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From: dyer@spdcc.COM (Steve Dyer)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Large linear memory space for 80x86 machines ?
Message-ID: <327@spdcc.COM>
Date: Tue, 20-Oct-87 01:10:56 EDT
Article-I.D.: spdcc.327
Posted: Tue Oct 20 01:10:56 1987
Date-Received: Wed, 21-Oct-87 05:59:36 EDT
References: <668@rna.UUCP>
Organization: S.P. Dyer Computer Consulting, Cambridge MA
Lines: 29
Keywords: XENIX 386

In article <668@rna.UUCP>, dan@rna.UUCP (Dan Ts'o) writes:
> 	What is the current state of large data array usage on 80x86 machines ?
> I want to write a (semi-real-time) C program that uses single data arrays that
> are 2-8Mb each. What about...
> 	- Running Xenix with protecte mode, possibility on a '386. Do any
> current or planned implementation of Xenix support such large linear address
> spaces ? How about virtual memory support ?

XENIX 386 is a true virtual memory OS and it supports the 32-bit linear
addressing of the 386 chip; the C compiler generates "small model" 32-bit code
without any overt manipulation of segment registers.  I don't remember what
the absolute maximum user process size is right now (stemming from their
virtual memory design), but the more immediate limits are a function of
the amount of physical memory you have and the size of the swap space you
have allocated.  I believe that the heuristic is that a process can occupy
all of physical memory (less the kernel) plus some percentage of the total
swap space, where both of these are parameters that can be tweaked to
customize an installation to its workload characteristics.

On my machine, an AT with an Intel Inboard 386, the system boots with the
message: "maximum user process size = 7771K", this with 5888K of physical
memory, of which 4844K is available for user processes and a 10mb swap
area.  I haven't made any attempt to optimize process size or reduce my
use of swap, although it certainly could be done with the instructions from
SCO.  I suspect you'll have no problem with your application under XENIX 386.
-- 
Steve Dyer
dyer@harvard.harvard.edu
dyer@spdcc.COM aka {ihnp4,harvard,linus,ima,bbn,m2c}!spdcc!dyer