IBM 5150 maximum memory?

Michael B. Brutman mbbrutman-cctalk at
Sun Apr 27 10:11:19 CDT 2008

Chuck Guzis wrote:
>> Date: Sat, 26 Apr 2008 23:15:41 -0500
>> From: Jim Leonard 
>> I disagree somewhat, if only to present the evidence of the PCjr.  Now
>> *that* was clearly meant as a home computer (cartridge slots, wireless
>> keyboard, small and light, built-in composite video)
> I use the introduction of the Peanut as proof that IBM was still 
> suffering from the shock of having customers flock to buy the PC.  As 
> they discovered, even the unwashed public had its limits as to what 
> could be passed off as an IBM product.  Or maybe it was just the 
> result of a drunken bet.  You forgot the complete lack of DMA and, 
> shall we say, "anemic" interrupt handling facility?
> How about the 1990's IBM "multimedia" computer system?  Use it as a 
> phone answering machine, fax machine, CD player, etc.  The only 
> problem being that you have to leave it on all the time.  Toshiba 
> made a similar blunder at about the same time with its Infinia 
> machines--only to abandon the desktop business altogether.
> Cheers,
> Chuck

You guys keep using the term IBM as the designer and manufacturer of the 
PC.  The stories I've heard was that it was more of a skunkworks project 
in a little division in a swamp in Florida.

The people designing the PCjr had a few competing interests:

- Inexpensive and less 'industrial' for the home market.
- Compete on the low end with the C64, in the middle with the Apple ][, 
and avoid competing on the high end with the PC
- Differentiate it from the PC clones

So they came up with a machine that had a base memory of 64K, was 
expandable to 128K, better graphics than most machines at the times, 
cartridges for the pre-K set, and a simplified keyboard.  Oh, and it ran 
most PC software.

If you judge it against the PC of course it falls short - it's not a PC! 
  Different design goals by much of the same group that designed the PC. 
  You could tell they were conflicted, or very forward thinking.  For a 
machine that only went to 128K, the standard BIOS knew to scan to 640K. 
  For a machine that only supported one parallel port and one diskette 
drive, the BIOS knows to look for 2 parallel ports and it has support in 
BIOS for three diskette drives.

The sidecar bus was a conflict of goals too - they were trying to make 
expansion 'easier' than on a PC, but still preserve expansion.  A lot of 
what the machine needed was already built-in or accomodated with special 
purpose slots on the motherboard.  In just the cabinet with no sidecars 
I can have 128K of memory, better than CGA graphics, an internal modem, 
serial port, ports for two joysticks, diskette drive and controller.  In 
1984 that was a pretty complete machine, except for the parallel port.

Things like dropping DMA, using NMI for keyboard decoding and having the 
video memory shared with main memory piss off engineers, not customers. 
  The single biggest complaint about the PCjr at the time was the 
keyboard, not the lack of DMA.

24 years later, with the exception of a few games there is very little 
software for the PC that I want to run on a Jr that doesn't run.  It's a 
pain to get extra memory on it, but once you clear that hurdle it was a 
very capable little machine.

(Nevermind that mine has clock/cal, SCSI hard disk, Ethernet, etc.)


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