[Simh] Simulating the PDP-15/76 Unichannel

[Rich Alderson]

At several points in the discussion, the KL-10 has come up as an
example of a system with a PDP-11 front end processor.  Reference
has been made to "shared memory" between the -11 and the PDP-10
engine which is the main reason for the existence of the system.

The PDP-11/40 front end and the PDP-10 engine do not share memory.
Data transfer takes place over a Unibus device (from the -11 side)
which resides in the I/O bay of the -10, called a DTE-20.  This is
a very simple device which ties into the interrupt systems of the
-10 and -11 with a "doorbell"; depending upon who is ringing whose
bell, data moves between the -11's memory and the -10's, but neither
side knows the address(es) in the other processor.

Note that the front end software is special, as well.  Both of the
loader programs, the older KLDCP which is retained by the diagnostic
package and the later RSX-20F[1,2,3], expect to deal with 18-bit
data on disk, not 16.

I'm not saying that a KL-10 emulation would not be interesting, just
pointing out that it has nothing in common with the Unichannel on
the PDP-15/76.

                                                                Rich

[1] Developed mostly from RSX-11M, with a few nods to RSX-11D, at
    least as far as the doc writers were concerned.
[2] If you boot an RSX-20F floppy on an 11/40 without a DTE-20, you
    will immediately receive a message on the console telling you
    that the DTE-20 is not found and the system will halt.
[3] NB:  The primary front end must be an 11/40.  There are up to 4
    DTE-20s in a KL-10, and the secondary systems can also be 11/34's,
    I believe, but I don't think they run RSX-20F, just special loads
    like DECnet or HASP/RJE software.