Date: Sat, 20 Dec 2003 18:31:57 -0800 (PST) From: "Bruce R. Montague" <brucem@mail.cruzio.com> To: chat@freebsd.org Subject: Re: A bit of trivia: what does usr stand for? Message-ID: <200312210231.hBL2Vv6d000245@mail.cruzio.com>
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I don't know anything about the derivation of the name "usr", but it is of mild interest that one reason 3-letter filename suffixes are so common (.exe, .jpg, .com, .txt, etc..) is that many old DEC machines (including the PDP-11) used a character set called "rad 50" (or RADIX-50, Radix 50, etc..). Rad 50 had 40 characters (50 in octal), and could store 3 characters in 16 bits. A number of filesystems used a single 16-bit word for the filename suffix in dir structures and such, and thus the 3 char maximum on suffix lengths. If you ever had to work with rad-50 at low-level, you are liable to remember it, because characters were not aligned on bit boundaries but had to be inserted and extracted using a process similar to that used for converting between base 10 numbers in ASCII text and binary numbers. DRI used 3-letter suffixes for CP/M, likely because the RT-11 system at the NPS that seems to have strongly flavored CP/M used RAD-50. MS DOS, of course, inherited the convention from CP/M. None of this still has anything to do with Unix's "usr", but 3 letter name limitations were common at one time. RSX-11, another PDP-11 OS, had a one word rad-50 3 character maximum on command names; this (or other similar systems) may have influenced the use of common 3-character names such as "ddb"... or maybe not. There were a lot of similar character sets - Univac Fielddata, CDC Display Code.... These were all different of course. Dealing with char conversion in mainframe days was a mess. Of course, today these days 3 char suffixes (.htm) are probably just common because the statistics work out nice. - bruce
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