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.
