Caller%20Must%20Do

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Caller Must Do

• Before it calls a procedure/function, it must
• Pass parameters
• Up to four parameters are passed by a0 - a3
• Save caller-saved registers on the stack
• It includes a0 - a3 (since the callee may
  change these values), s0 - s9, and ra
• Why ra?
• Execute a jal instruction, which jumps to the
  callees first instruction and save the next
  instruction in ra

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Caller Must Do cont.

• After the procedure/function call, it needs to
• Read the returned values from v0 and v1
• Restore caller-saved registers

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Callee Must Do

• Before it does its calculations, the callee must
  do the following
• Allocate memory for its frame by subtracting its
  frame size from the stack pointer
• Save callee-saved registers in the frame
• It must save s0 - s7, fp, ra before changing
  them
• ra needs to be saved if the callee itself makes
  a call
• When needed, establish the frame pointer fp by
  loading sp to it
• In this case, fp must be saved

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Allocating Space for Local Data on Stack

• In MIPS, local variables are also stored on the
  stack
• The segment of the stack containing a procedures
  saved registers and local variables is called a
  procedure frame (or activation record)

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Callee Must Do

• After its calculations and before it returns to
  the caller, the callee must
• Place the return values in v0 and v1 (if
  needed)
• Restore all the callee-saved registers that were
  saved at the procedure entry
• Pop the stack frame by adding the frame size to
  sp
• Return by jumping to the address in register ra
  using jr

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If There are Five or More Parameters

• We know what to do if we have up to four
  parameters
• What if a procedure needs five or more
  parameters?
• We pass the additional parameters using the stack

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Example Five Parameters
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Example Five Parameters cont.
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Nested Procedures

• When calling non-leaf procedures (procedures that
  can call other procedures), more care needs to be
  taken
• A best example in this case is a recursive call

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The Recursive Function

• The function is a caller as well as a callee
• The base case of the function is not difficult to
  write

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The Recursive Function

• The recursive part is not difficult
• What will happen?

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The Recursive Function

• The recursive part is not difficult
• What is wrong?
• We need to save a0 and ra registers since they
  are changed

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The Recursive Function
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The Stack During Recursion
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.data array .word 12, 34, 67, 1, 45, 90, 11,
33, 67, 19 msg_done .asciiz "done!\n" .text
.globl main main li a0, 4 jal
fact done li v0,4 la a0,msg_done syscall
jr ra fact addi sp, sp, -8 sw ra,
4(sp) sw a0, 0(sp) slti t0, a0, 1 beq
t0, zero, L1 addi v0, 0, 1 addi sp, sp,
8 jr ra L1 addi a0, a0, -1 jal fact lw
ra, 4(sp) lw a0, 0(sp) mul v0, v0,
a0 addi sp, sp, 8 jr ra