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Subroutines (Cont.)

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By software convention (not by hardware) registers have been assigned different roles:

• $t0 - $t9 — The subroutine is free to change these registers.
• $s0 - $s7 — The subroutine must not change these registers.
• $a0 - $a3 — These registers contain arguments for the subroutine. The subroutine can change them.
• $v0 - $v1 — These registers contain values returned from the subroutine.

The following are the guidelines for using the temporary and saved registers, according to the convention:

1. If the routine DOES NOT call any other routines, then use the temporary registers ($t0-$t9 or $8-$15 and $24-$25) without any restrictions.


2. If your routine DOES call another routine, then you cannot assume that values in the temporary registers will be left intact across the call (i.e., by convention, the called routine has a right to use these registers). Therefore, for any value stored in a temporary register that needs to be preserved across a call,
   
   
   • you must either save the register before the call and restore the value afterwards (caller-saved register), or
   
   
   • assign any such values to a saved register instead (but see below).


3. If you use any of the saved registers ($s0-$s7 or $16-$23), you must save their contents before you use them (callee-saved registers), and restore the values before returning.

Register values are usually saved on the system stack (i.e., the memory pointed to by $sp). Since saving and restoring a register is an expensive operation, it is a goal of the assembly programmer to design a register allocation scheme that minimizes the number of save/restore operations.

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