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This is an optimization for speed. Many subroutines need to use only a few registers for temporary values. They can use the T registers without the added expense of saving and restoring them. A subroutine that needs more registers or that needs to have values saved across a subroutine call may use some S registers, but must pay for them by saving and restoring them.


two linked subroutines

These rules are somewhat complicated. Here is a picture. It shows the four sections of subroutine linkage. The basic tasks of each section are:

Subroutine Call: Push all T registers that contain values that will be needed after the call. Put arguments in A registers. jal to the subroutine.
Prolog: If this subroutine calls other subroutines, push $ra. Push all S registers that the subroutine alters.
Body: Normal code, except that it must follow these conventions if it calls a subroutine. T registers and A registers can be used freely, as can any S registers that were saved in the prolog.
Epilog: Put return values in V registers. Pop any S registers. Pop $ra if it was pushed in the prolog. jr $ra back to the caller.
Regaining Control: Pop any T registers that were previously pushed.



Is there any limit in these rules about how many levels deep subroutine calls may be?