Path: utzoo!utgpu!watmath!att!bellcore!rutgers!iuvax!mailrus!tut.cis.ohio-state.edu!ucbvax!ucbarpa.Berkeley.EDU!wilson
From: wilson@ucbarpa.Berkeley.EDU (James E. Wilson)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Subtantiatng my criticism [really: VM on PDP 11/70]
Summary: PDP-11s *DO* have virtual memory
Message-ID: <30475@ucbvax.BERKELEY.EDU>
Date: 8 Aug 89 17:48:57 GMT
References: <13277@polyslo.CalPoly.EDU> <30466@ucbvax.BERKELEY.EDU> <1404@hydra.gatech.EDU>
Reply-To: wilson@ucbarpa.Berkeley.EDU (James E. Wilson)
Organization: University of California, Berkeley
Lines: 26

In article <1404@hydra.gatech.EDU> robinson@prism.gatech.EDU (Stephen M. Robinson) writes:
>Whoa...  Alright, yes, an 11/70 has separate instruction and address space.
>However, this does *not* constitute virtual memory with all the requisite
>disk space, paging mechanism (and caching, etc).  Not only that but "i-space"
>cannot be used for data and vice versa.  You *are* limited to 64K of program
>and 64K of data in user applications.  I thought the kernel was similarly
>limited but I guess not.  Sorry David, now *I* look somewhat foolish, eh?

Yes, you are looking very foolish indeed, since David is correct that the
PDP-11 supports virtual memory.  The PDP-11 has a segmented memory architecture.
It supports 8 segments of up to 8K each.  The virtual addresses used by the
CPU are translated into a physical address for accessing memory, by way of the
segment tables.  Caches and paging are not necessary for virtual memory.
The key point is that although the CPU uses 16 bit virtual addresses, a PDP-11
could have any amount of physical memory (up to 4 Megabytes on later models)
because of the address translation hardware (i.e. virtual memory).

Also, I would like to point out that the 64K address limit has nothing to
do with virtual memory.  After all, *ALL* machines have addressing limits,
the PDP-11's just happens to be smaller than most.  That is why DEC came out
with the VAX (Virtual Address eXtension), the VAX did not add virtual memory
(the PDP-11 already had it), what it did was eXtend the address range, and
make paging easier to implement.

Jim
wilson@ucbarpa.Berkeley.EDU