IBM 5150 maximum memory?

Eric Smith eric at brouhaha.com
Thu Apr 24 20:25:46 CDT 2008


Jules wrote:
> 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?)

Sure, but the MC68000 was *much* more expensive than an 8088 at that
time, and required fewer support chips, because Intel had a bunch
of peripheral chips that worked almost gluelessly on the 8088.

I'm not sure whether the MC68008 was available then, but with the
MC68000 the minimum memory configuration would have been 32KB rather
than 16KB, and memory would have had to be expanded in increments of
18 chips.

Most significantly, though was that IBM had already done 8086 designs,
and the PC design more or less leveraged what they already had.  I'm
sure they would have liked to have put in a better processor, but
since the PC was just supposed to be a quick-and-dirty hack to show
that they could build a microcomputer, I don't think they were all
that concerned about whether the decisions they made at the time
would have unfortunate consequences later on (e.g., the stupid
edge-triggered interrupts, where anyone with the slightest bit of
experience would have known to use negative level triggered interrupts).
They clearly didn't expect that the product they were designing for
release in 1981 was going to set the standard for the most popular
computer architecture on the planet for not just the next 27 years
but for the forseeable future beyond that.

Fortunately over time we have shed a lot of the legacy baggage,
including the aforementioned interrupt stupidity.  The x86 processor
has even become more friendly to programmers, in the form of the
32-bit flat mode addressing of the 386 and later, and even more so
in "long mode" on the x86-64.

Eric




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