Diagram

Those rules are complicated. In broad outline it works the same way as the previous chapter's stack-based linkage convention. But now, the subroutine prolog pushes room on the stack for local variables, and the epilog pops that room.

The picture shows the sections of subroutine linkage. The basic tasks of each section are:

Prolog: Push $ra and the caller's $fp. Push any S register the subroutine will alter. Make room for variables and initialize the subroutine's $fp and $sp.

Body: Normal code, except it must follow these conventions if it calls another subroutine. T and A registers can be used freely, as can any S registers that were saved in the prolog. Use disp($fp) to access variables on the stack.

Subroutine Call: Push any T registers that contain values that are needed. Put arguments in A registers. jal to the subroutine.

Regaining Control: Pop any previously pushed T registers.

Epilog: Put return values in V registers. Reset $sp. Pop any S registers. Pop the caller's $fp and $ra. jr $ra back to the caller.