On Wed, 15 Nov 2000, P.G. wrote:

> On Wed, 15 Nov 2000, Johnny Billquist wrote:
> > On Tue, 14 Nov 2000, John Hayward wrote:
> > 
> > > > The PDP-11 is a 16 bit machine, period. That means you can only address
> > > > 64k bytes. Nothing strange about that.
> > >
> > > As I recall some PDP-11s (11/70 I believe) had an instruction/data space
> > > separate mapping so that your instructions could be in 1 64k area and your
> > > data in another.
> 
> You had 3 64Kbyte (32K word) address spaces: Instruction, data and supervisor,
> The supervisor space was used to hold libraries. There were extensive overlays
> in the programs, the largest I saw IIRC was over 900 blocks (512 byte).
> RSX-11M+ was capable of handling the 3 address spaces.

The Kernel/Supervisor/User mode is a different issue and has nothing to do
with this. You can only be in one more at a time, and you only have 64 k
address range. The different modes only affect which set of registers used
by the MMU to map your virtual address to a physical address, nothing
else. And some machines only have kernel/user (no supervisor).
Incidentally, this coinside with which machines have split I/D space.

> > Correct. And it's a number of models, not just the 11/70. From my head:
> > 11/45, 11/50, 11/55, 11/70, 11/74, 11/44, 11/53, 11/73, 11/83, 11/84,
> > 11/93, 11/94.
> 
> I do not have such a good memory :-) But form this list 11/45 only can address
> 124Kword (248Kbyte) memory, so even if it had this abilty, it would could not
> be used too well. 11/44 and 11/70 had 22 bit memory addressing (I do not know
> the rest off hand).

Yes, the 11/45, 11/50 and 11/55 only have 18-bit addressing in the MMU,
but you still have the split I/D space, so that you can have, in theory,
128k bytes usable (as long as you keep it at 64k code and 64k data).

	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