one more question about pdp8/e console switches behaviour

From: Pete Turnbull <>
Date: Thu Nov 11 13:28:10 2004

On Nov 11 2004, 13:29, Gooijen H wrote:
> Hmm, is that behaviour of a real pdp8/e console logical?
> I am not saying it is wrong; perhaps the impementation DIGITAL
> made is a little strange ...?

I think it's perfectly logical. When you press EXAM, it increments the
address ready to read the next word. When you press DEP, it increments
the address ready for the next entry. Unlike a PDP-11, where sometimes
it increments, sometimes not, and you have to remember the rules :-)

It's just a different way of doing it. Remember this is a simpler
machine, designed some years before the -11.

> When you activate HALT (or SING STEP), the top row of lights
> show the Memory Address (aka the Program Counter, PC).
> When the knob is then set at "MD" the current instruction (at PC)
> is displayed on the 'data' row of lights below the Memory Address.
> So far so good (correct?).

Yes, if you HALT a running program, that's correct. It displays the
address just about to be used, logically, so that if you press CONT it
starts at the address which is displayed, just as if you did an ADDR
LOAD of that address, and then pressed CONT.

> When you press EXAM the *first* time, the data still shows what
> is at memory location [PC], but the Memory Address shows [PC+1] ...

Yes. It shows the word at the address that was displayed just before
you pressed EXAM. Having pressed EXAM, it then displays the next

> That would mean that the displayed data and the displayed Memory
> Address *can* be out of 'sync'. Only after you pressed EXAM at least
> once you are sure that the displayed data is from location minus one
> from the displayed Memory Address.
> Is my understanding correct?

It's not really out of sync. When running, or after a HALT, and BEFORE
you tamper with any switches, the display shows the current address and
the data at that address. Which is probably what you want, most of the
time. If you then press EXAM, you will get the data from whatever
address was current (and displayed) just before you press EXAM. If you
press DEP instead, the data you write would go to the address that was
current, and displayed, just before you pressed DEP. When you press
either of those keys, the address is incremented ready for the next

> Other question.
> Is the increment after EXAM (and DEP) *always* done on the pdp8/e?

It's always done. It's done as you press the switch down.

> The reason I'm asking is that on PDP-11's the repeated sequence
> EXAM / DEP / EXAM / DEP, etc. does *not* increment the address.
> Address increment is only done when EXAM is pressed after EXAM
> was pressed. Likewise for a DEP / DEP sequence.

PDP-8 users get the DEP right the first time, and don't need to check
it ;-)

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Thu Nov 11 2004 - 13:28:10 GMT

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