D&I of 4.3 BSD

[Marlon Pearson]

Reading through "The Design and Implementation of the 4.3 BSD Unix*
Operating System" I understand most of it so far but I would
like to ask a couple questions (not all at once). 

On p. 80 it is talking about low-level context switching. It reads
"... To make switching user structures as efficient as possible on the
VAX, the system maps the physical memory associated with each process's
user structure to a fixed location at the top of the user-mode virtual
address space. (This location is also at a fixed virtual address in the
kernel's address space.) Since the user-mode virtual memory mapping is
defined by values maintained in a process's PCB, switching between 
processes, and between user structures, simply becomes a matter of swapping
PCBs..."

I am confused about a couple things. I am familiar with mapping virtual
addresses to physical but not the other way around. So the first sentence
has got me. The last sentence I understand but the parenthesized sentence
I just kind of ignore because I figure once I understand the first I will
get that one too.

A further explanation would be welcomed.
Thanks, Marlon

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Marlon of the Jungle
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