[Sun Users' Group & elsewhere] n.
1. Software EXchange. A technique invented by the blue-green algae hundreds of millions of years ago to speed up their evolution, which had been terribly slow up until then. Today, SEX parties are popular among hackers and others (of course, these are no longer limited to exchanges of genetic software). In general, SEX parties are a Good Thing, but unprotected SEX can propagate a virus. See also pubic directory.
2. The rather Freudian mnemonic often used for Sign EXtend, a machine instruction found in the PDP-11 and many other architectures. The RCA 1802 chip used in the early Elf and SuperElf personal computers had a ‘SEt X register’ SEX instruction, but this seems to have had little folkloric impact. The Data General instruction set also had SEX.
DEC's engineers nearly got a PDP-11 assembler that used the SEX mnemonic out the door at one time, but (for once) marketing wasn't asleep and forced a change. That wasn't the last time this happened, either. The author of The Intel 8086 Primer, who was one of the original designers of the 8086, noted that there was originally a SEX instruction on that processor, too. He says that Intel management got cold feet and decreed that it be changed, and thus the instruction was renamed CBW and CWD (depending on what was being extended). Amusingly, the Intel 8048 (the microcontroller used in IBM PC keyboards) is also missing straight SEX but has logical-or and logical-and instructions ORL and ANL.
The Motorola 6809, used in the Radio Shack Color Computer and in U.K.'s ‘Dragon 32’ personal computer, actually had an official SEX instruction; the 6502 in the Apple II with which it competed did not. British hackers thought this made perfect mythic sense; after all, it was commonly observed, you could (on some theoretical level) have sex with a dragon, but you can't have sex with an apple.