UNIX and PC mem betwx 640K-1MB

[John R. Levine]

In article <3900 at jato.Jpl.Nasa.Gov> kaleb at mars.UUCP (Kaleb Keithley) writes:
>In article <30344 at cup.portal.com> cliffhanger at cup.portal.com (Cliff C Heyer) writes:
>>Compared to the 680X0 machines, I found myself
>>thinking that UNIX on the 80X86 architecture 
>>might be a bit more complicated because the
>>ROM and VRAM located above 640K.

>Not only is VRAM and ROM in certain locations on 680x0, but I/O is memory
>mapped, compared to 80x86 which has a separate I/O bus.

All 386 unices page, which means that they treat RAM as a pool of 4K
pages, the real addresses of the pages being nearly irrelevant.  The hole
in the address space causes a little extra startup complication as the
kernel makes the initial free memory list, but as architectural warts go,
it's not a big one.  Incidentally, 86 machines have a separate I/O space
but not a separate I/O bus.  There's only one bus, with a line that
indicates whether an address should be treated as an I/O or memory address.

>>Also does UNIX on a PC use the BIOS?

It can't, the BIOS code runs in real mode and unices run in protected mode.
The PS/2 series has an ABIOS which runs in 286 protected mode, but I haven't
heard of anyone who uses it other than IBM OS/2.  Given that AT clones don't
have an ABIOS, any protected mode system has to be able to run without an
ABIOS, so most of them don't bother to use it even if it's there.

-- 
John R. Levine, Segue Software, POB 349, Cambridge MA 02238, +1 617 864 9650
johnl at esegue.segue.boston.ma.us, {ima|lotus|spdcc}!esegue!johnl
Marlon Brando and Doris Day were born on the same day.