[Simh] PDP-8: The possibilities?
Ray Jewhurst
raywjewhurst at gmail.com
Thu Sep 8 23:07:08 EDT 2016
Lesson learned. I will stick to Bitsavers for my info. As far as rotate, I
did know about that and because of that and other reasons, I am going to
back off on the S and I think I will concentrate on the Straight-8.
On Sep 8, 2016 11:01 PM, "Johnny Billquist" <bqt at softjar.se> wrote:
> 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_aUXQo5hAHEt1
>> g 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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