COMPUTINGCOLUMN SECTION CONVERSATIONS TITLE Andrew S. Tanenbaum: The Impact of MINIX Charles Severance, University of Michigan Andrew S. Tanenbaum describes the motivation, development, and market impact of the MINIX operating system. he story of the evolution when teaching courses about OSes. crashing at random. The functionality of Unix and Unix-like At first, Tanenbaum used a booklet was there, but it would run for 15 min- operating systems written by John Lions ( John Lions’ utes and then crash. T is very convoluted. Commentary on Unix 6th ed., with Modern Unix-like OSes generally Source Code; Peer-to-Peer Communi- Tanenbaum knew he was close, derive from AT&T/BSD Unix or some cations, 1976) to teach his classes: but he had trouble solving that “one variation of Linux. But in the begin- last bug” so he wrote software to ning, there was only AT&T Unix, I was teaching a course on operating emulate the PC hardware: and if you trace the moment where systems using version 6 Unix; John the Linux-like OSes flickered to life, Lions wrote a book describing it line by I ran my operating system on a sim- you’ll find the MINIX OS. line that was very popular at universi- ulator for the PC that was entirely MINIX’s original purpose was ties. The bean counters at AT&T didn’t deterministic and reproducible, and simply to support university courses like this: having every computer sci- then after it crashed, I ran it again and that attempted to teach OSes. Es- ence student in the world learn about looked at the last 100,000 instructions sentially, it was a set of floppy disks its product was a horrible idea. Version to find out what happened. It took me that came with Andrew Tanen- 7 came out with a license that said, a little while to complete the simulator baum’s popular book, Operating “thou shalt not teach,” so you couldn’t and get it running. If I had been smart, Systems: Design and Implementa- teach version 7 Unix anymore. It has I might have invented VMWare in the tion, first published in 1987. I visited to be up there as one of the dumbest 1980s because I had done much of the him at Vrije Universiteit (VU), and mistakes in all of business history. work but my focus was on Unix. we talked about the motivation, development, and impact of that As an accomplished computer sci- His OS worked perfectly on the simple decision to write an OS to entist who had been teaching OSes simulator—it could run forever teach a course. Visit computer.org/ for many years, Tanenbaum knew and never crash. Seemingly, it only computingconversations to watch that writing a simple Unix-like OS crashed on the hardware. He was our discussion. wouldn’t be all that difficult: about to give up when he got a lucky break: EARLY DAYS I decided to write my own operating AT&T initially provided the source system that was compatible with V7 so I was confused until one of my stu- code to its Unix OS to universi- that I could teach a course on a Unix- dents, Robbert van Renesse, mentioned ties. It didn’t have an open source like operating system. I sat down and to me that when the Intel CPU chip gets license, but it was readily avail- began writing it, and I got it almost to hot, it gives interrupt 15. I wasn’t catch- able, which made it a natural choice the point where it worked, but it kept ing interrupt 15 because the manual 0018-9162/14/$31.00 © 2014 IEEE Published by the IEEE Computer Society JULY 2014 7 r7con.indd 7 6/25/14 4:48 PM COMPUTING CONVERSATIONS didn’t mention it. I put in a piece of academic exercises, while BSD Unix Because MINIX is based on a code to catch it, and sure enough, had 20 years of development and pro- very small kernel with most of within an hour of running it, I got, “Hi, duction experience. But AT&T wasn’t the OS functionality in user space I’m interrupt 15—this is impossible, yet done making strategic errors: processes, it’s possible to recover and you’ll never see this message.” from partial failures or even upgrade Kirk McKusick and others who wrote OS components on the fly without WRITING A NEW BOOK BSD formed a company called Berkeley taking the entire system down: Once MINIX was working, Tanen- Software Design Incorporated [BSDi] baum wanted to document it by to sell BSD Unix commercially. Their If you’re running some real-time system like a radio telescope or an electric power plant, you might not In the early days, there seemed to be no reason to view want to go down for upgrades, so MINIX or Linux as commercial products. a system that can update or repair itself while maintaining fairly high availability might have a niche. writing a book similar to the one phone number was 1-800-ITS-Unix. We’ve got it working in the lab now Lions wrote, something that ex- In another brilliant commercial move, for BeagleBoard, BeagleBone, and plained MINIX in great detail so that AT&T sued BSDi to get rid of it. AT&T BeagleBone Black and are about to it could be used to teach operating had no knowledge at all about how to release them as open source. systems courses: sell Unix, so it should have gone to the BSDi guys and said, “We want to buy Most chapters have three sections. your company—how much?” And it egardless of MINIX’s possi- The first is the general principles of a would have had successes: there would ble commercial potential, component, such as a file system. The have been no MS-DOS, no Windows, R there’s no question that second is how these general principles and the world would have been all when Andrew created MINIX, he apply to MINIX—how the file system is Unix. But instead, AT&T tried to stop set the wheels in motion for an organized and its key data structures. these guys, which blocked BSDi from OS market that isn’t the exclusive The third part of every chapter nar- going commercial for several years due domain of proprietary vendors. rates the source code. to this lawsuit. According to chaos theory, some- times when a butterfly flaps its Even though this was before While AT&T’s lawsuit effectively wings in Brazil it can cause a hur- the Internet, news of the new book stalled progress on commercializ- ricane in Florida. If Robbert had not spread rapidly. At one point, a book- ing BSD Unix, other OSes including casually mentioned interrupt 15, store invited him to give a talk that Linux began to establish their po- there would have been no MINIX and was so well attended it had to use sitions in the market. With all this no Linux and thus no Android (which the Santa Clara Convention Center as confusion and with Linux address- is based on Linux). The relative stock the venue. A worldwide best-seller, ing the market for an open source prices of Apple and Samsung might Operating Systems: Design and Imple- Unix-like system, Tanenbaum kept be quite different now. mentation, is now in its third edition. MINIX’s focus on education. LINUX A NEW FUTURE Charles Severance, Computing Linus Torvalds was a student who As the field has evolved, Tanenbaum Conversations column editor and picked up MINIX and started evolv- sees that there might be some value Computer’s multimedia editor, is ing the source code: in a highly reliable OS: a clinical associate professor and teaches in the School of Information Linus went out and specifically bought The EU decided to set up a couple of at the University of Michigan. Follow a PC for the purpose of running MINIX. programs where it could give a large him on Twitter @drchuck or contact He was in the MINIX Usenet news amount of money (about €2.5 million) him at [email protected]. group and spent a lot of time develop- to specific investigators and let them do ing Linux based on MINIX. their thing. I applied for this and got an ERC Advanced grant. With this fund- In the early days, there seemed to ing, we’re trying to go push MINIX, for Selected CS articles and be no reason to view MINIX or Linux embedded systems as well as the x86, columns are available for free at as commercial products. Both were especially in the area of high reliability. http://ComputingNow.computer.org. 8 COMPUTER r7con.indd 8 6/25/14 4:48 PM.