register dancing

register dancing n.  Many older processor architectures
   suffer from a serious shortage of general-purpose registers.  This
   is especially a problem for compiler-writers, because their
   generated code needs places to store temporaries for things like
   intermediate values in expression evaluation.  Some designs with
   this problem, like the Intel 80x86, do have a handful of
   special-purpose registers that can be pressed into service,
   providing suitable care is taken to avoid unpleasant side effects
   on the state of the processor: while the special-purpose register
   is being used to hold an intermediate value, a delicate minuet is
   required in which the previous value of the register is saved and
   then restored just before the official function (and value) of the
   special-purpose register is again needed.