Bringing Back the PDP8

Anne Thwacks writes " Andrew Grillet has decided that the Digial PDP8 - the first ever minicomputer, will rise from the dead. He is calling it the PDQ8. Sure others have done software emulations, and even hardware clones, but he is not just building a hardware clone, but trying to revive the whole idea of 12 bit computers!"

Comments

12 bits

[vasqzr]

I found that, where you are not primarily handling ASCII, 12 bits was a very good size.


Maybe someone would enlighten the rest of us on why a certain bit size is better than another, and why we currently use 8/16/32/64, instead of 12/24/48/96 ?

Re:12 bits

[FauxPasIII]

>> Maybe someone would enlighten the rest of us on why a certain bit size is better than another,
>> and why we currently use 8/16/32/64, instead of 12/24/48/96 ?

Because powers of 2 are easier to work with in binary.

Re:12 bits

[oliverthered]

8 bit is base 2

1 2 4 8 16 32 64 128 256 512

So I suppose 12 could be ....

1 3 6 12 24 .... hmmmmm..... now quite correct

maybe
1 2 3 6 12 24 ... 60 ... 420

anyway

12 is good because
12/6 = 2
12/4 = 3
12/3 = 4
12/2 = 6

Re:12 bits

[lobsterGun]

This is a story that a wisened old timer told me at a convention (the guy had a whit bushy beard so you know it has to be true).

Back in the day there were two schools of thought: the 8 bit byte and the 9 bit byte. The 9 bit byte school represented the same data as the 8 bit byte school (0 to ff), and used the extra bit for parity.

8 bit bytes led to 16 bit words etc...
9 bit bytes led to 18 bit words etc...

My reaction to the old time was to ask WTH would anyone do this?

His response was to go into a discussion of 36 bit computing. "Ya see son. When you have 32 bits you can divide those bits evenly 6 ways:
32 single bits,
16 2-bit groups,
8 4-bit groups,
4 8-bit groups,
2 16-bit groups,
1 32-bit group.

With a 36 bit system you can divide those bits evenly 9 ways
36 single bits
18 2-bit groups
12 3-bit groups
9 4-bit groups
6 6-bit groups
4 9-bit groups
3 12-bit groups
2 18-bit groups
1 36 bit group

using 36 bits give you more flexibility in addressing."

He went on to tell be stories about how they explioted the advantages of 36 bit computing back when he had worked at Compuserve and how sad he was that 36 bit systems had died.

It could very well be that 3 12-bit bytes are used to make up a 36 bit word... ...or it could be that that old timer was full of crap or off his meds or something. The world may never know (Much like the number of licks it takes to get to the center of a tootsie pop).

This is great news :)

[solostring]

I fondly remember the PDP8. My father had one installed in the garage when I was a kid, and had my first experience of programming on it when I was 8 or so.

On the subject of PDP8's, I was surprised to hear that they were used in communications in Hong Kong up until at least 1999 for a number of financial institutions. I worked with an old computer technician who earned a fortune maintaining these beasts. I wonder if they are still being used in HK after the Chinese reclaimation?.....

Why?

[BWJones]

but he is not just building a hardware clone, but trying to revive the whole idea of 12 bit computers!"

So, after reading the article, I am still trying to figure out....Why revive the idea of 12 bit computers? Other than nostalgia (which is why people still drive Studebakers, old Ferraris and old Porsche's I suppose), what is the point?

replica computers ... its logical innit?

[trash eighty]

people have been building replicas of old cars, boats and aeroplanes for years. i suppose its logical that people would start building replicas of old computers sooner or later. much computer hardware is boring these days, generic MBs and computers. ahh the good old days when we had some variety ;)

all power to him!

Re:ENIAC

[Ark42]

Funny..

I read this, then scrolled down to the bottom, and my quote of the moment said:
"Just think, with VLSI we can have 100 ENIACS on a chip!" -- Alan Perlis

Text based games

[Technician]

I remember playing the father of the Zork series of games on a PDP 11/35 which was the newer 16 bit machine. The game was called Adventure. We got the game on a 5 Meg 14 inch hard drive (RK05) and wasted a bunch of paper making moves on a keyboard/printer terminal. The scrollback feature of the hardcopy was great for finding your way out of a maze again. Adventure has since been ported to CPM and DOS. The game is still a great game and will challenge the thought process. Take a pencil and paper to keep from getting lost. There is no map. Do a google search to find this true classic game. You should be able to run it in a DOS window on Windows 95 before DOS and Windows 95 expire at the end of this year. I'm still trying to figure out who the shadowy figure is who tries to get my attention.

a PDP8 was my home machine in 1976

[MarkWatson]

But, it was just an Intersil Intercept Jr. single board version of a PDP - same instruction set though.

It was fairly easy to program for - I wrote a simple cross-assembler on a Dec-10 that would print out my assembler source with machine code (in octal). For short programs, it was fairly quick to enter the programs in octal. Since the Intercept Jr. was all CMOS, the programs would stay in memory as long as I wanted without runnng down the batteries.

Really, it was very cool, and fun.

-Mark

What a memory...

[NormAtHome]

We had one of these in high school (25 years ago), looking back I thought it was pretty cool but man that single DEC tape drive was a royal pain. We also had an old teletype with a paper tape punch, haven't seen one of those since. In my junior year we got a grant from the state and got a PDP-11 with dual 8 inch single sided floppies, now that was living!

Didn't these things have selectable word sizes?

[GreyPoopon]

I remember seeing one of these puppies when I was in high school. It was no longer in service, but still had most of its internal components. In fact, I still have one of the "flip chips" that I took from inside it. Anyway, I believe I recall a dial on the front of the main panel that allowed you to choose the word size. I can't remember whether 12 bits was the minimum or the maximum, though. Can anybody verify that this existed, or am I just remembering some strange dream?

PDP8? How about a PDP11???

[Spatch3]

You know, I work with PDP11's day in and day out. DEC's last PDP processor was, I believe, a PDP 11/93. It has a whopping 2 MB of RAM and 8 serial ports on the processor board! This was a double sized board only taking up 2 slots on the DEC backplane that took the function of 5 boards that took up 5 slots each. Of course the disk controller, clock card and other boards are separate. These boards are circa 1990. They are in a custom digital dictation system that can handle 64 simultaneous audio ports where people are either dictating or transcribing. The OS is the roll off your tounge: RSX11M+. These systems, which we are replacing slowly but surely, have been absolute work horses lasting for at least 10 years.

I jsut had to reboot one this morning... :)

Chris

PIC is a bit more practical

[Bruce Perens]

If you want to program old microcomputer architectures, learn the PIC microcontroller, which is based on some Control Data Co. Peripheral and Input Controller (I may have the "I" wrong in that acronym). It's available in 12, 14, and 16-bit flavors. It doesn't have much of a stack. And it has the virtue that, as a $1 microprocessor, it's still practical for many projects, while a PDP-8 is really an intellectual exercise at this point.

Bruce

I blew up a PDP8 - and survived

[Ella the Cat]

I'm old enough to have done an electronics project building a joystick interface for a PDP8 as an undergraduate. I spent ages soldering TTL chips and after a few weeks plugged the card in, to a strong smell of fish and burning insulation. It wasn't my fault, the slot in the edge connector was too wide, and every single connector on the backplane had shorted to every other. It was 6 months to get the machine repaired, so someone figured out they could take out the power transformer, scrape off the burnt mess, figure out how many primary and secondary turns were needed on the transformer, then wind them on using a reel of wire and a lathe. They go the machine going, someone else filled the board slot with epoxy and cut a new slot. My project was saved! A few weeks later i reached round the back of a PDP8 to unplug a power connector, grabbed the live pin, but was saved because my arm was earthed to the PDP8 case. I love that machine, I still have the instruction set on a sheet of paper.

I loved the PDP-8 but I'm not convinced...

[dpbsmith]

Until I actually started programming a PDP-8 (in assembly language, of course), I would never have believed that you could program comfortably in such a seemingly restricted instruction set. And, conversely, when I moved to a PDP-11, I thought I was going to revel in the freedom and power of all those instructions, all those registers, those addressing modes, those index registers... and the ability to access 65536 bytes directly.

If C is "high-level assembly language," then the PDP-11 is "a computer that directly implements C."

To my surprise, though, I didn't really find that a lot was gained. Programming a PDP-11 didn't really FEEL much easier or more powerful than programming a PDP-8. And it was amazing how much every program expanded in size. It's been said that the PDP-8 instruction set was the most core-efficient ever devised, and I'd believe that.

On the other hand, when I tried programming a 6502, which on the face of it doesn't SEEM that much more restricted than a PDP-8, I just about went bananas.

Having said all that, I'm still not sure I see the point. The sweet design for a computer has to depend on the economics of the hardware around it. Who cares? Even IF the "core-efficiency" thing were true, and even IF you could use standard RAM with a 12-bit processor and not waste any bits, and even IF it turned out that the PDP-8 design were, say, 30% faster and used 30% less RAM for a given program than x86... how could it matter?

If the Alpha, which really WAS a superior design, wasn't superior enough to overcome Intel marketing, customer inertia, and only the normal amount of mismanagement, how can a PDP-8 be anything more than a curiosity?

Re:TRS80

[ch-chuck]

DOn't laugh - IMSAI, which predates the TRS80, is soon to sell, for under $1000, a series two with 20Mhz Z8S180, 1 meg static memory, AND toggle switches and flashing lights! CP/M never ran so fast.

W95 and DOS will not expire at the end of the year

[kfg]

Only official support for them will. DOS will never 'expire.' It's done. Of course you can't buy MS-DOS anymore, but there's always DR-DOS and FreeD0S, both still supported.

W95/98, on the other hand, will actually expire some years in the future. I discovered this on a reinstall that went bad. Windows simply refused to install. Having a Gateway at the time I called tech support and the issue was tracked down to a buggy BIOS (gotta watch for those updates) that had reset my system clock to a future time.

"Ah, there's your problem. Windows has a 30 year time bomb built in so it thinks it's expired."

Ummmmmm, good to know. I guess that's how long we've got to port all our favorite W95/98 games to Linux ( or maybe Plan 9).

KFG

PDP-8 microprocessors

[panurge]

This is so long ago I can't even remember the manufacturer, but there were at least 2 CMOS implementations of a PDP-8 processor. Has anybody observed that the instructions were based around core memory, so the accumulator was cleared when it did a store? If you could get the core again, a home computer with a PDP-8 processor and core memory would really benefit from a modbox with windows and lights.

For some industrial control jobs, something like a PDP-8 or PDP-11 is in many ways ideal because you can see everything that goes on. It is actually possible for one person to understand the hardware, the microcode, and every single bit of the software. For me, that is the great pleasure of small embedded designs. I really think it would be good to have a teaching tool for CS that actually meant that the student could do a project and have a complete overview of the entire thing in this way. I'm far from knocking progress, but there are comments on this thread that are a bit about the kind of alienation we have now between hardware and software - most people have no real idea at all what the hardware does, and use terms like "cache" without even stopping to think about what is going on. So yes, let's have someone build an understandable modern PDP-8. It's less weird than the RCA1802 and easier to get your head around than the 8080.