no significant bugs in 80286 virtual memory

[John Plocher]

In article <326 at siswat.UUCP> buck at siswat.UUCP (A. Lester Buck) writes:
>There have been several postings in comp.unix.microport recently
>intimating that the "experts" know of a serious bug in the 80286.

I'm not an expert here, but this is what I've been told by those (both here
and elsewhere) who do know...

>	exception handling routine, the contents of the CX register may be
>	unreliable.  (Whether CX contents are changed is a function of bus

CX is used by many programs in REPeat loops, string copies, and the like.
Having CX not be reliable in exception handling routines makes it hard to
handle every case (trapping writes to screen is a major one, altho not
really related to VM).

>	[ seven types of protection violation listed that no operating
>	system should want to restart, like reading from execute-only
>	code, or direct I/O at the wrong priority ]

But the main problem is that since things are screwed up at this point
(and are screwed up differently on different versions of the chip), writing
an exception handler which is robust in the face of such "unknowns" is
-what would be a good word?- non-trivial :-)

>Is dynamic stack growth essential to Unix?

Not really, but having it makes life easier.

This is really moot, tho, because the 386 has none of these problems [but
it has a few of its own], and a 386 box onow costs the same as a 286 system
did a year ago.  See also the points made about the "fun" involved in paging
64K segments in previous postings.

>A. Lester Buck		...!uunet!nuchat!moray!siswat!buck

   -John (Not a 286 internals expert) Plocher