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]