[Simh] PDP-8: The possibilities?
Johnny Billquist
bqt at softjar.se
Thu Sep 8 23:01:04 EDT 2016
On 2016-09-09 04:40, Ray Jewhurst wrote:
> My source pertaining to the serial bus on the S being the basis for the
> Omnibus I got straight from the FAQ. To be more specific it pertained to
> the easy configurability of the 2 busses.
The FAQ isn't always right. :-)
I don't think it makes any sense at all to compare the 8/S with the
Omnibus. I just did a quick check, and some controllers were usable both
on the straight 8 and the 8/S, such as the PC02, PC03, CR03C, AA01A and
probably others... So I would say that the straight 8 and 8/S was pretty
much the same, as far as peripherals were concerned. And as far as I
remember, these were pretty much the same on the 8/I as well. Check the
manuals if you want more details. :-)
And I just found the difference in the OPR instruction for the 8/S
compared to other models, that I had some vague memory of.
On the 8/S, the Increment AC bit cannot be combined with any rotates,
since they are both done in the same clock cycle.
As far as I can remember, only the 8/S have this property, and on other
models, it is a clearly defined sequence of the different OPR bits, with
increment happening before rotates.
Johnny
>
>
> On Sep 8, 2016 10:27 PM, "Johnny Billquist" <bqt at softjar.se
> <mailto:bqt at softjar.se>> wrote:
>
> 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, simh-request at trailing-edge.com
> <mailto: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
> <mailto:raywjewhurst at gmail.com>>
> To: simh<simH at trailing-edge.com <mailto:simH at trailing-edge.com>>
> Subject: [Simh] PDP-8: The possibilities?
> Message-ID:
>
> <CAMFEAABLe-s+qSZmm4AXyR8Pqhx3dPkiaDJb_aUXQo5hAHEt1g at mail.gmail.com
> <mailto:CAMFEAABLe-s%2BqSZmm4AXyR8Pqhx3dPkiaDJb_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
>
>
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> --
> Johnny Billquist || "I'm on a bus
> || on a psychedelic trip
> email: bqt at softjar.se <mailto:bqt at softjar.se> ||
> Reading murder books
> pdp is alive! || tryin' to stay hip" - B. Idol
>
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--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt at softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
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