bit bang

bit bang n.  Transmission of data on a serial line, when
   accomplished by rapidly tweaking 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
   {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.