[TUHS] Unix for PDP11/20 w/o mmu or paging option

[Noel Chiappa]

    > From: Clem Cole

    > The KS11 MMU for the 11/20 was built by CSS ... I think Noel has done
    > more investigation than I have.

I got a very rough description of how it worked, but that was about it.

    > I'm not sure if the KS11 code is still there. I did not think so.

No, the KS11 was long gone by later Vn. Also,I think not all of the -11/20
UNIX machines had it, just some.

    > The V1 work was for a PDP-7

Actually, there is a PDP-11 version prior to V2, canonically called V1.
The PDP-7 version seems to be called 'PDP-7 UNIX' or 'V0'.


    > I'm fairly sure that the RK05,  used the RK11-D controller.

Normally, yes. I have the impression that one could finagle RK03's to work on
the RK11-D, and vice versa for RK05's on the RK11-C, but I don't recall the
details. The main difference between the RK11-C and -D (other then the
implementation) was that i) the RK11-C used one line per drive for drive
selection (the -D used binary encoding on 3 lines), and ii) it had the
'maintenance' capability and register (al omitted from the -D).

    > The difference seems to have been in drive performance.

Yes, but it wasn't major. They both did 1500RPM, so since they used
the same pack format, the rotational delay, transfer rate, etc were
identical. The one peformance difference was in seeks; the
average on the RK01 was quoted as 70 msec, and 50 msec on the
RK05.


    > Love to see the {KT11-B prints] and if you know where you found them.

They were sold on eBait along with an -11/20 that allegedly had a KT11-B. (It
didn't; it was an RK11-C.) I managed to get them scanned, and they and the
minimal manual are now in Bitsavers. I started working on a Tech Manual for
it, but gave up with it about half-way done.

    > I wonder if [our V1 source] had the KS-11 stuff in it. 

No; I had that idea a while back, looked carefully, our V1 listings
pre-date the KS11.


    > From: Roland Huisman

    > There is a KT11B paging option that makes the PDP11/20 a 18 bit
    > machine.

Well, it allows 2^18 bytes of main memory, but the address space of the
CPU is still2^16 bytes.

    > It looks a bit like the TC11 DECtape controller.

IITC, it's two backplanes high, the TC11 is one. So more like the RK11-C...
:-)

    > I have no idea how it compares to the later MMU units from the
    > software perspective.

Totally different; it's real paging (with page tables stored in masin
memory). The KT11-B provides up to 128 pages of 512 bytes each, in both Exec
and User mode. The KT11-C, -D etc are all segmentation, with all the info
stored in registers in the unit.

    > I wonder is there is some compatibility with the KT11-B [from the KS11]

I got the impression that the KS11 was more a 'base and bounds' kind
of thing.

	Noel