one more question about pdp8/e console switches behaviour

From: Gooijen H <GOOI_at_oce.nl>
Date: Thu Nov 11 06:29:39 2004

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 ...?

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

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?

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

again, TIA,

- Henk, PA8PDP.


> The position is always relevant, but you don't set it to AC. You set
> it to MD (Memory Data). And when you press EXAM (or DEP), the address
> displayed on the upper row will increment (but the data displayed will
> remain the same, the data for the address that was current when you
> pressed EXAM/DEP).
Received on Thu Nov 11 2004 - 06:29:39 GMT