Subject: PDP-8 Front Panel

From: [email-masked] (Bruce Grant)
Date: 14 May 1994 01:08:37 -0400

I recently saw a (kinda fuzzy) photo of a PDP-8 front panel, where a few
of the labels were not readable.  I'd appreciate it if someone familiar
with the PDP-8 could answer a couple of questions about it:

1)  What do the 8 switches in the lower right do?  Labels appear to be

         Start   Load    ?     ?    Cont  Stop  Sing.   ?
                   ?                            Step    ?

2)  There appears to be a column of lights on the right edge of the 
    panel labeled:

                         FETCH
                         EXECUTE
                         ?
                         BREAK 

                         ?
                         PAUSE
                         RUN

    What are the missing labels?

3)  What do the 'Data field' and 'Inst field' lights and switches refer to?

Thanks,

Bruce Grant ([email-masked])

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From: [email-masked] (Tom F Karlsson)
Date: 16 May 1994 10:35:01 GMT

>          Start   Load    ?     ?    Cont  Stop  Sing.   ?

Must be the panel of the original PDP-8.
(is it a jpg picture, posted a while ago on the net ?)

The switches are:

Start	Load     Deposit   Examine   Continue   Stop   Single   Single
	Address					       Step	Instruction

> 2)  There appears to be a column of lights on the right edge of the 

FETCH
EXECUTE
DEFER  <- but this one I'm not sure about.
BREAK

ON
PAUSE
RUN

The only picture I have (besides the small ones in div. user handbooks) is
the jpg picture. Even after reanalysing the picture, editing colors and such,
it was hard to see..
Hopefully we have someone out there with a real machine who can tell us ?

>From the PDP-8/L handbook:

DATA FIELD SWITCH
The DF switch serves as an extension of the SR to load the DF by means of
the LOAD ADDRESS key. The DF determines the core memory field of data
storage and retrieval.

INST FIELD SWITCH
The IF switch serves as an extension of the SR to load the IF by means of
the LOAD ADDRESS key. The IF determines the core memory field from which
instructions are to be taken.

Sounds perhaps a little bit confusing :-)

/Tom

--
|  Tom Karlsson           email: [email-masked]      phone: +46 18 260097  |
|  Student of Computer Science @ Uppsala University, Sweden.               |
|  Secretary of Update.   email: [email-masked]                         |
|           "Intelligence is to do stupid things in a smart way"           |

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From: [email-masked] (gordon hlavenka)
Date: Thu, 19 May 1994 16:52:43 GMT

>Sounds perhaps a little bit confusing :-)

Not _too_ confusing; as a geeky high-school freshman in 1972 even I
could figure it out.  Now, we have an example in the X86 architecture:

The 8 series has a 3-bit CS register (called IF) and a 3-bit DS register
(called DF).  12-bit addressing gives you 4K segments, 12 bits wide.
The switches described above allow you to load the CS and DS from the
front panel.

The difference between pdp8 "segments" and X86 segments is that the
pdp8 segment registers simply extend the address by three bits.  This
is actually a lot simpler to understand than the Intel gyrations.

--
----------------------------------------------------
Gordon S. Hlavenka           [email-masked]
Proud father of Daniel Scott     born August 9, 1993