Source : Free On-Line Dictionary of Computing

register dancing
     
        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.
     
        [{Jargon File}]