Programs larger than real memory on an 80286 ???

[Jim O'Connor]

In article <9391 at ico.ISC.COM>, rcd at ico.ISC.COM (Dick Dunn) writes:
: In response to a question about requiring the whole process to be in memory
: on a 286, John Plocher wrote:
: > 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...
: 
: Jim O'Connor responded, from the 80286 OS Writer's Guide:
: > "The memory legitimately addressed by the tasks running on an 80286 (the
: > [italics] virtual memory) may exceed the actual memory available...
: and then claims:
: > The rest of the chapter goes on to explain, in quite a bit of detail, how to
: > support demand paging.  I believe your statement above to be incorrect.
: 
: Mr. O'Connor finished with the put-down:
: > In the future, it would be appreciated if you had all your facts straight when
: > justifying design decisions.  Of course, if the data published by Intel is
: > incorrect, the pie is on my face instead. :-)
: 
: The data published by intel is not (in this case:-) incorrect, but the egg
: is certainly on your face.  You owe John Plocher an apology, and YOU need
: to have YOUR facts straight before you flame.  The 286 provides limited
: support for virtual memory, but no support for paging.  "Virtual memory"
: is *not* the same as "demand paging".

First of all, the statement above was not meant as a put-down.  I worked with
that article with the intent of NOT making it sound like a flame.

Mr. Dunn has a point, however.  In my analysis of uport's design decision, I
mixed the terms "virtual memory" and "demand paging", which I am aware are
not the same thing, and as such, was not correct in my own statements.
The 80286 does not directly support "demand paging", which is the easiest way
to provide "virtual memory".  However, my original intent was to offer evidence
that the 80286 *does not require a process to be completely in RAM* in order to
execute. My reading of the original article lead to me believe that Mr. Plocher believed that it did.  For not clearly stating this intent, I do apologize.

However, unless the hardware bug several people mentioned (which inhibits
restartable instructions) has still not been fixed, I believe that some form of
virtual memory can be had with the 80286.  As several people have pointed out,
it would probably not be worth the effort, but it cannot be said that it is
impossible.
---
James B. O'Connor		+1 615 821 4090 x651
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