Uses for IPF/32b PPC style segments?

One obvious use is to reserve a Region Register (IPF) for kernel
memory
space (providing a benefit similar to x86's global bit). It would
probably be
too wasteful to use one for Thread Local Storage (the only advantage
being that fixed addresses could be used, if such can be counted an
advantage); with 16 PPC segment registers it might not be that bad.

Shared libraries could perhaps also be allocated a region/segment
(again
allowing TLB entry sharing across otherwise independent address
spaces).

(IPF also ties base page size [which is the only page size for linear
page tables] to the Region Register, so they are useful for providing
memory areas with different page sizes.)

Given the cost of changing a segment/region register (a syscall) and
the
small number of active segments (16)/regions (8), such does not seem
especially attractive for a Single Address Space OS.

Of course, one does not have to use unique regions (28b addresses
might be a bit limiting even for a 32b system, but 61b addresses
should
be large enough for many years), but the slight reduction in context
switch overhead does not seem like a significant benefit.


Paul A. Clayton
just a technophile

.

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