Very Computer

Hi, In 4.3BSD Unix, there is a data structure called Usrptmap, which
is in addition to the user page tables. I don't quite understand the
purpose of this data structure. Why isn't it sufficient to allocate
only User page tables in kernel virtual address space using a resource
map?

Any enlightenment on this is greatly appreciated.

David

Department of Computer Sciences
University of Texas at Austin

Usrptmap[] is the page table that maps the user page tables.  This
first-level page table must be in physically contiguous memory (per
the VAX architecture).  The easiest way to do that was to make it
static duration.  The kernel memory allocators do not arrange for
contiguous physical pages.
--
In-Real-Life: Chris Torek, Lawrence Berkeley Lab CSE/EE (+1 510 486 5427)

Bill Jolitz (yes, that Bill Jolitz :-) ) pointed out that this is
wrong:  the VAX hardware looks up user page tables in kernel virtual
space.  It is only the Sysmap[] PTEs that must be in physically
contiguous space.

He added that:

Quote:>In the Univ. Utah emulation of the "old" VM, it was easier to arrange
>things with consecutive physical, which might be where the confusion
>comes!

(I have never dealt with the old Utah code myself, so I think I just
must have confused the VAX page lookup with other multilevel PTE
schemes.  This kind of hardware lookup eventually runs out of levels
and has to fall back on some simple scheme, usually `physical base
address plus offset'.  There are other methods, such as Sun's weird
Sun-3 and SPARCstation MMUs and MIPS and HP-PA software translation
schemes, which give you a different set of drawbacks. :-) .)
--
In-Real-Life: Chris Torek, Lawrence Berkeley Lab CSE/EE (+1 510 486 5427)