 1 MB to 2.5 MB. Sep 19, 2017 at 19:52
  • The link no longer works :( Sep 28, 2022 at 1:58

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