Register Problem

In the Simple Linkage convention of the previous chapter, registers $s0―$s7 must not be altered by a subroutine. But this restriction creates a problem when subroutines call other subroutines.

Say that main calls subA and that subA calls subB. subA can't save any values in $s0―$s7 (because it is not allowed to alter them). But any values it saves in $t0―$t9 might be clobbered by subB (because subB is allowed to alter them). In effect, subA can't use any registers! Not good.

The solution is to allow subA to use $s0―$s7. However, before using one of these registers, subA must save its value on the stack. Later, when subA returns to its caller, it must restore the register to its initial state.