Programs larger than real memory on an 80286 ???

[John Plocher]

In article <235 at extro.ucc.su.oz> glenn at extro.ucc.su.oz (G. Geers [ext 3241]) writes:
>I am curious to know why Xenix 2.1.3 sets a maximum user process size
>restriction (329 kb with 640 kb installed on an AT).
>Does microport also have this restriction ???
>In particular, 'compress' (16 bit) has a bss of about 400 kb so will this
>run on either microport or xenix on an 80286 machine with only 640 kb of
>real memory ?

Microport Unix V/AT requires that the complete image of a process be in
memory while it is being executed.  This is because the 286 does not 
support demand paging like the 386 does.  On a 640K system, the kernel
uses most (if not all :-) of the memory in the system and leaves very
little for user programs.  The quick and easy solution is to add more memory
(compress runs with 1Mb albeit very slowly) - Microport recomends a minimum
of 1Mb, with 1.5 to 2Mb giving even better performance.  Sure, Unix runs in
640K, but when `size /unix` gives you a number near 250K and there are 50K
of disk buffers, that leaves you with about 340K of user memory.
This means that you *can* run unix, and quite a few programs, but vi, compress,
and other "big" programs just won't fit.

Compress, when compiled for 12 or 13 bits max, is smaller and may run in a
640K machine.  But in any case I'd recomend adding at least 512K, if not 1Mb
of memory to your machine.

   -John Plocher