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From: anton@mips.complang.tuwien.ac.at (Anton Ertl)
Newsgroups: comp.arch
Subject: Re: Why I've Dropped In
Date: Sat, 14 Jun 2025 15:53:52 GMT
Organization: Institut fuer Computersprachen, Technische Universitaet Wien
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anton@mips.complang.tuwien.ac.at (Anton Ertl) writes:
>Stephen Fuld <sfuld@alumni.cmu.edu.invalid> writes:
>For the 8086 architecture, the effective addresses are reg, reg+const
>or reg+reg (with severe restrictions on the registers usable for that;
>the 8086 does not have GPRs).

There is also absolute addressing on the 8086, and IA-32 (that
encoding was repurposed for RIP-relative addressing on AMD64 IIRC).

And I completely ignored the segment registers, which were intended to
to provide a virtual-memory/relocation mechanism, but in MS-DOS
programs were used as a cumbersome way to access more than 64KB of
memory.  Only in stuff like PC/IX was it used as intended AFAICT.

The tragedy continued with the 80286, which now supported protected
segments with up to 64KB, well suited for the kind of usage in PC/IX
(but apparently that never had a 286 port) and actually used in Xenix,
but which was against the grain for the kind of usage in MS-DOS.

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