bit bang

Transmission of data on a serial line accomplished by

rapidly changing a single output bit, in software, 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

wannabees.

Bit bang was used on certain early models of Prime

computers, presumably when UARTs were too expensive, and on

archaic Zilog 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.

[Jargon File]