The SPARC architecture has 32 integer registers, divided as follows:

- global registers (g0-g7)
- input (i0-i7)
- local (l0-l7)
- output (o0-o7)

Parameter passing and return is done using the output registers, which are
generally considered scratch registers and can be corrupted by the callee. The
caller must take care of preserving them.

The input and local registers are callee-saved, but we have hardware assistance
in the form of a register window. There is an instruction to shift the registers
so that:
- o registers become i registers
- local and output registers are replaced with fresh sets, for use by the
  current function
- global registers are not affected

Note that as a side-effect, o7 is moved to i7, this is convenient because these
are usually the stack and frame pointers, respectively. So basically this sets
the frame pointer for free.

Simple enough functions may end up using just the o registers, in that case
nothing special is necessary, of course.

When shifting the register window, the extra registers come from the register
stack in the CPU. This is not infinite, however, most implementations of SPARC
will only have 8 windows available. When the internal stack is full, an overflow
trap is raised, and the handler must free up old windows by storing them on the
stack, likewise, when the internal stack is empty, an underflow trap must fill
it back from the stack-saved data.