The calling convention

• A calling convention defines how subroutines are called.
• For mips this includes:
  • How registers are used, and when they must be preserved
  • Where and how parameters are passed
  • How the stack is managed.
• Stack Management:
  • See the back of the green sheet.
  • An activation record or frame
  • The frame contains all storage a subroutine requires
    • Variable storage (local variables)
    • Register preservation
    • parameters.
  • The called routine must manage the stack
  • The end of the frame is pointed to by the stack pointer.
    • The stack pointer is adjusted as needed by the routine. (More on this later)
    • On exit
      • addiu $sp, $fp, 4
  • The beginning of the frame is pointed to by the frame pointer
    • Upon entry:
      • sw $fp, -4($sp)
      • subiu $fp, $sp, 4
    • Upon Exit:
      • lw $fp, 0($fp)
  • For a non-leaf routine, the $ra must be preserved
    • Upon Entry:
      • sw $ra, -4($fp)
    • Upon Exit:
      • lw $ra, -4($fp)
• Other registers which might be saved
  • Look at the register information on the front of the green sheet.
  • Preserved across a call refers to who is responsible for saving the registers.
  • As we have seen, $sp, $fp, and $ra must be preserved, or saved and restored by the called routine.
  • We only need to do this if we change the values of a register.
    • So if you want to use $s0, you must:
      • Save it on the stack.
      • use it
      • Restore it when you are finished with the routine.
  • The opposite is true of the registers that are not preserved across a call
    • If you use $t0, and make a routine call
    • The routine might also use $t0, and it is not the responsibility of the routine to preserve this register.
    • Therefore if you want $t0 preserved across a call you must
      • Save $t0 on the stack
      • Make the routine call
      • restore $t0
• Parameters:
  • Registers $a0 through $a4 are used for the first four parameters
  • Additional parameters are placed in the last portion of the calling routine's stack in reverse order (back of green sheet)
  • Return values are stored in $v0 and $v1
• Here is a complete example