Int 13h buffer 64k boundaries

Fred Cisin cisin at
Thu Apr 19 19:16:50 CDT 2018

>> 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.

On Thu, 19 Apr 2018, Chuck Guzis via cctalk wrote:
> 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.

Of course not.  But WHY didn't DOS programs, such as FORMAT, check whether 
their buffers were in usable places?   Not a common problem in DOS 1.0, 
but by about DOS 3, DOS was much less likely to be entirely in the bottom 

> 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.

"Wisdom comes from experience. Experience is often a result of lack of 
wisdom."    - Terry Pratchett

Although I wanted to know some, I was brought up with NO background in 
hardware nor electronics!
Is it OK to be envious?

My parents were dismayed when I left aerospace FORTRAN programming and 
went into auto repair ("I'll get back into computers when I can afford a 
tabletop computer of my own.  Less than 10 years.")  That started to turn 
around when I was successful, and started supplying them with all of their 
cars.  ("I bought this Karmann Ghia for a few hundred dollars, and did a 
lot of work on it.  I think that you will enjoy it.")

I drooled over S100, and bought the first TRS80 to show up at the store 
($400, since I had learned enough to be able to hook up a tape recorder 
and CCTV monitor).

> So, at the time, looking at the 5150, it was an overpriced primitive
> implementation using a 1970s CPU.

Even I could see that Segment:Offset was a kludge to get a MB of memory in 
a 64K machine.

> Many people at the time thought it
> would be less popular than the 5100.

Well, it certainly SOLD way more.  But, I doubt that I could barter it to 
John Titor for a one way ride back 55 years.

> 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".

I made a decision in August, 1981 to buy a 5150.
"It probably won't be as good as many others, but, being from IBM, within 
a decade, most computers will be copies of it, with only a niche market 
for anything else."
I was pleased that Apple survived.

Grumpy Ol' Fred     		cisin at

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