A digital data processor having a main pipeline to which a side pipe is loosely
coupled. In particular, the side pipe is coupled to the main pipeline at a point
after which an instruction entering the side pipe cannot cause an exception. When
such an instruction enters the first stage of the side pipe, a copy or "ghost"
of this instruction is created. While the actual instruction flows down the side
pipe, this ghost instruction is allowed to flow independently down the main pipeline
as if it were a non-squashable no-op. When the ghost reaches the retirement stage
of the main pipeline, it is retired in normal program order, regardless of the
status of the actual instruction. However, in addition, each system resource that
is still waiting for a result from the actual instruction is marked appropriately.
When the actual instruction finally completes in the side pipe, the only consequence,
other than those local to the side pipe itself, is that any results are forwarded
to the awaiting resources. If it should happen that a resource has been reallocated
to a more recent instruction while the actual instruction is still in flight, the
forwarded result is discarded upon arrival; otherwise, the resource is updated
using the forwarded result. As a result of employing such a pipe coupling mechanism,
the number of pipe stages that must be traversed by instructions other than those
requiring a side pipe resource may be reduced. One disclosed example of such a
side pipe is a multiply accumulate unit suitable for use in digital signal processing
applications, whereby the performance of control type instructions is greatly improved.
