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: Fri, 11 Sep 1992 20:47:02 GMT
X-Posted-From: connection.prospect.com
X-Posted-Through: sol.ctr.columbia.edu

dawill@hubcap.clemson.edu (david williams) writes:
>
>[The IBM PC/AT]
>  
>    First off:  the memory structure.  XT's only had 20 address bits 
>to play with (8088/86), so the 640K user 384K bios/expansion area made
>sense.  Along comes the 286 with 24 address lines (16Meg memory space)
>and what does IBM do?  They keep the ROM area in exactly the same space
>that the XT had it, namely at the top of the FIRST MEG.  What a wreck.
>It should be at the top of the 16th Meg in memory, like any intelligent
>person would put it, which would leave the rest of the memory open for
>other things, and not the screwed-up EMS memory like they have now.
>

I believe that you are wrong about this.  The 286 can operate in two modes,
Real and Protected.  In Real Mode, the CPU can still access only 1MB RAM,
so the ROMs and memory mapped hardware had to stay where they were.  In
Protected Mode, all addresses that programs generate are virtual, so it
doesn't really matter where this area physically resides.  Furthermore, even
if there were some way to move the area out to the top of the 16MB range,
Real Mode would still be limited to 1MB, a barrier which would soon become
just as restrictive as the 640KB, and EMS/XMS would have been created anyway.

>
>   I can hear you guys yelling in the background for the fact that 
>the AT ran as a virtual 8086, and can only address the first meg in that
>mode.  Geez, the stupid 286 has all sorts of stuff to help get around
>that problem.  They couldn't re-write the OS to take advantage?  Didn't
>IBM release a new version of DOS for the AT introduction anyway (1.2M
>disk support?)  around this time?  Oh yeah, and take a look at how the
>AT handles the A20 line for memory!  The keyboard controller handles 
>memory accesses?!?!?!  Give me a break.
>

If you knew anything about the way the 8086 and the 286 work, you'd realize
that nothing could have been done.

>
>  IMHO, the only thing right about the AT was the 16-bit bus, which
>should have been introduced in the origianal XT anyway (if it had used
>a 8086 that is.)  Everything else - CMOS, drive types, and so on - was
>a big mistake.
>

Why?  Would you rather have to set DIP switches on the motherboard for
configuration?  What's wrong with CMOS RAM?  Drive types I'll agree with.

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