The Jargon File, Version 2.9.10, 01 Jul 1992. VariousЧитать онлайн книгу.
0. One speaks of setting and clearing bits. To {toggle} or `invert' a bit is to change it, either from 0 to 1 or from 1 to 0. See also {flag}, {trit}, {mode bit}.
The term `bit' first appeared in print in the computer-science sense in 1949, and seems to have been coined by early computer scientist John Tukey. Tukey records that it evolved over a lunch table as a handier alternative to `bigit' or `binit'.
:bit bang: n. Transmission of data on a serial line, when accomplished by rapidly tweaking a single output bit at the appropriate times. The technique is a simple loop with eight OUT and SHIFT instruction pairs for each byte. Input is more interesting. And full duplex (doing input and output at the same time) is one way to separate the real hackers from the {wannabee}s.
Bit bang was used on certain early models of Prime computers, presumably when UARTs were too expensive, and on archaic Z80 micros with a Zilog PIO but no SIO. In an interesting instance of the {cycle of reincarnation}, this technique is now (1991) coming back into use on some RISC architectures because it consumes such an infinitesimal part of the processor that it actually makes sense not to have a UART.
:bit bashing: n. (alt. `bit diddling' or {bit twiddling}) Term used to describe any of several kinds of low-level programming characterized by manipulation of {bit}, {flag}, {nybble}, and other smaller-than-character-sized pieces of data; these include low-level device control, encryption algorithms, checksum and error-correcting codes, hash functions, some flavors of graphics programming (see {bitblt}), and assembler/compiler code generation. May connote either tedium or a real technical challenge (more usually the former). "The command decoding for the new tape driver looks pretty solid but the bit-bashing for the control registers still has bugs." See also {bit bang}, {mode bit}.
:bit bucket: n. 1. The universal data sink (originally, the mythical receptacle used to catch bits when they fall off the end of a register during a shift instruction). Discarded, lost, or destroyed data is said to have `gone to the bit bucket'. On {{UNIX}}, often used for {/dev/null}. Sometimes amplified as `the Great Bit Bucket in the Sky'. 2. The place where all lost mail and news messages eventually go. The selection is performed according to {Finagle's Law}; important mail is much more likely to end up in the bit bucket than junk mail, which has an almost 100% probability of getting delivered. Routing to the bit bucket is automatically performed by mail-transfer agents, news systems, and the lower layers of the network. 3. The ideal location for all unwanted mail responses: "Flames about this article to the bit bucket." Such a request is guaranteed to overflow one's mailbox with flames. 4. Excuse for all mail that has not been sent. "I mailed you those figures last week; they must have ended in the bit bucket." Compare {black hole}.
This term is used purely in jest. It is based on the fanciful notion that bits are objects that are not destroyed but only misplaced. This appears to have been a mutation of an earlier term `bit box', about which the same legend was current; old-time hackers also report that trainees used to be told that when the CPU stored bits into memory it was actually pulling them `out of the bit box'. See also {chad box}.
Another variant of this legend has it that, as a consequence of the `parity preservation law', the number of 1 bits that go to the bit bucket must equal the number of 0 bits. Any imbalance results in bits filling up the bit bucket. A qualified computer technician can empty a full bit bucket as part of scheduled maintenance.
:bit decay: n. See {bit rot}. People with a physics background tend to prefer this one for the analogy with particle decay. See also {computron}, {quantum bogodynamics}.
:bit rot: n. Also {bit decay}. Hypothetical disease the existence of which has been deduced from the observation that unused programs or features will often stop working after sufficient time has passed, even if `nothing has changed'. The theory explains that bits decay as if they were radioactive. As time passes, the contents of a file or the code in a program will become increasingly garbled.
There actually are physical processes that produce such effects (alpha particles generated by trace radionuclides in ceramic chip packages, for example, can change the contents of a computer memory unpredictably, and various kinds of subtle media failures can corrupt files in mass storage), but they are quite rare (and computers are built with error-detecting circuitry to compensate for them). The notion long favored among hackers that cosmic rays are among the causes of such events turns out to be a myth; see the {cosmic rays} entry for details.
The term {software rot} is almost synonymous. Software rot is the effect, bit rot the notional cause.
:bit twiddling: n. 1. (pejorative) An exercise in tuning (see {tune}) in which incredible amounts of time and effort go to produce little noticeable improvement, often with the result that the code has become incomprehensible. 2. Aimless small modification to a program, esp. for some pointless goal. 3. Approx. syn. for {bit bashing}; esp. used for the act of frobbing the device control register of a peripheral in an attempt to get it back to a known state.
:bit-paired keyboard: n. obs. (alt. `bit-shift keyboard') A non-standard keyboard layout that seems to have originated with the Teletype ASR-33 and remained common for several years on early computer equipment. The ASR-33 was a mechanical device (see {EOU}), so the only way to generate the character codes from keystrokes was by some physical linkage. The design of the ASR-33 assigned each character key a basic pattern that could be modified by flipping bits if the SHIFT or the CTRL key was pressed. In order to avoid making the thing more of a Rube Goldberg kluge than it already was, the design had to group characters that shared the same basic bit pattern on one key.
Looking at the ASCII chart, we find:
high low bits
bits 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001
010 ! " # $ % & ' ( )
011 0 1 2 3 4 5 6 7 8 9
This is why the characters !"#$%&'() appear where they do on a Teletype (thankfully, they didn't use shift-0 for space). This was *not* the weirdest variant of the {QWERTY} layout widely seen, by the way; that prize should probably go to one of several (differing) arrangements on IBM's even clunkier 026 and 029 card punches.
When electronic terminals became popular, in the early 1970s, there was no agreement in the industry over how the keyboards should be laid out. Some vendors opted to emulate the Teletype keyboard, while others used the flexibility of electronic circuitry to make their product look like an office typewriter. These alternatives became known as `bit-paired' and `typewriter-paired' keyboards. To a hacker, the bit-paired keyboard seemed far more logical —- and because most hackers in those days had never learned to touch-type, there was little pressure from the pioneering users to adapt keyboards to the typewriter standard.
The doom of the bit-paired keyboard was the large-scale introduction of the computer terminal into the normal office environment, where out-and-out technophobes were expected to use the equipment. The `typewriter-paired' standard became universal, `bit-paired' hardware was quickly junked or relegated to dusty corners, and both terms passed into disuse.
:bitblt: /bit'blit/ n. [from {BLT}, q.v.] 1. Any of a family of closely related algorithms for moving and copying rectangles of bits between main and display memory on a bit-mapped device, or between two areas of either main or display memory (the requirement to do the {Right Thing} in the case of overlapping source and destination rectangles is what makes BitBlt tricky). 2. Synonym for {blit} or {BLT}. Both uses are borderline techspeak.
:BITNET: /bit'net/ [acronym: Because It's Time NETwork] n. Everybody's least favorite piece of the network (see {network, the}). The BITNET hosts are a collection of IBM dinosaurs and VAXen (the latter with lobotomized comm hardware) that communicate using 80-character {{EBCDIC}} card images (see {eighty-column mind}); thus, they tend to mangle the headers and text of third-party traffic from the rest of the ASCII/RFC-822 world with annoying regularity. BITNET is also notorious as the apparent home of {BIFF}.
:bits: n.pl. 1. Information. Examples: "I need some bits about file formats." ("I need to know about file formats.") Compare {core dump}, sense 4. 2. Machine-readable representation of a document, specifically as contrasted with paper: "I have only a photocopy of the Jargon File; does anyone know where I can get the bits?". See {softcopy}, {source of all good bits} See also {bit}.
:bitty