PDP-1 Spacewar! program internals
Lee Courtney (ACM)
lee_courtney at acm.org
Wed Oct 22 11:26:53 CDT 2008
Hi Al,
Very nice interview. Is there an audio of the entire interview available on the Museum web site, or Youtube?
Lee Courtney
--- On Tue, 10/21/08, Al Kossow <aek at spies.com> wrote:
> From: Al Kossow <aek at spies.com>
> Subject: PDP-1 Spacewar! program internals
> To: classiccmp at classiccmp.org
> Date: Tuesday, October 21, 2008, 11:00 AM
> Here is part of an oral history I recorded with Steve Russel
> last
> August which discusses how the Spacewar! program works
> internally.
>
>
> Exerpt of an oral history of Steve Russell
> August 2008
> CHM Reference Number X4970.2008
> (C) 2008 Computer History Museum
>
>
> Russell: Anyway, the PDP-1 arrived, and Marvin Minsky
> wrote the tripos
> demonstration, generally called the Minskytron,
> and there was the famous weekend
> where the mob of undergraduates transcribed the
> macro assembler from TX-0 to the
> PDP-1, because they didn't like FRAP. And
> then fairly quickly thereafter, they
> wrote DDT and connected up the macro symbols to
> DDT. So that was all sort of in
> place by the middle of the fall of 1961. And the
> combination of the Minskytron and
> having DDT with interactive debugging with
> symbols was very tempting. I don't
> remember the exact order of things, but I'm
> pretty sure I started talking up a better
> demonstration program than the Minskytron, and
> eventually, Alan Kotok went up
> to Maynard and collected the sine and cosine
> routines from DECUS, presented
> them to me, and said, "Okay, here are the
> sine and cosine routines; now what's your
> excuse?" And I discovered I had run out of
> excuses; I had to actually think. And so
> I started work and figured out the basic trick
> of Spacewar! display which is that you
> only need to calculate a unit vector pointing in
> the direction of the spaceship. And
> you can express everything else the spaceship
> does, and the outline of the spaceship
> in terms of that unit vector, suitably scaled.
> So it's basically a lot of addition in the
> usual program upkeep.
>
> Kossow: Do you want to just give an overview, then, of how
> Spacewar! actually
> works?
>
> Russell: It's one big loop, and the loop is on the
> displayable objects. And I called
> them displayable objects, although I didn't
> know about object orientation or object-
> oriented programming at the time.
>
> Kossow: So you have the sun--
>
> Russell: Colliding objects, not displayable objects. The
> colliding object is a space
> ship, there are two of those. And that has a lot
> of extra data with it. It shares the
> position and velocity tables with all of the
> torpedoes and explosions that are running
> around. So there's just one big loop through
> the colliding objects, and it looks at all
> the higher-numbered colliding objects to see if
> there's a collision, using an octagon
> because you don't need to calculate the
> square root of anything, you can do that by
> work on X difference, Y difference, and X+Y
> difference--
>
> Kossow: So the bounding box for the collision detection is
> an octagon?
>
> Russell: Yes. So it goes through, it sees if this object is
> colliding with any higher-
> numbered object. If it is, it replaces the
> calculation routines. That's another thing
> that every colliding object has, is a
> calculation routine. It replaces the calculation
> routine with the explosion calculation routine.
> And then things take care of
> themselves. Then, after it's decided whether
> it's an explosion or not, it goes off to
> the calculation routine. And the calculation
> routine updates the position, since all
> colliding objects have velocity; and if it's
> a spaceship, it worries about reading the
> controls and updating the other things about the
> spaceship in deciding whether to
> launch a torpedo or not. And if a torpedo needs
> to be launched, it searches up the
> colliding object table for an empty slot,
> indicated by having no calculation routine.
> It searches up the table for an empty slot, puts
> a torpedo calculation routine there,
> and the spaceship position plus the suitable
> increments, so it won't run into its
> torpedo, and of the velocity of the spaceship
> plus an increment for the torpedo, and
> it goes on. When that the main loop gets done,
> you go off and do some star display
> and display the sun, and calculate -- and part
> of the spaceship calculation is to
> calculate the effect of gravity on the
> spaceship. Originally, there wasn't any gravity,
> and I had an interpreter, which interpreted the
> outline description, and Dan
> Edwards, sometime in late 1961 or early 1962,
> looked at that code and decided if he
> could write a special purpose compiler which
> would compile precisely the right
> code, and proceeded to. And there's one
> compiled outline for each spaceship; each
> spaceship actually does half of the spaceship
> outline and then you twiddle the
> vectors and do the other half. That keeps the
> display running just as fast as it can.
> That gives time to calculate the effect of
> gravity on the two spaceships, but not on
> the torpedoes. So we decided that they were
> photon torpedoes not affected by
> gravity.
>
> Kossow: So when the explosion routine starts, it continues
> calculating motion, so
> the explosion moves?
>
> Russell: Yes. If you see two spaceships collide, if you
> watch closely, you will see
> that there are two explosions that continue off
> in the direction that the two
> spaceships were going. There is another number
> in the table for all colliding
> objects, which is the size. And this is, roughly
> speaking, proportional to the amount
> of computing it takes to compute that object.
> And at the end of the loop, as you go
> through the main loop, you accumulate the sizes
> also; and so at the end of the loop,
> there's fritter away time loop that attempts
> to keep the frame rate approximately
> constant. It doesn't do a wonderful job;
> it's visually adequate, but God help you
> when you try to take a movie of it.
>
> Kossow: One of the complaints with all modern kids trying
> to play it now is
> that it's TOO SLOW.
>
> Russell: Kids who are used to something like Asteroids seem
> to think that. But
> when we do the demos, we get a number of people
> who seem to be still quite
> addicted to it with the old, slow version. Now,
> that was always a complaint; the
> reason that all the parameters got accumulated
> in the first page of the listing, which
> says you can put that first page of the listing
> of the console, and anyone who wanted
> to try a different set of parameters could. But
> the ones that were compiled in or
> assembled in were the ones that I thought were
> good. Now it turns out I'm not a
> representative arcade game player, and so my
> version of the parameters is slower
> and gives more opportunity for marksmanship than
> the arcade version.
>
> Kossow: Right, that's the whole thing with gravity and
> doing the, what's that called,
> where you whip around the sun? Does it have a
> name, where you whip around the
> sun and you shoot?
>
> Russell: The closest name is the "CBS maneuver"
> is what happens when two lazy
> experts fight each other, which is they both
> turn at right angles to the sun, and fire
> for 3.5 or 4 seconds, so they're now in
> stable orbits, and they know it. And then they
> turn at each other and start trying to place
> torpedoes where they think the other one
> is going to be. And so the trails turn into an
> eye around the sun, and CBS used that
> as a logo, so it got called the "CBS
> maneuver". You can-- one of the spaceships can
> go the other way around the sun, so that both
> ships meet on the same side of the
> sun. But that seems usually to have less chance
> of winning. Not much less, but
> somewhat less.
>
> Kossow: It's better that you stay on opposite sides,
> then?
>
> Russell: Yes.
>
> Kossow: And then at some point, you added hyperspace?
>
> Russell: Yes, and we realized that that was going-- I
> don't remember whether we
> actually had it. I may have had it for a little
> while with no limit, but it became very
> clear that someone who didn't understand
> could use hyperspace to escape their
> proper justice forever, and so we added the
> unreliability of hyperfield generators
> very quickly. One thing that Asteroids and the
> arcade versions, the later arcade
> versions of Spacewar! added, which actually was
> a big help, especially with their
> high acceleration rates, was "training
> mode" where space was actually viscous. So
> if you got your ship accelerated so that it was
> going across the screen so fast you
> couldn't understand what was going on, if
> you took your hands off, the situation
> would gradually become understandable. I
> don't think I would have been persuaded
> to do that in the original Spacewar! because it
> was unrealistic. But it definitely
> made it easier to learn.
>
> Kossow: So are there any other favorite anecdotes about
> Spacewar!, or just the
> spread of Spacewar!?
>
> Russell: Well, the "imitation is the sincerest form
> of flattery"-- many people saw
> Spacewar! as, some people ask for copies of the
> source, and of course we gave them
> out because we very briefly considered trying to
> sell Spacewar!. We realized the
> only possible customer was Digital Equipment,
> and we also, on a little reflection,
> that they were too cheap to do it. So we gave it
> out to anyone who wanted it, and
> some people got the listing and thought about
> it, and by reading it-- a lot of people
> simply saw the game and had a computer that
> wasn't the PDP-1 but did have a
> display, and implemented Spacewar! their own
> way. I suspect most of them figured
> out the "basic trick", but I'm not
> sure.
>
> Kossow: What "basic trick" were you thinking of?
>
> Russell: That you could do everything based on the
> spaceship unit vector. How are
> you fixed for the source code for different
> implementations of Spacewar!?
>
> Kossow: We have a few. I don't know if we have the
> PDP-10 version. We have the
> 12 version, and I think we have a PDP-7 version.
>
> Russell: I think Bob Saunders wrote the PDP-7 version. I
> think one 6 version
> simply ran on the PDP-1 simulator. I think there
> must have been others, but I don't
> know. Something for some history grad student to
> pursue. When we were running a
> demo for the Yelp event, there was one woman who
> had done Spacewar! in turtle
> graphics as a high school programming project,
> which she wasn't too happy with.
> She seemed to like the demo of the original
> Spacewar!.
>
> Kossow: So turtle graphics running on a micro or something
> like that?
>
> Russell: I didn't quiz her. I didn't have the
> opportunity to quiz her further.
>
> Kossow: So she thought this version was better?
>
> Russell: No, she just thought it was nice to see the
> original.
>
> Kossow: So she was in her mid-20s?
>
> Russell: 20s or early 30s; probably 20s. The DEC field
> service story where the
> DEC production people got into the practice of
> loading Spacewar! the last thing
> before PDP-1 shipped, and field service would
> then unpack it, make sure that
> nothing horrible had happened, tryed turning on
> power, and starting Spacewar!.
> And if it worked, they would call the customer
> over and say, "See, it works." If it
> didn't work, then they'd worry about it.
> In the restoration project, we had a little
> reflection and we decided that probably if
> Spacewar! works, just about everything
> works, as far as machine instructions go. It
> doesn't guarantee all the I/O gear works,
> but it does multiply and divides, and just about
> every instruction. So a lot of people
> implemented Spacewar! just from knowing that it
> existed and having seen it maybe
> once.
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