Dec Rainbow prehistory

From: Sean 'Captain Napalm' Conner <spc_at_conman.org>
Date: Wed Nov 19 18:24:38 2003

It was thus said that the Great Tom Jennings once stated:
>
> On Tue, 2003-11-18 at 22:29, Dave Mitton wrote:
>
> > >Some BOZO designer had, I assume arrogantly, put all the video
> > >interrupts (vert, horiz retrace, etc) starting at 00020h, which everyone
> > >knows is where MSDOS has it's software interrupts. They were in a PAL or
> > >something, and could not be moved.
> >
> > Well, I won't defend the quirks of the Rainbow... (more later)
> > But you're forgetting that the Rainbow 100A design pre-dates the IBM PC, it
> > was first designed as a CP/M machine. MS-DOS came later.
>
> My apologies to the engineer, I should not have insulted him/her even
> from afar, I have no idea what their criteria might have been, but I
> still maintain it was very short sighted. I assume it was decreed from
> 'above'.

  Why was it short sighted? Intel reserved all interrupts between 0 and 31,
leaving 32 (or 20h) to be the first interrupt available for programmer or
system use. Since the design was before MS-DOS, it doesn't seem all that
far fetched to start the hardware interrupt vectors at 20h. If anything,
the engineer was following the guidelines set by Intel; IBM, on the other
hand, ignored Intel.

> Whether MSDOS 'winning' (sic) the OS war of the time (vs. CP/M-86) was
> obvious or not, MSDOS was in no way ignorable. The 8088 card added to
> the Rainbow design post-dates MSDOS (and certainly post dates 86DOS,
> which MSDOS 1.25 is, bought from Seattle Computer Products (man I wish I
> had saved those 86DOS 0.86 and 1.x manuals!), which *someone* at DEC
> must have known about. And if they didn't they weren't paying attention.
> It's hard to imagine they chose interrupts 0020h - 002fh by accident;
> it's just Good Practice to avoid all possible conflicts, especially when
> the cost is essentially zero to do so.

  When was the Rainbow introduced? MS-DOS (PC-DOS) 1.0 wasn't introduced
until August of 1981, and it was in August or September of 1980 that
Microsoft licensed QDOS from Seattle Computer Products; I'm not sure when
QDOS was first introduced, but my guess is around 1979 or there-abouts.
Also, DEC was primarily a minicomputer manufacturer so it may not be that
far fetched that engineers there might not be terribly familiar with state
of the art (hah!) microcomputer products at the time.

  -spc (When will this stuff dislodge from my brain? 8-)
Received on Wed Nov 19 2003 - 18:24:38 GMT