IBM 5150 maximum memory?

Michael B. Brutman mbbrutman-cctalk at
Fri Apr 25 21:56:21 CDT 2008

I'm kind of surprised that this hasn't come up yet.

The original BIOS and the 2nd revision BIOS (Oct 1981) both had a cute 
little bug in them where they would only count up to 544K of installed 
memory.  You might have 640K and have all of the switches set correctly, 
but the BIOS bug caused the count to stop at 544KB.

This little bug burned me for a few days.  It's documented in Computes! 
Mapping the PC and PCjr.


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