Subject: PDP-8 Front Panel

From: :[email-masked]
Date: Tue, 17 May 94 02:51:00 -0500

BG>Message-ID: <[email-masked]>
BG>Newsgroups: alt.sys.pdp8
BG>Organization: UMCC, Ann Arbor, MI, USA

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

BG>     What do the 8 switches in the lower right do?  Labels ARE

BG>         Start   Load    Deposit  Examine   Continue   Stop  Sing(le).   
BG>           |      address    /       \         |                Step    
              |        |      (SR in memory)   from stop            |    
              |        |       value  add. content         Execute 1 instru.
         (program   (in SR to                                    
         at address  Prog. Countr)[Switch Register is 12 swchs to left]
         in Switch                [cheapest way to input one instruct'n]
          Register)               [or datum at a time]

BG>2)  There appears to be a column of lights on the right edge of the 
BG>    panel labeled:[with the major machine states]

BG>                       / FETCH content of address
BG>                 MAJOR   EXECUTE that instruction
BG>                STATES   DEFER   to Extended Arith. Element option
BG>                       \ BREAK   stop then do it again
                            
BG>                         ION [interrupt enable flip/flop on]
BG>                         PAUSE [temp. delay]
BG>                         RUN  [what it should be doing]

BG>3)  What do the 'Data field' and 'Inst field' lights and switches refer 
                 SETS PAGE IN CORE FOR DATA AND INSTRUCTIONS
                REMEMBER, THE BASIC MACHINE IS 4K IN 1K PAGES!
                 
Bruce: I hope this clears things up a little. The idea was you could run the 
machine entirely from hand-assembled programs inputted to front panel, and 
when the machine died, go to a specific location in memory (and by looking 
at the accumulator and instruction register) to try to debug the thing. In 
fact, until the later 8 machines (and the price of ROM falling) you had to 
toggle in a short loader program to read the rest of the loader (paper) tape 
that would then let you run a one-shot app like the MIT chess program which 
needed every bit of the machine to make it work or a primitive OS that could 
handle 128K DECTapes, 32K hard disks (18" wide, x 1/2" thick) and later 128K 
hard drives in the same size factor and weight.

                                                dmr
                                       [[email-masked]]

  000 001 010 011 100 101 110 111 <how the triads of SR switches are set for 
   0   1   2   3   4   5   6   7          octal machine>

[[email-masked]]
---
 * WaveRdr 1.0 [NR] * UNREGISTERED EVALUATION COP

---

From: [email-masked] (Johnny Billquist)
Date: 19 May 1994 02:51:10 +0200

>          Register)               [or datum at a time]

          ^^^^^^^^^^

          Wrong. The program is started at the current address,
          the switch register is the physical switches, whose
          value can be loaded into the current address with
          LOAD ADDR

>                REMEMBER, THE BASIC MACHINE IS 4K IN 1K PAGES!

No, the basic machine is 4K in 128 word pages, and this has nothing
to do with the DF and IF.
There is an option whereby you expand the pdp8 to 32K words,
separated into 8 4K fields. IF and DF indicates which field is
currently selected for instruction and data respectively.

>hard drives in the same size factor and weight.

The MIT chess program referred to is probably MAC-CHESS (I think it
was called, long before apple started to call everything MAC-XXX).
This program ran on the pdp6, not the pdp8.
-- 
Johnny Billquist                  || "I'm on a bus
CS student at Uppsala University  ||  on a psychedelic trip
email: [email-masked]       ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol