Seibel: After school you got hired directly into the Bell Labs; how did thathappen? It doesn’t sound like you were a classical academic researcher atthat point in your career.

Thompson: I just drifted. It was hard to describe. I certainly wasn’t inschool in any real sense. In the formal sense, yes, I was. One of myprofessors, who is actually a very good friend, sicced the Bell Labs recruiteron me. But I wasn’t looking for a job. In fact, I had absolutely no ambitions;nothing. And he made me appointments to see him in his little recruitingbooth, and I either slept through them or told him I wasn’t interested. Andhe kept after me. At some point he called me and said that he wanted tocome over and see me. So he came over to my apartment. And said that hewanted me to come out and interview at Bell Labs. I told him no. And hesaid, “It’s a free trip. You can do what you want to out there.” And I say,“Well, up front I’ll just tell you that I’m not interested in a job. I’ll be glad togo for a free trip ’cause I have friends on the East Coast. I’ll go visit them.”And he says, “Fine.” So that was the interview that I got into. And I wentand spent my two days at Bell Labs and then rented a car and went up anddown the East Coast visiting my high-school friends that were spread out allover everywhere.

Seibel: Obviously there was something that the folks at Bell Labs saw inyou and said, “We should get this guy into our lab.”

Thompson: I don’t know their side of it. My side of it is that these arepeople that I was reading the papers of in the classes I was taking/teaching.And I knew them by name and reputation. And they were still doing funthings. To me, work was work and these guys weren’t working. They werehaving a good time. Just like school.

Seibel: And so what kind of things did you do when you first arrived there?

Thompson: Bell Labs was in the MULTICS project and I was hired in towork on MULTICS. And I did. I played with the machines, booted upMULTICS, and did my little piece of it. At some point, Bell Labs decided thatMULTICS wasn’t for them, and they backed out of the project.

But they had these MULTICS machines which were special-purposemachines that were just sitting around idle until someone could cart themaway. So for approximately a year I had this machine that was monstrous.There are maybe two or three of us that used it. So I started doingoperating-system stuff, trying to get a little operating system up and running.

It was insanely hard because it was a real complex computer. But I got it upwhere it would sit there and say hello on 50 Teletypes around the building.And then it went out the door. So I shopped around then and found someother unused machines and essentially built Unix on these very, very smallPDP machines.

Seibel: Did you have the time to do that because your bosses knew thatthat’s what you were doing and said this is a good research project, or wasit just because you were in between jobs?

Thompson: No, I was sort of incorrigible, to be honest. I suspected that Iwould eventually get fired, but it didn’t bother me. We were supposed tobe doing basic research but there was some basic research we should bedoing and some basic research we shouldn’t be doing. And just coming outof the ashes of MULTICS, operating systems was one of those basicresearch things we shouldn’t be doing. Because we tried it, it didn’t work, itwas a huge failure, it was expensive; let’s drop it. So I kind of expected thatfor what I was doing I was going to eventually get fired. I didn’t.

Seibel: How do you design software? Do you scribble on graph paper orfire up a UML tool or just start coding?

Thompson: Depends on how big it is. Most of the time, it sits in the backof my mind—nothing on paper—for a period of time and I’ll concentrate onthe hard parts. The easy parts just fade away—just write ’em down; they’llcome right out of your fingertips when you’re ready. But the hard parts I’llsit and let it germinate for a period of time, a month maybe. At some pointpieces will start dropping out at the bottom and I can see the pyramid buildup out of the pieces. When the pyramid gets high enough in my mind, thenI’ll start at the bottom.

Seibel: But you’re not just building leaves—you know the structure they’regoing to fit into.

Thompson: Suppose someone is describing something to me frompostulates like, “Here’s a computer and here are the op codes.” I canvisualize the structure of programs and how things are efficient or inefficientbased on those op codes, by seeing the bottom and imagining the hierarchy.And I can see the same thing with programs. If someone shows me libraryroutines or basic bottom-level things, I can see how you can build that intodifferent programs and what’s missing—the kinds of programs that wouldstill be hard to write. So I can envision that pyramid, and the problem is totry and decompose it and get the bottom pieces.

Modern programming scares me in many respects, where they will just buildlayer after layer after layer that does nothing except translate. It confusesme to read a program which you must read top-down. It says “dosomething.” And you go find “something.” And you read it and it says, “dosomething else” and you go find something and it says, “do something else”and it goes back to the top maybe. And nothing gets done. It’s justrelegating the problem to a deeper and deeper level. I can’t keep it in mymind—I can’t understand it.

Seibel: So why not still read bottom-up? The leaves are there, somewhere.

Thompson: Well, you don’t know what are leaves and what aren’t. If it’swell described you can read the English and get it and then you don’t haveto read the code. But if you’re actually just given a bunch of code and told,“Read it and try and make it better or try and make it do something else,”then typically I read it top-down.

Seibel: Do you ever write down anything before you start writing code?

Thompson: I usually write down data structures before I write downcode. I don’t write down algorithms—no flowcharts, or stuff like that. Butthe stuff you have to refer to on almost every line of code—data structures.

Seibel: If you’re writing a C program, does that mean C code that woulddefine those data structures?

Thompson: No, little boxes with arrows and stuff.

Seibel: So you’ve got this big picture, the pyramid. How much do you stickto that plan once you start coding?

Thompson: I don’t stick to code. If I find a different partitioning halfwaythrough, I’ll just hack and go over it. A lot of people I know, when theywrite a line of code, it’s concrete from then on for the rest of life, unlessthere’s a bug. Especially if they write a routine with an API and scribble theAPI somewhere on an envelope or an API list—then that’s it. It’ll neverchange, no matter how bad it is. And I’ve always been totally willing to hackthings apart if I find a different way that fits better or a different partitioning.I’ve never been a lover of existing code. Code by itself almost rots and it’sgotta be rewritten. Even when nothing has changed, for some reason it rots.

Seibel: How do you decide when code needs to be thrown away?

Thompson: When it’s hard to work on. I do it much quicker than mostpeople do. I’ll throw away code as soon I want to add something to it and Iget the feeling that what I have to do to add it is too hard. I’ll throw it awayand start over and come up with a different partitioning that makes it easyto do whatever I wanted to do. I’m really quick on the trigger for throwingstuff out.

Seibel: Is that true working with other people’s code as well?