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.

Here is a picture. It shows the sections of subroutine linkage. The basic tasks of each section are:

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

Prolog: Push $ra and the caller's $fp. Push any S register the subroutine will alter. 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. Variables on the stack are accessed using disp($fp).

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.

Regaining Control: Pop any previously pushed T registers.