Bob, you are right in that the PDP-5 used address 0 for the PC, and put
the saved PC at address 1 at interrupts, which is not compatible with
the PDP-8, and means any interrupt driven code will not work across the
PDP-5 and PDP-8.
Not sure what you mean by 0/1 for interrupts. Maybe you mean that 0 is
where the pre-interrupt PC is saved, and execution starts at 1?
Anyway... As far as the early PDP-8 models go, the 8/S is the odd ball
out. As far as I can remember, a bunch of OPR combinations did not work
the same (or at all) on the 8/S, compared to any other PDP-8 model. So
special care needs to be taken when you write something for an 8/S.
Apart from that, the machines are mostly upward compatible, indeed. The
Omnibus machines added a few new things, but yes, you normally use
various undocumented opcodes to tell the machines apart. RAR RTR is the
one I know the best, but there are probably others too.
Kermit-12 is a good source if people want to check how to tell which
model it is running on, since that program does a pretty decent job of
identifying pretty much all machines.
Ray Jewhurst mentioned that the serial bus of the 8/S was the basis for
the Omnibus - that is backwards and wrong in several ways. First of all,
I'm not sure the bus was serial on the 8/S. The CPU was serial.
Second, the Omnibus is most definitely not serial, and I also seriously
doubt there are any relationship at all between the Omnibus and anything
on the 8/S. Third, I have some vague memory that the Negibus was used on
8/S, but I should probably look that up.
Johnny
On 2016-09-09 03:53, Bob Supnik wrote:
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, [email protected] wrote:
Message: 1
Date: Thu, 8 Sep 2016 18:57:52 -0400
From: Ray Jewhurst<[email protected]>
To: simh<[email protected]>
Subject: [Simh] PDP-8: The possibilities?
Message-ID:
<camfeaable-s+qszmm4axyr8pqhx3dpkiadjb_auxqo5hahe...@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
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|| on a psychedelic trip
email: [email protected] || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
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