From: dawill@hubcap.clemson.edu (david williams)
Subject: Re: PC (OS/2) / Amiga debate -- hopefully a rational debate, an I an ideali
Organization: Clemson University
Date: Fri, 11 Sep 1992 21:30:26 GMT

jerry@connection.prospect.com (Jerry Shekhel) writes:

>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.

   The data books I have indicate the 286 reset address is at FFFFF0 -
clearly a 24 bit address.  I *know* the chip comes up in real mode, 
and this mode is limited to 1M.  There's something a hair shy of
vertical here.  I guess I need to do some more research on this topic,
but I still stand by my claim.

>>
>>   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.

   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.)

   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)

>>
>>  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.

   No, I'd rather have an intelligent autoconfig like the amiga has.
Have you ever used a real AT CMOS?  You don't get to hit some keys at
boot time to get a nice setup program - You had to run a SETUP.EXE
program off of a disk that came with the motherboard.  Suppose you had
a machine, and the batteries for CMOS memory dies. (As they were wont
to do)  You had to go dig up the disk, and run the CMOS program again.
Whats that?  Lost your disk?  TOO BAD, you loose.  
   The newer machines with built-in CMOS setup were a godsend.

>| Jerry J. Shekhel  | 
-- 
      Dave Williams
        dawill@hubcap.clemson.edu
           "Huh?  What?  Could you repeat the question?"

Newsgroups: comp.sys.amiga.advocacy