7 May
2018
7 May
'18
5:36 p.m.
From: Johnny Billquist
My point being that ... pages are invisible to the
process segments are
very visible. And here we talk from a hardware point of view.
So you're saying 'segmentation means instructions explicitly include segment
numbers, and the address space is a two-dimensional array', or 'segmentation
means pointers explicitly include segment numbers', or something like that?
I seem to recall machines where that wasn't so, but I don't have the time to
look for them. Maybe the IBM System 38? The two 'spaces' in the KA10/KI10,
although a degenerate case (fewer even than the PDP-11) are one example.
I'm more interested in the semantics that are provided, not bits in
instructions.
It's true that with a large address space, one can sort of simulate
segmentation. To me, machines which explicitly have segment numbers in
instructions/pointers are one end of a spectrum of 'segmented machines', but
that's not a strict requirement. I'm more concerned about how they are used,
what the system/user gets.
Similarly for paging - fixed sizes (or a small number of sizes) are part of
the definition, but I'm more interested in how it's used - for demand loading,
and to simplify main memory allocation purposes, etc.
> the semantics available for - and _visible_ to -
the user are
> constrained by the mechanisms of the underlying hardware.
That is not the same thing as being visible.
It doesn't meet the definition above ('segment numbers in
instructions/pointers'), no. But I don't accept that definition.
All of this is so similar to mmap() that we could in
fact be having this
exact discussion based on mmap() instead .. I don't see you claiming
that every machine use a segmented model
mmap() (and similar file->address space mapping mechanisms, which a bunch of
OS's have supported - TENEX/TOP-20, ITS, etc) are interesting, but to me,
orthagonal - although it clearly needs support from memory management hardware.
And one can add 'sharing memory between two processes' here, too; very similar
_mechanisms_ to mmap(), but different goals. (Although I suppose two processes
could map the same area of a file, and that would give them IPC mapping.)
Noel