segments and Unix

[mwm at eris.UUCP]

Just a few quick observations on segments:

1) Segments are not a new thing - Burroughs has been selling them on
their large systems for over a decade now (two decades, maybe?).

2) The thing that everybody who works with unix should think of when
the word "segments" comes up (after eighty-eighty sux, of course :-)
is "Multics," followed by "slow." But Multics tried to support far
more than is being discussed here.

3) Segments are a good thing, but only if you've got enough to be
usefull (enough to store arrays as illith vectors), and each one is
big enough to be ditto. "Enough" varies with time, of course.

4) You don't have to have a time overhead for having segments. After
all, a VAX has segments already.

5) You don't have to have broken pointer semantics if you use
segments. Scattering them around in a large, sparse address space
works fine.

6) 32 bits isn't a big enough address space. You can (maybe) make
something usefull out of it, but it probably won't be usefull in a few
years.

7) The memory cost for segments should be small, and may be zero,
depending on what kind of architechture you're trying to cram them
into.

8) Segments are coming to Unix. See either MACH or the Karels&McKusick
paper on the new BSD virtual memory system.

	<mike