[Simh] PDP-8: The possibilities?

[Bob Supnik]

The PDP-5 is, in fact, not all that compatible, because it used memory 
location 0 as the PC, pushing the interrupt locations to 1/2, instead of 
0/1. So any program requiring interrupts will not work on a -5 vs an -8. 
The PDP-5 had an IO halt/restart facility, modeled on the PDP-1 and 
dropped from the PDP-8, which allowed an IOT to "wait" for completion 
without looping and testing a flag. It does not seem to have supported 
an EAE or extended memory.

The PDP-8 family (8, 8/S, 8/I and variants, 8/E and variants, 8/A) are 
superset compatible for defined operations. It's possible to tell them 
apart based on their behavior on undefined operations. The code for 
identifying a PDP-8 is out there, but I don't have it at hand. I 
remember that the behavior of RAL RAR and RTL RTR was one way of telling 
the 8, 8/S, and 8/I apart.

Most of the work for supporting models would be in the peripherals, 
particularly the ones that are 'compatible' across the line (reader, 
punch, terminals, clock). The pre-Omnibus machines used the older style 
IOP1, IOP2, IOP4 pulse methodology; the Omnibus machines can decode all 
8 possible combinations. Beyond that, peripherals tended to be distinct: 
the RK8 for the 8/I vs the RK8E for the Omnibus machines; the Type 552 
DECtape controller for the -5 and -8 vs the TC01/TC08 for the later 
machines.

The "CMOS 8s" are a whole different kettle of fish. They were only used 
in word processing/DECmate systems and had many unique features.

/Bob

On 9/8/2016 9:10 PM, simh-request at trailing-edge.com wrote:
> Message: 1
> Date: Thu, 8 Sep 2016 18:57:52 -0400
> From: Ray Jewhurst<raywjewhurst at gmail.com>
> To: simh<simH at trailing-edge.com>
> Subject: [Simh] PDP-8: The possibilities?
> Message-ID:
> 	<CAMFEAABLe-s+qSZmm4AXyR8Pqhx3dPkiaDJb_aUXQo5hAHEt1g at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> After both reading and participating in some recent discussions, I got to
> thinking that maybe the array of PDP-8 models could be better represented.
> I say this because from what I have read very early PDP-8 code is not  100%
> compatible with later models conversely the PDP-5 is compatible with the
> early code and likewise uses a negibus like the Straight-8. I thank this
> could be a rewarding experience for some of us and since I can't work I
> would be able to help coordinate, write pseudo code and beta test. If
> anyone is interested in this let the discussion begin.
>
> Thanks
> Ray