Operations in the video of the PDP-8/I front panel

This Web page supports the PDP 11/20 exhibit developed by Herb Johnson. Last edit December 5 2008. Herb Johnson

Here's a Web link to a video of the operation of a DEC PDP-8 console. Thanks to David Gesswein of pdp8online.com, who produced the video in 2008 to assist the exhibit project.

The PDP-8 is a smaller "cousin" of the PDP-11, similar in age and function but which runs different software. The video shows the operation of that console, in a manner similar to the 11/20's console, to start up the computer.

The video shows the front panel in operation by a person. He is starting the PDP-8 and "telling" it to get information from the harddrive. He is loading into the computer via the front panel, whas is called the bootstrap code needed to operate the disk drive attached to that computer. "Bootstrap" is a 19th century term, as in "lift yourself by your own bootstraps", or shoelaces. That means starting something up from nothing or nearly nothing.

The video shows a series of manual operations on the front panel switches. The lights on the front panel shows how the computer responds. The front panel directly accesses memory, and shows or modifies data at the addresses shown. The panel works in binary because the switches and lights are either "on" or "off" - binary "one" or "zero". The numbers in the video titles, are descriptions with numbers in base eight or octal, because octal conveniently represents the binary values which are entered into the front panel switches, and which are displayed in binary on the on/off lights of the front panel lights.

In manual operation, the front panel switches are set to an address, the address is loaded with the "load address" switch. Then the switches are set to the data value, and the data is loaded with the "deposit" switch. If data is to be loaded into the next address, the address is automatically updated so you only need to set the switches to the data, and then use the "deposit" switch to load that data, and to advance the address again.

Data loaded in memory locations are either PDP-8 program instructions - the "binary code" that the computer runs - or they are data, used by those instructions. Also, after a program is loaded, you have to load the starting binary address of that code, and "run" it by using the "run" switch. Then the CPU takes over and runs the programs, which in "bootstrap" operation normally load other programs, which load in more programs, until you have your main program or operating system running.

Thanks to David Gesswein for authoring and producing this video, of his own operation of his PDP-8/I system. His Web page of that system and including this video is at this Web link.

Front panel operations

Set address to 0200
lift switch #? up for a "1", all others down for "0", press "load address".

Load 0200 with 6603, DMAR (start disk transfer)@ lift switches up or down for a "1" or "0", press "deposit". @ The front panel logic advances to the next address.

Load 0201 with 6622, DFSC (skip next instruction when done)@ lift switches up or down for a "1" or "0", press "deposit", advance to the next address.

Load 0202 with 5201, JMP .-1 (to DFSC)@

Load 0203 with 5604, JMPI .+1 (to address in next location)@

Load 0204 with 7600, start address of Disk Monitor System@

Set address to 7750@

Load 7750 with 7576, (- words to transfer)@

Load 7751 with 7756, (addr-1 to transfer to)@

(load address and start the program???)

Copyright © 2008 MARCH, licensed to Herb Johnson. Video produced and owned by David Gesswein of pdp8online.com.