You've got to be pulling my chain... (Ethernet)

From: Allison J Parent <allisonp_at_world.std.com>
Date: Sat Apr 11 13:58:57 1998

< Alright, so what we have is the last 4KW used up for stack
< space, register mapping, and IO mapping. I would guess the

and also boot proms. Default boot on PDP11 is 173000Q. The first
page is where all the vectors for the interrupts and traps are.

< MMU... do I have it right? Even on a machine with a full
< 22bits of address lines on the backplane, like an 11/73, the
< CPU still only has a 16bit address space. The faq doesn't make
< it terribly clear what happens if you want to open up any
< arbitrary window.

Think of the 4mb (q22) as linear physical space. At any time the CPU/MMU
can allocate contigious or scattered blocks of that space as logical
physical space. To do a "long jump" a local jump to a system space is
done, memory management code is run and then a jump to the now available
code (in logical space) is done. One of the background jobs would be to
swap out old segments that are unused to make room for current processes
and swap them back if the non current process should wake up.

The cpu is always in logical 16bit space but the windows (multiple)
can be moved around.

An ascii graphic would show the CPU 16bit space as several blocks
mapped to multiple blocks scattered in a larger space. The cpu
literally never leaves the range of a 16bit space but instead trade
out chunks of it for different ones out of an available pool. You never
actually jump put of logical space only shuffle what physical memory is
part of that space.

Allison
Received on Sat Apr 11 1998 - 13:58:57 BST