sdm7g@galen.med.Virginia.EDU (Steven D. Majewski) (04/10/91)
In summary:
There are at least two kinds of segments (or segmented addressing):
A) SEGMENT or BASE REGISTER + OFFSET addressing.
Opinion seems to be: all things being equal, this is a *BAD THING*.
B) SEGMENTS as in a (named) address space, possibly with Mem.Mgmt.
protection and maybe even capabilities.Generally seen as a *GOOD
THING* .
Observations:
1) You can have A .and. .not. B ( e.g. Intel 8086 )
2) You can have B .and. .not. A ( e.g. VAX - implemented as part of
the MMU - not the CPU )
( ...or BOTH or NONE obviously ... )
3) ALL THINGS ARE NOY EQUAL:
Two major constraints are compatibility and SILICON|$$|(some finite
resource).
IF you are starting a design from scratch and plan to turn it into
silicon around '93, then 64 bits is the way to go. IF TODAY what
you have to (affordably) choose from is addressing 2^32 bytes TOTAL
or addressing 256 segments of 2^32 bytes, who doesn't want the
extra space ?
IF you believe in the RISC design philospohy of: follow the 80%/20%
rule & let the compiler (writer) worry about "easy to program" - let's
make that thing FAST, THEN IF segmented memory organization gives a
performance edge, then it must be a GOOD THING.
( I don't know if that is the case, but that hasn't been what people
have been arguing about here ! )
4) B ( above ) should PROBABLY be done as part of memory managemant,
however past history has shown that method to be often non-standard
even within the same CPU arch.and not easily programmed. IN FACT IT
IS USUALLY *NOT* PROGRAMMED AT ALL - which is why programmers like
a linear address model - the don't have to program it. But when we
start to want an Object Oriented OS with Capabilities & persistant
objects, we will not only want that 64-bit address space - we will
want some way to program it. And we will probably want to program
access control on some level finer that the process. I'm not sure
how to do that with only a MMU and not some sort of segmentation
that restricts what is addressable.
5) But otherwise, I hate segments (as in A) TOO, so don't flame me!
--
========= "If you've got a hammer, find a nail!" - George Bush =========
Steven D. Majewski University of Virginia Physiology Dept.
sdm7g@Virginia.EDU Box 449 Health Sciences Center
(804)-982-0831 Charlottesville, VA 22908