7 ms·
Building a vacuum tube computer
- PaulHoule 7y agoRISC-V has pretty big registers and a lot of them, so you will need more tubes than you otherwise might...
- causality0 7y ago>but electron speed in cupper is about 15-20cm per nanosecond so it is usually not a big factor. I'm going to be extremely pedantic for a moment, but this is a case where there's a bit of a difference between electricity and electrons. The speed of electricity, aka electrical energy, aka electric field propagation through copper is 15-20cm per nanosecond. The speed of electrons, aka drift velocity, is far slower and governed by the current and conductor cross-section. Vd=I/neA. For a 91w TDP desktop CPU at maximum current draw, the drift velocity through the power connector would be about 245cm per hour. Yes, per hour. For AC circuits the drift velocity is effectively zero because the electrons vibrate back and forth around their starting position.
- NikolaNovak 7y agoThis blew my high-school-physics mind; is there a quick primer / ELI5 on this somewhere? Are you saying that if I connect a battery to a lightbulb, "electricity" moves at speeds we're accustomed to, but "electrons" will only move from one side terminal of battery, through a bulb, to the other side, way way slower than laypeople think? Or did I misread and oversimplify your message? :|
- rrss 7y agoyeah, electrical power propagates way faster than electrons drift (and the power flux is primarily outside the wire, but that's a fun story for a different day). One way to think about it is as a newton's cradle sorta deal. It's not perfect, but fairly close. Also maybe flicking a jump rope - energy propagates, but the bits of rope don't move from beginning to end.
- dillonmckay 7y agoBefore I read your comment, I thought the description sounded like Newton’s Cradle. Flicking a jump rope sounds more akin to AC to me, though. Maybe, pulsed DC? What about a train or bumper car analogy (not joking)? I would really like to see this as a visualization.
- kirrent 7y agoAbsolutely. Electrons are accelerated by the field until they hit something. The average velocity of all the electrons due to the competing acceleration and collisions is the drift velocity and it's very slow. On the other hand, electric fields move quickly. At some large proportion of the speed of light depending on the particular materials of the wire and some other factors.
- userbinator 7y agoThink of a large tube filled with tiny balls. If you push on one end, the other end moves almost immediately, but the balls themselves are barely moving.
- 7y ago
- PhilWright 7y agoMaybe they only recently decided to target RISC-V but the ALU design does not cover all the functions needed. It is missing implementation of the shift instructions, for that they need a 32 bit 5-level barrel shifter. Plus they also need a comparison block which is quite complex on its own. Given the number of years it has taken to get to this point, if could be 2050 before it is working.
- Animats 7y agoYou don't need a barrel shifter. You can shift one bit at a time. It's slower, but takes far fewer components. Capacitor memory addressed with tubes seems a strange choice. DRAM is capacitor memory, of course. Atanasoff, who had a a sort of computer in 1939, had capacitor memory, but he had to use a drum rotary switch to address it. Memory was the big problem in the early days. IBM had an electronic multiplier before WWII, and plugboard-programmed machines, but no good memory elements. (Just registers with motor driven wheels and clutches and contacts.) Pilot ACE had a delay line (slow, serial access), the Manchester Baby had a Williams tube (too expensive per bit, but random access), and the EDVAC had mercury tank delay lines (slow, serial access, and toxic). Whirlwind (1951) had the first core memory (expensive per bit, but got cheaper over time.) Core would be a reasonable choice for a tube system. Addressing is XY, so you need O(sqrt(N)) tubes. Memory was a million dollars a megabyte as late as 1970.
- Taniwha 7y agoProbably more, we bought 1.5Mb of core for our B6700 in the mid to late 70s for ~US$1.25
- segfaultbuserr 7y ago> You don't need a barrel shifter. You can shift one bit at a time. It's slower, but takes far fewer components. I think I've finally found someone to ask this question. How many transistors and space did it take to implement a 8-bit barrel shifter in the 1980s? I found the computing capabilities of 8-bit CPUs in retrocomputers, like the Z80 or 6809, were actually not too bad. With shift-and-add, a lot of computation can be done effectively, but their biggest limitation is the lack of a barrel shifter. Without one, no constant-time bitshift is possible with variable step, you have to shift 1-bit at a time. Performance could be boosted dramatically if a single 8-bit barrel shifter is included, and would open the avenue of a lot of optimization techniques for graphics as well. Why didn't they include one? Especially when you consider that the Z80 was an "ultimate upgrade" of the Intel 8080, and the 6809 was the "ultimate upgrade" of the 6800. Was it technical limitations, i.e. even a 8-bit barrel shifter is still too expensive for a 8-bit chip? Or was it that the cost of additional shift instructions decoding/processing is much higher than the barrel shifter itself? Or was it the lack of demand, i.e. it could be done reasonably well with acceptable cost, but simply no commercial reason in the 8-bit era to add one?
- jhallenworld 7y agoI would not target RISC-V- they need to review much simpler machines, like Princeton IAS or Manchester Baby, and they should try a bit serial design. https://en.wikipedia.org/wiki/Manchester_Baby https://en.wikipedia.org/wiki/Manchester_Baby "By June 1948 the Baby had been built and was working.[23] It was 17 feet (5.2 m) in length, 7 feet 4 inches (2.24 m) tall, and weighed almost 1 long ton (1.0 t). The machine contained 550 valves (vacuum tubes)—300 diodes and 250 pentodes—and had a power consumption of 3500 watts." Also look at super tiny tubes: https://tubes-store.com/index.php?cPath=30_101 https://tubes-store.com/index.php?cPath=30_101
- tachyonbeam 7y agoThe super tiny tubes are cute. They look like they have to be soldered in though. Could be annoying to replace when they break down? The bigger tubes are socketed.
- klodolph 7y agoThere are special types of high-reliability tubes used for computers. If I recall correctly, one of the tradeoffs is that they use a different alloy for the wire, which makes the wire more expensive to manufacture because the process wears out the tooling more quickly. I can’t find the article where I found this information, though.
- C1sc0cat 7y agoThe key to long life for valve (tube) computers was never turning it off.
- klodolph 7y agoThat’s not really the key. When ENIAC was first run, several tubes would fail each day—and yes, it was turned on all the time. They addressed these problems by adjusting heater currents. In general, there’s a tradeoff here because you don’t want to strip the cathode (which can wear out a tube quickly) and you want good performance from the tube. The on-off cycle effect on filaments is also largely due to surge currents with a cold tube, at least for typical designs, and there are various techniques to limit the surge currents which will extend tube life.
- rrss 7y agoSome of the early tube computers had some really interesting circuits. One that comes to mind is the use of "kirchoff adders," which are more-or-less analog adders with easy one-bit DACs and ADCs on the inputs/outputs. I can't remember if this ended up in the "final" IAS machine, but I'm pretty sure it was used in the MANIAC and is described in the "Second Interim Progress Report on the Physical Realization of an Electronic Computing Instrument": https://library.ias.edu/files/pdfs/ecp/secondinterimpro02inst.pdf https://library.ias.edu/files/pdfs/ecp/secondinterimpro02ins....
- ncmncm 7y agoSeems like a PDP-8 would be a smarter start. Only 8 instructions, IIRC, and a few 12-bit registers. And it can emulate a PDP-11, so (the emulator) could run UNIX.
- segfaultbuserr 7y agoDefinitely. BTW, PDP-11 (and PDP-8?) uses 74 logic chips, not discrete transistors, so I would say it's already complex enough for tubes.
- VLM 7y agoThe original -8 (no slash anything) used discrete transistors and DTL logic. Going the opposite direction I always wanted to reimplement the flipchip modules using SMD technology just for fun. A discrete transistor SMD PDP8 that fits on a desk would superficially seem realistic, at least for a large enough desk. Honestly I just don't want to pay for the connectors and wire the backplane connectors. The SMD components could turn 60s era "handheld" flipchip modules into postage stamps, but no technological progress has happened for wire wrapped backplanes so my SMD design's backplane would remain exactly the same size and complication as the original -8, unfortunately. I'd have a giant tangle of wire wrap wires and a couple hundred postage stamp size "nano-flipchips". Probably 50 pounds of interconnect copper and 1 pound of circuit boards, LOL. This is whats likely to get Op. Its easy to make an adder or a latch, but takes an entire spool of wire to connect one up to the overall circuit. Lots of work. Sure, if you use 1950s germanium transistors, RTL and DTL logic is no fun anymore. I always wanted to try RTL and DTL using something more fun, like 2010s era microwave low power amplifier FETs. I would not like to pay for it, but it would be a lot of fun to build a 50 GHz ALU or similar. I have no idea what I'd do with it other than say "wow" a lot, which makes it an ideal hobby experiment.
- juliangoldsmith 7y agoAs far as backplanes go, you could go with FPC cables instead of wire-wrapping. That would fit the new components theme, but would require actual circuit boards for the backplanes.