pipeline
pipelined
pipelining

    A sequence of functional units ("stages")
   which performs a task in several steps, like an assembly line
   in a factory.  Each functional unit takes inputs and produces
   outputs which are stored in its output buffer.  One stage's
   output buffer is the next stage's input buffer.  This
   arrangement allows all the stages to work in parallel thus
   giving greater throughput than if each input had to pass
   through the whole pipeline before the next input could enter.

   The costs are greater latency and complexity due to the need
   to synchronise the stages in some way so that different inputs
   do not interfere.  The pipeline will only work at full
   efficiency if it can be filled and emptied at the same rate
   that it can process.

   Pipelines may be synchronous or asynchronous.  A synchronous
   pipeline has a master clock and each stage must complete its
   work within one cycle.  The minimum clock period is thus
   determined by the slowest stage.  An asynchronous pipeline
   requires handshaking between stages so that a new output is
   not written to the interstage buffer before the previous one
   has been used.

   Many CPUs are arranged as one or more pipelines, with
   different stages performing tasks such as fetch instruction,
   decode instruction, fetch arguments, arithmetic operations,
   store results.  For maximum performance, these rely on a
   continuous stream of instructions fetched from sequential
   locations in memory.  Pipelining is often combined with
   instruction prefetch in an attempt to keep the pipeline
   busy.

   When a branch is taken, the contents of early stages will
   contain instructions from locations after the branch which
   should not be executed.  The pipeline then has to be flushed
   and reloaded.  This is known as a pipeline break.

   (1996-10-13)