Int 13h buffer 64k boundaries

Chuck Guzis cclist at
Thu Apr 19 18:16:21 CDT 2018

On 04/19/2018 12:14 PM, Fred Cisin via cctalk wrote:

> I have no difficulty admitting that I didn't, and don't, have
> Chuck's level of experience and knowledge. My entire venture into
> microcomputers was a hobby that got out of hand.
It's not so much expertise, but where you start your investigations.

Right when I peered into the 5150, I saw the 8237 DMA controller (first
cousin to the 8257) and recognized it from my 8-bit (8085) days.  It was
immediately obvious that IBM had taken a bunch of legacy 8 bit
peripheral chips and shoved them into the PC.   In fact, the 5150 was
surprising in that how primitive the engineering was--something you
didn't expect from a high-tech pioneer like IBM.  So the DMA address
space had to be 16 bits with simple bank select--using a disk controller
chip that was design to be used with 8 inch drives.

The Technical Reference BIOS listing confirmed the suspicion that the
5150 implementation couldn't cross 64K banks.  It had nothing to do with
DOS, per se.

At the same time the PC debuted, we were working with early steppings of
the 80186, which did feature two channels of 20-bit address DMA--and 16
bit bus width to boot.

So, at the time, looking at the 5150, it was an overpriced primitive
implementation using a 1970s CPU.   Many people at the time thought it
would be less popular than the 5100.

Rather than buy my first 5150, I was strongly drawn to the NEC APC. For
about the same price as an outfitted 5150, you could buy a true 16 bit
box with 8" disk drives and really nice graphics that was built like a
battleship.  The only problem is that nobody had ever heard of it.

But IBM had the golden reputation.  Many people at the time,
particularly the older ones, didn't talk about "computers" so much as
"IBM machines".


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