Programming the PDP11, part 2 of 4Show Video Details ↓ Tara: All this work for such a short program? Man: Yeah, you're right that is kind of a pain. That's why DEC developed their paper tape software. Tara: What's paper tape? Man: It's very similar to the punch cards that I'm sure you've used. I have some over there if you want to hand it to me. Yeah, thank you. If you look at one of these tapes, well, here it is. Tara: What are the holes for? Man: Ah, good question. The paper tape holds binary 0s and 1s. A hole represents a 1 and each position that does not have a hole represents a 0. The paper tape reader uses light to sense the presence or absence of holes. There are 8 positions across a row of tape therefore each row holds 1 byte of data. The entire tape then is a collection of bytes. Tara: So this tape is kind of like a file? Man: Exactly. Tara: How do you keep all these tapes straight? How do you know what each is named? Man: Ah, good question. We either get a label and type the name of the tape on it or, if I'm really kind of lazy I can get out my marker and just kind of write the name of the tape on it. It's really kind of neat because we can use as many letters as we want and any kind of font that we want and it's probably better than some of the current operating systems like they write for the PDP-11 that restricts you to have like 6 character names and a 3 character extension like PROG dot BAS. In fact, I really can't imagine anybody is going to put up with that kind of naming scheme for much longer. There's got to be a better way to do it. Anyway, the nice thing about these tapes is you can write however many characters you want to on it. Tara: So when you want to get rid of a file, do you just put it in the trash? Man: You bet. And the good thing is that if we accidentally delete a file I can recover it from the waste basket as long as a janitor hasn't emptied it, then it's kind of too far gone. But it is kind of a neat idea. Tara: Just like windows. Man: Ummm. Right. I'm not quite sure what you mean by windows. It does get pretty hot in the summer in here and I guess some windows would be nice. Tara: Uh, right. Dave: So what was that you were saying about loaders? Man: Ah, right. A loader's a program that takes bytes from paper tape and loads them into memory. Usually the paper tape contains a machine language program and once that's loaded into memory we can then execute the program. The problem is, how do we get the loader into memory to begin with? Tara: I'm confused. Doesn't the computer already know how to program from the paper tape? I know the computer that I use boots the disk without doing anything special. Man: No. The only programs that the computer can execute are the ones that have been loaded somehow. And until we get the loader itself loaded into memory, we can't read any paper tapes into memory. As far as your computer, you must be using one of the new ones where the loading program is kind of already hard wired in. Dave: You know, he's right because in your computer the loader's in the ROM so you never really have to work with it directly. Man: What? Dave: Well could you just tell us how you get the loader into memory? Man: Good question. We toggle it. Here, look at the PDP-11 programming card. It contains a copy of the bootstrap loader. All we have to do is toggle this program into memory and we're in like Flynn. Tara: So now that we have that done, what kind of programs do we have on paper tape? Man: Let's see. I have 3D tic-tac-toe, we have a star trek game... Dave: Look, these programs and games and stuff sound interesting and all, but we really don't have time for that right now. Could you just show us how you write programs on this computer? |