[Simh] Simulating the PDP-15/76 Unichannel

[Mark Pizzolato - Info Comm]

On Monday, March 16, 2015 at 2:19 PM, Bob Supnik wrote:
> This has been on the wish list for a decade.
> 
> The 15/76 Unichannel was a big PDP-15 that used a PDP-11/05 as an IO
> processor to get access to inexpensive Unibus peripherals, particularly
> the RK11-E/RK05, which supported 18b data, the LP11, the CR11, and
> various plotters (not supported in SimH). XVM/DOS and XVM/RSX supported
> the Unichannel, and standard DOS probably did as well.
> 
> The 15 and the 11 are crosscoupled through shared memory. Except for
> 4K-12K(W) of local memory, all of Unibus address space is mapped into
> the 15's main memory. DMA transfers from the RK11-E could transfer 18b
> data (using the Unibus parity lines for extra data lines); programmed IO
> transfers from the IO processor transferred 16b data, zeroing out bits
> 0-1 on the 15.
>
> Control is done by a DR15-C parallel interface cross-connected to two
> DR11-C's in the 11. The 15 created task blocks in shared memory and
> transfers an 18b pointer to the 11 via the DR15/11 parallel connection.
> The 11's IO program, called PIREX, executes the directive and sends an
> API interrupt back when done.

Clearly the DR15/11 paradigm passing through PIREX is necessary for DMA devices, but how is programmed I/O handled.  

With the whole Unibus space addressable from the 15 side, is there code on the 15 which actually does programmed I/O directly to the Unibus devices?  If programmed I/O is done directly by the 15, then this whole problem is very different than one which relates to shared memory access.  

- Mark