Dumb OS question

It's not a dumb question! It is, in some respects, a very controversial question. In very general terms, a real time operating system is what is used when a response from the computer to an external event is required in a "timely" fashion. For example, if it's a fly-by-wire jet, and the pilot pulls the control stick back, you don't want the computer to delay in making the control surfaces move because the co-pilot is doing some lengthy navigational or fuel consumption calculations at the same time. For a multi-tasking operating system, this generally means two things: 1. When an external stimulus comes in, there has to be some way of making sure it will interrupt tasks of lower priority. 2. After the higher-priority task is started, it has to finish in a fairly predictable amount of time. Depending on the nature of the real-time task and/or the person defining "real-time", you also often find some further requirements. There are also folks who split the definition into "soft real-time", where it's OK if you're occasionally late with an answer or occasionally take too long to respond, and into "hard real-time", where the computer is never ever allowed to be late in responding or answering. Traditional Unices are not, by any measure, real-time operating systems. There are Unix-like OS's that are called "real-time operating systems", but many of the "hard real-timers" will disagree with this. A more modern approach to "real-timeliness" considers not only the main CPU, but also subsidiary CPU's and systems. For example, if a response to a stimulus requires doing any disk reads or writes, you may need to thoroughly know the worst-case response time of the disk drive. Modern disk drives (or even many disk drives from a decade ago) have microprocessors in them and for a truly safety-critical application it may be necessary to thoroughly review and qualify the firmware that's in the disk drive to be sure that it will always respond in a certain minimum amount of time. Some real-time operating systems can be pressed into service as general-purpose multi-user OS's. For example, RSX-11M. Others make quite nice single-user development platforms - for example RT-11. Modern network-in-the-kernel OS's are very difficult to turn into "real-time" systems with much functionality. You ever have a NFS server go down and tie up a campus full of workstations for minutes at a time? -- Tim Shoppa Email: shoppa(a)trailing-edge.com Trailing Edge Technology WWW: http://www.trailing-edge.com/ 7328 Bradley Blvd Voice: 301-767-5917 Bethesda, MD, USA 20817 Fax: 301-767-5927