On Tue, 14 Nov 2000, BADL wrote:

> I used to have a PDP 11/44 about 10-12 years ago.  If I remember
> correctly, it only supports segmented addressing mode, much
> like that of the 80286.  And the size of the segments might be
> limited to something like 64k? *guess*

Well, yes and no. I can understand that people might regard the MMU as
segmented addressing, but I don't agree with that.

The PDP-11 is a 16 bit machine, period. That means you can only address
64k bytes. Nothing strange about that.
Then you have an MMU which divides your 64k into eight pages of 8k bytes.
These pages can be relocated anywhere in physical memory, so that given a
16 bit virtual address and the MMU relocation registers, you can find out
the physical address, which on an 11/44 is 22 bits.
Apart from the relocation, you can also set different protections on
different pages, and also limit the actual size of a page to less than 8k,
and even decide in which direction the page grows.

Unless my brain fools me, segmented addressing works slightly different.
I'm definitely no expert on the x86 architecture, so please correct me if
I'm wrong.
In a segmented architecture you have one (or  perhaps two) registers which
adds a constant to all your memory references. This register might also
have some granularity to it. You cannot protect parts of the memory, and
the code/data still have to be contigous, wherever it happens to be
located. Also, I believe anyone can play with the segment registers, they
are in no way protected agains manipulation.

Yuk!

	Johnny

Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt@update.uu.se           ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol