Re: 80286 protected mode

mitchalsup@aol.com (MitchAlsup1) writes:

Is protected mode not "how Pascal" thinks of memory and objects
in memory ??

You can map the object of standard C (essentially what one malloc()
call gives you, or what a variable contains) and the equivalent of
Pascal into a segment on the 286, but then that object is limited to
64KB in size, and the program is limited to 8192 objects.  And the
program runs very slowly.  So I doubt that Pascal compiler
implementors though that the 80286 is their dream machine.  Especially
given that you can implement Pascal just as well with less performance
disadvantages on hardware with flat memory (and still perform bounds
checking where necessary).  In particular, I doubt that Turbo Pascal
used the 286 in this way.

Whereas by the time 286 got out, everybody was wanting flat
memory ala C.

It's interesting that, when C was standardized, the segmentation found
its way into it by disallowing subtracting and comparing between
addresses in different objects.  This disallows performing certain
forms of induction variable elimination by hand.  So while flat memory
is C culture so much that you write "flat memory ala C", the
standardized subset of C (what standard C fanatics claim is the only
meaning of "C") actually specifies a segmented memory model.

An interesting case is the Forth standard.  It specifies "contiguous
regions", which correspond to objects in C, but in Forth each address
is a cell and can be added, subtracted, compared, etc. irrespective of
where it came from.  So Forth really has a flat-memory model.  It has
had that since its origins in the 1970s.  Some of the 8086
implementations had some extensions for dealing with more than 64KB,
but they were never standardized and are now mostly forgotten.

- anton
--
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>