[Simh] The missing PDPs

[Paul Koning]

> On Aug 12, 2019, at 10:17 PM, Johnny Billquist <bqt at softjar.se> wrote:
> 
> ...
> The PDP-12 poses a more interesting problem, as it is a dual CPU thingy with shared memory and two totally different instruction sets, but some interaction between them...

I've never programmed one but I have a handbook somewhere, so from that memory: it's not a "dual CPU" in the usual sense of two separate engines running in parallel, interconnected by memory or by I/O channels.  Instead, it's more like compatibility mode in early VAXen, where you can flip between two instruction sets through a mode control.  So the basic structure of a SIMH cpu simulation is unaffected; what changes is that there are two sets of opcode interpreters.

The I/O devices are also different; LINCtape is physically very similar to DECtape but the programming interface is rather different (significantly more advanced -- if I remember right, it will search for the requested block for you rather than putting that job in the driver software).

	paul