Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!brutus.cs.uiuc.edu!ux1.cso.uiuc.edu!aries!mcdonald
From: mcdonald@aries.scs.uiuc.edu (Doug McDonald)
Newsgroups: comp.sys.ibm.pc
Subject: Re: re 640K limit
Message-ID: <1990Jan24.174315.13698@ux1.cso.uiuc.edu>
Date: 24 Jan 90 17:43:15 GMT
References: <4668.25aed7f2@uwovax.uwo.ca> <21990004@hpvcfs1.HP.COM>
Sender: news@ux1.cso.uiuc.edu (News)
Organization: Univ. of Illinois at Urbana-Champaign, Chemical Sciences
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In article <21990004@hpvcfs1.HP.COM> johne@hpvcfs1.HP.COM (John Eaton) writes:
>In order to understand some of the limitations of the PC you must remember
>how the 8086 came to be. Intel was on a roll with the 4004/8008/8080 and
>decided to go all out and design a micro that really had some POWER. It
>was to be something that could challenge the minis and become the chip
>of the 80's. I believe it was originally called the "432". But they still
>needed to keep business coming in the door until this wonder chip was ready
>so they decided to do a quick and dirty enhancement of the 8080. This was the
>8086. It's only purpose was to keep the 8080 family alive until Intel could
>deliver its real processor. Its hard to fault the designers for any design
>decisions that were appropriate for it's expected lifespan.
>
Yes, but why did they have to botch it? One tiny, simple change
in it, that would not increase (it probably would DECREASE) the
gate count, and it would have been MUCH better: make the segment
registers a simple extension of the address - that is, the
8086 would have used only 4 bits of the segment registers,
but the 80286 would use 8 and the 80386 all 16. 

A second change to the 8086, that would have added two instructions, 
would have been to add "add carry to segment register" and
"subtract carry from segment register" instructions.

IT would still get flamed for segments, but at least the 80286 
would have been able to run 8086 code directly.

Doug McDonald