Re: [TUHS] Paper discussing Unix boot process?

I was going to mention the Multics init PLM! :-) Needless to say, it's probably not a good candidate for the original goal - a document describing how an OS boots - it's simply too complicated for ordinary mortals! (Reading it makes _my_ head hurt!) There are a couple of reasons it's so complex. Multics is not a monolitic OS, the way most versions of Unix are (although I gather this is no longer quite true of Linux); the OS isn't this large blob of bits you load into memory and start. Its structure makes heavy use of the segmentation model supported by the hardware. Moreover, although the first segments loaded aren't paged, many of the later ones are. (This makes sense in the context of the times; with limited core main memory, you wouldn't want to devote massive chunks of main memory to have the entire OS always resident.) However, this all makes for a more complex booting process; the standard Multics boot tape (a Multics System Tape) contains many modules, which get linked in individually at boot time. (In V6 terms - the version I'm most familiar with - it's as if a Unix boot tape contained 'lib1' and 'lib2', and the bootstrap included a linker to build the OS binary in memory.) And in fact the modules come in in tranches, and some of the earlier one are available for use in loading later tranches (e.g. paging). This does have some advantages, though; e.g. the MST is the same for all Multics machines (including the initial boot of a new machine); the system is customized to the particular configuration during the bootstrap process. This is actually not too crazy, there are reasons this makes sense. For one, the whole 'information utility' concept (where Multics admittedly went down the wrong path, in terms of the future of computers); a single giant machine (multi-processor, multi-memory bank, multi-I/O controller). The thing is that any of these could be switched out if it developed a fault, but then you have to be able to boot that new configuration. (In particular, Multics wasn't a master-slave multi-processor system, they're all the same; but only one CPU runs for most of booting, the others are started and added once the system is running. But the 'bootstrap CPU' might change if the original bootstrap CPU develops a fault...) Noel