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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!"
...and I building a 3.5 GHz TRS80 with a GIG of RAM and 2 5 1/4" 80 GB floppys. its the cats ass
Many of you probably have used Xilink's 1000, 2000, or 4000-series FPGA card during laboratories for your undergrad classes.
Well, if you'd like, you can follow this design of an FGPA implementation of the original PDP-8 computer!
If you've used Verilog (a hardware design programming language), like I have, you can even download all the code!
If you celebrate Xmas, befriend me (538
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 ?
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?.....
-- 7 string electric violin + live loop samplers
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?
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Well, I found this old link in my bookmark manager. It details the history of EMS (sound studio) in vivid detail, including a listing of all their original equipment.
The interesting part is that they posted high resolution images of their setup, which includes PDP-8 microcomputers!
The image: http://www.ems-synthi.demon.co.uk/studiopz.gif
The PDP-8s:
Left side - Teletype for PDP8
Left bay - PDP8/L Computer ("Leo") 4K x 12 bits (=6K bytes) 1.3 s cycle (0.77MHz), 32K Hard Disk Store
Center left bay - PDP8/S Computer
If you celebrate Xmas, befriend me (538
12 bit is much better for patriotic Americans.
Think on it, power of two is a far to simplistic and dare I say it European system for the patriotic American. In Europe they use metres, kilometers, grams and kilgrams. All this regulation of structure around a number like 10 is typical of Europeans. Americans use sensible systems like 14 pounds (abbreviated sensibly to lbs as pounds clearly contains the letter l) to a Stone and 16 Ounces (again with a sensible abbreviation of oz) to a pound. Who needs these ridiculous regimented European systems that dicate that everything must follow a sensible pattern?
Patriotic Americans arise. 12 bits to a byte, 7 bytes to a word, 13 words to a sentence and 1764 bits to a chain.
An Eye for an Eye will make the whole world blind - Gandhi
Probably OS/8, which if I remember correctly, was a multi-user operating system. You can run a multi-user system on a PDP-8 with 32KW of core.
Mea navis aericumbens anguillis abundat
On a related note, I'm going to be designing my own 32 bit system. It's going to be pretty cool having an asthetically pleasing case, and run most all of the common software out there, but make the operating system run on top of bsd. Then I'll make really high-end systems, and education type systems, and laptops.
Now I'm 95% of the way done with this whole project so I've hired an advertising firm to come up with some commercials. I figure I'll show joe average sixpack switching from the normal x86 windows machine, to my machine, I'll call them 'Switch-Ads'.
My proprietary systems will never run on anything else, and you will be forced^H^H^H^H^H^H encouraged to only buy via our website.
I'll call them MOC's ... and the company will be named Orange.
Ignore the "p2p is theft" trolls, they're just uninformed
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.
The truth shall set you free!
Now, I'm no expert on legacy computers, but a machine with 32KW of core sounds dangerous...suddenly my 80W Athlon looks puny!
In the bottom of a box somewhere in my basement, I've still got the BASIC source code for the Star Trek game we used to run on our high school's PDP-8. For each player's turn, it printed out the map of the current galactic sector along with any Klingon ships on the line printer.
It's funny, I remember when we played that game we felt like we had godlike control over a mysterious and powerful machine. Now when I play computer games, I mostly feel like a twitching moron.
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?
GreyPoopon
--
Why is it I can write insightful comments but can't come up with a clever signature?
Some mentioned earlier on this thread jokingly about making a super fast computer based on old architecture.
Has anyone actually done that? Has anyone actually taken say, a Tandy Color Computer 3's hardware and boosted it up to something approaching our current standards? I'm not talking emulation on a x86 platform. I mean fully working with a processor with a native OS.
Those architecture are so simple, with kernels so small you could print the hex binary out on a couple of pages. Imaging how fast an accounting package would be on a 1 gHz, or even a 200 mHz.
I know this maybe off topic, but if someone could resurrect a 12-bit system to a more modern standard, why not other system. DOS is still viable in certain circumstances, why not these platforms.
Think about an 8-bit controller with a serial connection, flash memory, and a RCA video out jack that is based on a C64. There is a TON of documentation for programming on something like that. Linux guruâ(TM)s could use C/C++ and Windows users could use Commodore BASIC.
Oh well thatâ(TM)s just my ramblings.
You say things that offend me and I can deal with it. Can you?
What else would be a 12-bit OS?
And of course
"with your bare hands?"
yes
you stand amazed as the dragon lies dead at your feet.
Bugger graphics you can't beat a maze of twisty passages, all different.... or was it a twisty maze of different passages.
An Eye for an Eye will make the whole world blind - Gandhi
Bruce
Bruce Perens.
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?
"How to Do Nothing," kids activities, back in print!
OS/8 was a single user system. I wrote a multi-user kernel that ran multiple copies of OS/8 customized to hook into device drivers supported by my kernel. It supported virtual paging using a hardware hack first suggested by Richard Lary (the author of OS/8). We ran four or five users on a 32KW 8/E with a couple of RK05 drives.
We never distributed it because paging performance without the hardware hack was very bad (every CDF instruction needed to be trapped and mapped in software) and the hardware hack was only developed for the 8/E and piggy-backed on one of the system boards destructively (i.e. once modified your 8/E wouldn't run without our hardware).
But it was used internally by the company I developed it for until about ten years ago.
Back then, the size of core memory was generally measured in machine words, thus in the case of a 12-bit machine like the PDP/8 with 32 k-words, the core would be: 32 x 12 bits == 384 k-bits, or 48 k-bytes.
In Soviet Russia, Jesus asks: "What Would You Do?"
I remember when we played that game we felt like we had godlike control over a mysterious and powerful machine.
That must've been a nice feeling, I bet. In these days, the mysterious and powerful machine has god-like control over you!
In Soviet Russia, Jesus asks: "What Would You Do?"
As nice as a base-3 system my be in theory, there are very good reasons for sticking to a base-2 system in hardware. As we are moving to smaller and smaller fabrication processes, it is necessary to lower the supply voltage Vdd. For example, now that we are approaching the 0.1um and 900nm levels (at least in research labs), Vdd is getting down around 1 Volt. However, the Vt (the threshold voltage needed to turn "on" a MOS transistor) stays the same, because it is determined by physical properties of silicon (mostly). That means we're losing headroom. To implement ternary logic, we would need 3 different voltage levels. We're simply running out of room to do things like that. You need to leave a noise margin around your "1" and "0" values for reliable operation. (For example, if Vdd=1V, you might consider 0.0-0.4 = "0", 0.6-1.0 = "1". Then a logic gate that "sees" 0.0-0.5 interprets it as "0", etc. If you had a "0" that was really 0.4V, you would hope that "noise" wouldn't bump it up above 0.5V, or else it would look like a "1".)
The threshold voltage for transistors is somewhere under 0.2-0.3V usually (depening on the technology & lots of other parameters). So, you absolutely need a 0.6V supply. (0-0.3 = "0", 0.3-0.6 = "1".) Unfortunately, even with Vdd=1V, you'll get voltage drops happening throughout the chip ("IR drops" - as in I=current times R=resistance) so that the 1V may only look like 0.8V to some parts of your circuit.
From the above discussion, it should be obvious that there really isn't room to shoehorn in a third voltage level. Also, a nice feature of CMOS design is that when a gate is sitting in a "0" or a "1" state, it is drawing no (well, negligible) power. Power is only dissipated while a value is switching from a 0/1 or vice versa. Off hand, I can't think of a way to do that with a third logic-value. Consider drawing even a tiny amount of current while a gate is sitting at logic "2" (or whatever you want to call the 3rd value). 1mA (milliAmp) times 1 million transistors on a chip = 1000 Amps. That chip's going to get a little hot!
Ok, so you've probably got at least two questions, which I will try to answer in advance. If you've got other questions - I'll just let someone else tackle those.
Q1) Why don't we just use a higher Vdd (supply voltage)?
A1) If you're using smaller transistor widths, you simply can't. When you use a really thin gate (i.e. 0.1um) on a transistor, the breakdown voltage of the gate is reduced. If you use a higher voltage, the transistor melts. (You could use larger transistors, but that kind of defeats the whole purpose! We make transistors smaller because we can fit more on a chip, and they operate faster and use less power.)
Q2) Can't we lower the threshold voltage?
A2) Yes, to some extent. (It's not always easy.) But we don't want to. Even when a transistor is "off", there is still a very small amount of leakage current flowing through it. If you reduce the Vth, you also increase the amount of leakage current. In older technologies, this hasn't been much of a problem, because the leakage current was so small in comparison to the dynamic power consumption. But as we are putting more and more transistors on a chip, the leakage power consumption in modern chips can easily add up to 30%-40% of the total power consumption. There's also another reason. If you did that, you would be lowering your noise margin. And you don't really have much control over the noise (which is why it's called that). If you reduce noise margins too much, you'll find it almost impossible to create a circuit that actually functions reliably.
Well, I hope that satisfies some of you (and doesn't get the rest of you too upset). VLSI circuit fabrication is a really neat field. Some of the tricks that are being used these days to fabricate that chip sitting in your computer and get it running at 2GHz (or aren't they up to 3GHz now?) are quite amazing - they're doing their best to cheat physics! Using a ternary counting system to build computers may have a lot of nice theoretical properties, but I can't see it displacing binary any time soon, except possibly in some really specialized applications. (There are always exceptions.)
That's my $0.03 worth. (Hey, I typed a lot. I think that's worth at least $0.01 extra. Maybe $0.025?.) Any errors in the above are mine, but I won't admit it.
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.
Panurge has posted for the last time. Thanks for the positive moderations.