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Retrocomputing

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    re: it's as if all files are memory-mapped Not 'as if' - they are. Storage is addressed through virtual memory. If you really have to, you can build 'read' and 'write' routines on top of that. Commented Jul 20, 2020 at 21:14
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    re: Whether this is superior to the traditional file-based model … Multics has a file system; in fact it invented the hierarchical file system. The debate is rather between read/write primitives and memory mapping. Memory mapping is attractive, but it seems to me your file size has to fit in your address space. If it won't, then you (the system designer) have to invent some way to move a 'window' of address space through the file, and suddenly it's not as clean as it was. Commented Jul 20, 2020 at 21:20
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    By "as if" I meant with w/ mmap() or equivalent; I've clarified that. I never said Multics lacked a file system, and the links I provided indicate it was the first with an HFS. By "file-based model" I meant accessing disk via open/read/write/close primitives rather than memory addressing. (I would think that's clear enough considering the context.) "The debate is rather between read/write primitives and memory mapping" is, in my opinion, only rewording what I said above. Commented Jul 20, 2020 at 22:27
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    @another-dave - Multics' hardware used 36-bit addresses and words, but unfortunately those 36-bit addresses are split into 18-bit segment number and 18-bit offset, which means the segment size limit is 2^18 words or 1.25MiB. As you suggest, this is too small for practical memory mapped operations; if the processor had been designed with segments as a separate part of the address, allowing a full 36 bits of offset, this would have been fine for any plausible application at the time, but it seems like the need to retrofit segmentation onto a preexisting 36-bit processor crippled the design. Commented Jul 21, 2020 at 3:56
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    @occipita In those days, the segment size was extremely large. Hardly anybody wrote code using megabyte contiguous data structures: only a handful of big machines had that much memory. Commented Jul 21, 2020 at 17:55