PDP-8 Replica: The PiDP-8

The Grandfather of Personal Computing
(and embedded microcontrollers too)

The PDP-8 was basically the PDP-1 from 1959, now stripped down to the bare minimum of parts that still made a working computer. DEC had no idea that was about to create the concept of the personal computer with this. It thought of industrial controllers - if these signals in, then these signals out. Open the valve when the pressure gets too high. Stop the heater when the temperature rises. And also, as a laboratory computer, doing perhaps some extra math.

In praise of Glorious, Glorious Minimalism


Our goal is not only to show computer history, but to keep hands-on experience alive. In the case of the PDP-8, that will be to make you feel two beautiful benefits of minimalism.

First, minimalism makes a computer extremely understandable. The Blinkenlights front panel of a PDP-8 shows all the information in the CPU, and the CPU can easily be understood down to the most elementary level. It makes the PDP-8 extremely likable, and extremely educative. Yes, it works like any other computer, just with less to get confused about. It is why the PDP-8 is often still used in Computer Science courses!

Second, minimalism forces creativity. Programs on the PDP-8 need to be concise and inventive. And the PDP-8 sparked so much of that, that programmers ended up doing never-imagined things with the machine, just because it was the only computer they could afford to use.
OS/8, an operating system much like the later MS-DOS, running good compilers, applications and lots of games. Graphics games even proved possible. And then there were some multi-user operating systems that gave four users concurrent access to Algol or Basic. It's one of these things you have to experience hands-on. Using a 4K computer with not even a stack pointer in its CPU, yet delivering a nice Basic and operating system for four people.


This is why so many people love the Eight. It is tiny enough to fully understand without a degree in CS, it was forced to be capable enough to be a personal computer. The experience will leave you with another thought. If this was all possible in 1969 on a computer 1/10th of an MS-DOS PC, why were we stuck with MS-DOS crap a whopping decade later? Because - computer evolution is crude and important software know-how that was already developed got lost in the chaos of the microcomputer revolution!

We leave this section with the quote of an 88 year old PDP-8 expert, who automated a steel factory with dozens of them. When he was shown OS/8 for the first time in 2017, he was stunned. In his mind, being an industry automation guy, the PDP-8 was just a very bulky Arduino-style microcontroller. And here it was loading a game of chess from an operating system! He had never even thought of more then 'if pressure sensor above 80, open the valve'... And the same was true for DEC management when they first introduced their glorious, lovely Eight to the world. Computer evolution was unpredictable.

    



Youtube: Ignore the possibility of using the PiDP-8 as a personal computer, with a comfortable terminal. Christopher Masto demonstrates how it worked in the early days of the PDP-8: how to operate the front panel and how to program on it.

It helps to read the Wikipedia article on the PDP-8 CPU, and then to have the PDP-8 Pocket Reference Card at hand.

Technical Specs


So the 18-bit PDP-1 architecture was cut down to only 12 bits. Bits in a CPU are used to encode an instruction number plus a memory reference (plus two other bits, see here). To get at least some memory space visible to the CPU, the PDP-8 was shrunk to have only 8 instructions. To count to 8 requires only 3 bits. 12 - 3 - 2 = 7. So the PDP-8 had only 7 bits left to 'see' 128 words of memory around the memory address from where it happens to load its instruction! Memory itself was still a good-sized 4,096 words of memory but the programmer needs to keep his subroutines to 128 bytes - one 'page' in memory. We should mention that later on, memory got expanded from 4 to 32k words (48k in modern 8-bit bytes) using bank switching, and that made OS/8 nice. But the 128 byte pages remained.

Although the Eight only had the 8 instructions (that is, by the way, not why it is called the Eight. It's just that this is DEC's Eighth computer design), two of those instructions were very versatile. One for I/O, one for a whole mix of micro-actions in the CPU hacked into the 7 free bits. DEC wrangled all it could from the few bits in the CPU. The I/O options were surprisingly rich. Our replica is fully loaded with not only a paper tape simulator (USB sticks act as remarkably high capacity paper tapes), but also simulated magnetic tape drives, the special DECtapes, floppy disks, removable hard disk packs, a fancy vector graphic terminal - and if you want to make Frankenstein monster out of the PiDP-8, we sneaked a modern I2C port into its guts for hacking purposes.

    




A PDP-8/I in its rack with a paper tape reader and two DECtape units for storage.

Why you need one at home

Because everyone likes puppies, and the PDP-8 is a puppy. Extremely educational, because easy to understand. If there are only 8 instructions, a small machine-language program is not exactly difficult to write. The Eight is easy to love, and with over a hundred programs, languages, games, and high-res graphics demos, it’s fun. A computer with a fraction of the power of a microcontroller chip that has the chutzpah to be a personal computer, just because genius programmers made it do impossible things. But its historical significance should not be ignored – this was the birthplace of the personal computer.

Replica Details

The PDP-8 came from the mid-1960s, and we felt that a bamboo case would fit that period's atmosphere. See the pictures, we can only hope you agree. The PDP-8/I was the particular submodel we chose to replicate, because it still has the 'everything in the CPU is visibe' front panel, and comfortable switches. Actually, they are switches from a washing machine, DEC was looking for nice switch parts without too much cost. Custom-made replicas of these ancient switches complete the look and feel of the replica.

The simulator is a modified version of the PDP-8 simh simulator (whose beginnings, amusingly, itself date back to the times of the PDP-10) written by Bob Supnik. Normally, it runs on a built-in Raspberry Pi. Even the smallest Pi makes light work of the simulation, so in fact, you can concurrently use the Pi Inside as a regular Linux system. Giving the PiDP-8 more justification to find a home in the living room; it can double as network storage, media server or whatever else you’d like to do with a Pi.

With some hidden combinations of the front panel switches, the PiDP-8 can also quickly reboot in configurations that reflect different periods in its life. From a late, luxury OS/8 session you can reboot down to a bare-bones system with only paper tapes.

That is, in fact, the last 'hands-on experience that makes you feel how it was' stories we want to leave you with. Grab some empty USB sticks, and think of them as paper tapes... now try to develop a program in the year 1968. Toggle in the bootstrap on the front panel, so the PDP-8 can read a paper tape with the teco editor. Put in a new paper tape and write your source code. Yet another tape, load the assembler. Yet another tape, save the assembled program. Load the tape again to run it. It crashes, use the front panel as a debugger. Start all over again to fix the bug. The benefit: you will now understand why a Basic interpreter did not begin as a toy language on a home computer, it was a sensational innovation. You have to go through the pain to really understand that.

And the PiDP-8 will happy administer the pain to you. Before you boot up comfortable OS/8 again, never to look back. But now you know how it felt!



    



This is a simple kit to build, even if you never soldered before.

See the PiDP-8 Project site for details.

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