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a = a + 1;

Answer:

lw      $t0,12($fp)    # get a
addiu   $t0,$t0,1      # a + 1
sw      $t0,12($fp)    # a = 

Frame-based Linkage Convention

stack frame

A real-world linkage convention allows many types of objects to go into a stack frame. The following rules assume that the stack only stores 32-bit values. There are other features of real-world linkage that the following does not have.

Calling a Subroutine (done by the caller):

  1. Push any registers $t0-$t9 that contain values that must be saved. Push the registers in numerical order.
  2. Put argument values into $a0-$a3.
  3. Call the subroutine using jal.

Subroutine Prolog (done by the subroutine):

  1. Push $ra (always).
  2. Push the caller's frame pointer $fp.
  3. Push any of the registers $s0-$s7 that the subroutine might alter.
  4. Initialize the frame pointer: $fp = $sp - space_for_variables. The "space for variables" is four times the number of local variables. (Remember that subtracting from $sp grows the stack, and that our variables are always four bytes wide).
  5. Initialize the stack pointer: $sp = $fp.

Subroutine Body:

  1. At this point the stack looks like the picture.
  2. The subroutine may alter any T, A, or V register, or any S register that it saved in the prolog.
  3. The subroutine refers to local variables as disp($fp).
  4. The subroutine may push and pop values on the stack using $sp.
  5. If the subroutine calls another subroutine, then it does so by following these rules.

Subroutine Epilog (done at the end of the subroutine):

  1. Put return values in $v0-$v1
  2. $sp = $fp + space_for_variables.
  3. Pop into $s0-$s7 any values for them that were previously saved in the frame (in step6).
  4. Pop the caller's frame pointer into $fp.
  5. Pop $ra (always).
  6. Return to the caller using jr $ra.

Regaining Control from a Subroutine (done by the caller):

  1. Pop any registers $t0-$t9 that the caller previously pushed.

QUESTION 6:

When the caller gets control back, are its frame pointer and stack pointer the same as when it called the subroutine?