From: jerry@connection.prospect.com (Jerry Shekhel)
Subject: Re: PC (OS/2) / Amiga debate -- hopefully a rational debate, an I an ideali
Organization: Molecular Simulations, Inc.
Date: Sat, 12 Sep 1992 20:02:35 GMT
X-Posted-From: connection.prospect.com
X-Posted-Through: sol.ctr.columbia.edu

dawill@hubcap.clemson.edu (david williams) writes:
>>
>>If you knew anything about the way the 8086 and the 286 work, you'd realize
>>that nothing could have been done.
>
>   I am way too familiar with the 8086 and 286, regretfully.  AND
>SOMETHING COULD HAVE BEEN DONE.  Hell, Intel went to all the trouble
>to build the capability into the 286, and the gang at IBM had the perfect
>opportunity to make the AT an awesome machine (with some cooperation
>from Microsoft to make MS-DOS take advantage of large memory.)
>

There was simply no way.  The whole problem was with large-model programs
(ones with multiple data segments).  Large-model compilers generated code
which performed segment arithmetic -- they assumed a direct relationship
between the contents of segment registers and physical addresses.  Such
a relationship does not exist in Protected Mode, so large model DOS programs
simply could not run in Protected Mode.  Therefore, DOS had to remain
a Real-Mode operating system, which meant that the 1MB limit remained.
Another problem was that by the time the AT arrived, there were already too
many programs which wrote directly to video RAM instead of calling the BIOS
for display.  This meant that the video RAM had to stay where it was --
between 640KB and 1MB.

The lack of any sort of compatibility between Real and Protected Mode on the
286 is one of the reasons many called it a "brain-dead" CPU.

The 386, of course, solves these problems, and MS-DOS 7.0 is supposed to be
the first version with architectural changes, including a linear 32-bit
address space.  I'll bet that 32-bit DOS (7.x) + 32-bit Windows (4.x) will
be cheaper and sell better than NT.  BTW, all of these products are currently
under development, but I wouldn't be too surprised if Big Bill suddenly turned
around one of these days and said, "Just kidding!  You thought that was for
real?!".  He's pulled that one before.

>
>And don't give me the cop-out that 286 segment programming was too
>difficult.  PDP-xx people had been doing segmented addressing for years
>at this point.  The GIMIX 6809-based machines had support for 2M main
>memories (on a processor without support for large memories!  64K only)
>

286 (Protected Mode) programming was not too difficult; it was just
*impossible* to run DOS programs in Protected Mode, and only in
Protected Mode could the 286 address more than 1MB RAM.

>
>The newer machines with built-in CMOS setup were a godsend.
>

Yep.

>
>      Dave Williams
>
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| Jerry J. Shekhel  | Molecular Simulations | Time just fades the pages      |
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