IBM 5150 maximum memory?

Jules Richardson jules.richardson99 at
Thu Apr 24 17:07:07 CDT 2008

Chuck Guzis wrote:
> Date: Thu, 24 Apr 2008 13:45:10 -0500
> From: Jules Richardson 
>> Indeed - in the context of the discussion I got involved in (which was
>> actually about memory prices, not the PC specifically), we were just
>> interested in what could be done with a 5150 *when it was new* - and I
>> think all that IBM offered then was the 64K boards (and of course third
>> parties didn't exist!)
> Yup, but vendors like Quadram and Everex came along pretty quickly.  
> Lots of folks realized that the 64K limitation was a huge one.

Definitely - I assume IBM's thinking was the memory expansion boards were the 
way forward (rather than thinking '64KB is plenty' - which would have plainly 
been stupid).  And why put more memory on the system board when you could sell 
customers memory expansion boards as an option? :-)

>> You know, I had a thought - I wonder if those 64K boards can't be
>> jumpered beyond the 256KB boundary? Maybe that's why I'm remembering a
>> 256KB limit on the original machines (and using original IBM expansion
>> boards). Getting around that would mean physically hacking the address
>> lines/decoding of the boards...
> There were also some hacks, since 64K DRAMs were available when the 
> 5150 was launched (why IBM didn't design the planar with jumpers to 
> select memory type is beyond me). 

Why they didn't do a lot of things when it came to the PC's design is beyond 
me (even down to the choice of CPU - wasn't the m68k generally available by 
the 1981 launch of the 5150?)

Some of the choices seem to have been typical revenue maximisation, but there 
appears to have been some real boneheaded design decisions in there too (even 
in a "the PC was supposed to just be an intelligent terminal" context, let 
alone as a standalone desktop machine)

> If you were handy with a soldering 
> iron and an Xacto knife, you could cut-and-jumper your way to 256K 
> planar memory.  The big pain was the soldered-in first row of 16K 

Y'know, I did have a document describing such a trick - I just had a look and 
have absolutely no idea where I've filed it, though :(

> I was happy to retire my 5150 and get a genuine Taiwanese clone mobo 
> with 256K and 8 slots.

Yeah, I acquired a clone from Comcen somewhere along the line, and that was 
more reasonable in terms of abilities - but in terms of vintage computer 
enthusiasm (rather than serious development machine) I'd still like to get 
hold of another genuine 5160 I think.



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