Name: EDSAC Simulator - Computerphile
Uploaded: 2020-03-27T17:07:36.000Z
Duration: 18 min 29 s
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We have here an emulator for the Cambridge EDSAC. It is a lot more

Adverbs of manner

user-friendly on this screen than it would have been initially when, remember,

you are dealing with five hole paper tape. That's what your program went in on

Your answers came back out on five-hole paper tape, which it was your job to put

through a teleprinter machine, which chattered away, read your tape, and

printed out the answers. Fortunately we have the printed out answers shown in

front of us. Here is the standard display of this emulator, so handily written by

our friend Martin Campbell-Kelly from the University of Warwick. You can

download this for yourself, it's free to use, and just play with it to your

heart's content. And it's very nice to have the

opportunity to play with a really genuine early first-generation computer.

>> Sean: We've been talking about John von Neumann or Johnny von Neumann  >> DFB: Call me Johnny!

>> Sean: Call me Johnny. Yeah! So EDSAC's being built in Cambridge? But how does it end up getting

connected to von Neumann? Is that what we're going? >> DFB: Yes, that's right. It's

the other way around of course - that the person who built EDSAC, Maurice

Wilkes, exact contemporary of Alan Turing. Both got first-class degrees in

mathematics. He was the one, Maurice Wilkes,  who went to

John Von Neumann's Summer School, in 1946, at the University of Pennsylvania, [in] the

Moore School of Engineering. Poor old Maurice had such difficulties with

raising the money, with getting his passport straight and all sorts of other

issues, that he actually arrived four weeks late, for a six-week course!  One has

to remember, of course, in those days, it was not straightforward. You were talking

about ships, you know, going via Iceland, and heaven knows where, and train

journeys to finish off, down to Pennsylvania. But anyway, I think he must

have had - what was the technology? -  Roneo'd or Cyclostyled notes - when he got

there. He got copies of the notes and was able to follow exactly what was going on,

learned about the proposed  von Neumann architecture, knew

that there was going to be this thing called EDVAC -  being built in Pennsylvania

and he and all the other participants were more or less I think, invited to go off, do your

own thing, support the cause of general purpose,

binary-based, digital computers; all electronic, not electromechanical - for the

inner electronics. And, in fact, it's worth saying something about this whole

business about "What did it cost you to be all electronic in those days"? I would

remind you that Tommy Flowers' decision, with Max Newman, to build Colossus out of

thermionic valves - that's vacuum tubes for those of you still in North America ! -

it did cost you. Yes, you could switch them on and switch them off. You could use them as

logic elements. But boy did they consume electricity! And then, of course, you

have to have air conditioning to get rid of the heat that they'd produced!

All the time what held things up was  you wanted to use them as switching

elements, but you really paid the price in terms of power consumption. So Tommy

Flowers and his special purpose Colossus machine - six kilowatts. EDSAC, now  being

rebuilt in Cambridge (correction: at Bletchley Park) The EDSAC rebuild should go live this coming summer, 2018.

Watch out for that. A good time to visit the National Museum of Computing which

is co-located on the Bletchley Park site. Power consumption of EDSAC? 11 kilowatts.

For comparison:  power consumption of ENIAC the gunnery-trajectory

special-purpose computer 1946, University of Pennsylvania, USA. I bet you can't

remember the number, Sean! It's horrifying! >> Sean: 20?  >> DFB: 150 kilowatts! All of that using

thermionic valves for the logic elements inside your computer yes that could

certainly be done. The big problem was memory.

There was really no obvious technology for memory. You get round it on the

Colossus machine by saying well your data, which you want to remember, just

cycles around on a continuous loop [of paper tape] The program - well that's easy, that's not

on tape because that is such a special purpose machine you key in the

program on switches  -  uniselectors - so your memory there (for your data)  was

paper tape. But if you didn't want paper tape whizzing round at 30 miles an hour

what other options were available to you? And the answer is - not many. And as we'll

find out, on EDSAC, with Maurice Wilkes coming out of a radar background, he knew

that in the early days of radar, when pulses came along - and these are

interesting pulses could it be an approaching German bomber - you wanted to

not only get hold of the pulse but to hang on to it, and remember it, so you

could analyze its shape and  so on And the way they did that is the most

amazing technology. It's called mercury delay lines. The bleep pulse that you get

was put, basically, through a little loudspeaker so it went "bleep". But instead

of transmitting through air, so you could hear it, it was transmitted down a column

of mercury. Why? Because sound travels a lot slower in mercury than it does in

air and at the other end there was a microphone, at the other end of your tube,

which heard the bleep and displayed it on an oscilloscope screen for you. But

then recycled it - took it back to the start of the mercury tube and sent it

down again so you had a continuously circulating system that showed you what

your pulses were and the idea was to use that same technology to represent the

0s and 1s in computer memory, which they did. It was not very satisfactory, in

so many ways, but you just had to do your best.

>> Sean: Presumably it deteriorate didn't it, this sound ... ? >> DFB: No, because when you

detected it, at the other end, you amplified it before you fed it back

again. So you could keep the purity of the pulse going for as long as you

wanted to. That was the idea. So, the story continues. Maurice Wilkes came back to

England and in a testament, really, to Cambridge's being fairly well-heeled

and to their faith in Maurice and to their need to be at the forefront of all

this stuff, they - by "they" I mean the University and the Mathematics

Department at Cambridge - agreed to bankroll Maurice to build one of these

new-fangled machines provided that when it was completed it could offer a

service to the University. This is very different to what happened in most of

the universities, where electronics engineers were just allowed to build

systems where you [the engineers] just played with them, and didn't really offer a computing

service to the community at all. They were just pure research machines. Maurice found

himself a very good hardware engineer called Bill Renwick

who was just excellent. You had to have this. You had to have a real devoted

electronics engineer, who knew what they were doing, to get all of this working.

When you look at it you've got this box - it's hardware, you've got to get it

working. You've got to get instructions in it; you've got to make it work but you

need a software person. And you need the world's first ever system-software

person - and they'd better be good! Somehow these people seem to come along, as and

when they're needed, I can never understand quite why. And the answer at

Cambridge was a really talented person - who I did know but rather slightly -

called David - David Wheeler. We can now say that as part of the celebrations

this summer, when the EDSAC rebuild is switched on and works perfectly(!) ... To go

along with that a very good friend of ours on Computerphile, Professor Martin

Campbell-Kelly, developed an emulation of EDSAC and how it works. So, rather than

talking in abstract about EDSAC let's get stuck in and run some programs on

EDSAC and see what you're up against with a very genuine first-generation