IBM 5150 maximum memory?

Eric Smith eric at
Thu Apr 24 12:39:23 CDT 2008

Jules wrote:
> Does anyone recall what the maximum memory was for an original IBM 5150 PC
> at launch time?

My vague recollection is somewhere around (but NOT equal to) 512KB, but
I could be wrong.  The memory expansion cards IBM sold at the time
weren't sufficient to get there, but third party cards would (and beyond).

> The way I recall it, IBM only offered 64KB expansion cards back in the day
> (256KB ones came later) and the 5150 would only take four of them (wasn't
> the fifth expansion slot wired differently or something)?

You could put five of them in, but then you had no video display and
no floppy controller.  Thus normally you didn't see more than three
of them in one machine.

> Or was there some kind of
> maximum limit dictated by the 5150's BIOS?

The BIOS would only test the amount of RAM specified by a DIP switch
on the planar (IBM-speak for "motherboard"), and the switch settings
didn't go to 640KB.

At least one online source claims that the switch settings did go
to 640KB.  Possibly that was only on the later revision of the
planar and BIOS?

The company I worked for at the time bought a PC on the day of release
(August 12, 1981), specifically to develop third-party peripherals and
software.  The company was one of the first to market with a "combo
card", but was never as successful with it as AST, Quadram, etc.


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