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From: anton@mips.complang.tuwien.ac.at (Anton Ertl)
Newsgroups: comp.arch
Subject: Re: the 286, Byte ordering
Date: Sun, 05 Jan 2025 17:51:34 GMT
Organization: Institut fuer Computersprachen, Technische Universitaet Wien
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jgd@cix.co.uk (John Dallman) writes:
>An idea: The target markets for the 8080 and 8085 were clearly embedded
>systems. The Z80 and 6502 rapidly became popular in the micro-computer
>market, but the 808[05] did not.

The 8080 was used in the first microcomputers, e.g., the 1974 Altair
8800 and the IMSAI 8080.  It was important for all the CP/M machines,
because the CP/M software (both the OS and the programs running on it)
were written to use the 8080 instruction set, not the Z80 instruction
set.  And CP/M was the professional microcomputer OS before the IBM PC
compatible market took off, despite the fact that the most popular
microcomputers of the time (such as the Apple II, TRS-80 ad PET) did
not use it; there was an add-on card for the Apple II with a Z80 for
running CP/M, though, which shows the importance of CP/M.

Anyway, while Zilog may have taken their sales, I very much believe
that Intel was aware of the general-purpose computing market, and the
iAPX432 clearly showed that they wanted to be dominant there.  It's an
irony of history that the 8086/8088 actually went where the action
was.

Intel released the MCS-51 (aka 8051) in 1980 for embedded systems, and
it's very successful there, and before that came the MCS-48 (8048) in
1976.

>Intel may still have been thinking in
>terms of embedded systems when designing the 80286.

I very much doubt that the segments and the 24 address bits were
designed for embedded systems.  The segments look more like an echo of
the iAPX432 than of anything designed for embedded systems.

The idea of some static allocation of memory for which segments might
work may come from old mainframe systems, where programs were (in my
impression) more static than PC programs and modern computing.  Even
in Unix programs, which were more dynamic than mainframe programs had
quite a bit of static allocation in the early days; this is reflected
in the paragraph in the GNU coding standards:

|Avoid arbitrary limits on the length or number of any data structure,
|including file names, lines, files, and symbols, by allocating all
|data structures dynamically. In most Unix utilities, "long lines are
|silently truncated". This is not acceptable in a GNU utility.

>The IBM PC was launched in August 1981 and around a year passed before it
>became clear that this machine was having a huge and lasting effect on
>the market. The 80286 was released on February 1st 1982, although it
>wasn't used much in PCs until the IBM PC/AT in August 1984.

The 80286 project was started in 1978, before any use of the 8086.
<https://timeline.intel.com/1978/kicking-off-the-80286> claims that
they "spent six months on field research into customers' needs alone";
Judging by the results, maybe the customers were clueless, or maybe
Intel asked the wrong questions.

>The 80386 sampled in 1985 and was mass-produced in 1986. That would seem
>to have been the first version of x86 where it was obvious at the start
>of design that use in general-purpose computers would be important.

Actually, reading the oral history of the 386, at the start the 386
project was just an unimportant followon of the 286, while the main
action was expected to be on the BiiN project (from which the i960
came).  Only sometime during that project the IBM PC market exploded
and the 386 became the most important project of the company.

But yes, they were very much aware of the needs of programmers in the
386 project, and would probably have done something with just paging
and no segments if they did not have the 8086 and 80286 legacy.

- anton
-- 
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>