Question

What needs to save the following registers to the stack, when calling a function in MIPS. Answer “caller" for the procedure making a function call, “callee" for the function being called, or “N.A" for neither.

$0

$v*

$a*

$t*

$s*

$sp

$ra

Answer 1

There are only two instructions necessary for creating and calling functions: jal and jr.

Suppose If we follow the register conventions when calling functions, we will be able to write much simpler and cleaner MIPS code.

Here we have to save the registers given in the problem. First draw the table with function call for each register as per the question:

$0	$v*	$a*	$t*	$s*	$sp	$ra
N.A.	caller	caller	caller	callee	N.A.	caller

Now assume a function foo that calls another function bar, which is known to call some other functions.

foo can takes one argument at a time and then will modify and use $t0 and $s0.

bar takes two arguments, and returns an integer, and uses $t0-$t2 and $s0-$s1.

Now create a table or boxes and then just before bar calls a function draw a possible ordering of the stack.

The top left box is the address of $sp. When foo function is first called, and then the stack goes downwards, continuing at each next column. Here we will Add ‘(f)’ if the register is stored by foo and ‘(b)’ if the register is stored by bar. The first one is written in the following ways:

1	$ra(f)
2	$s0 (f)
3	$v0 (f)
4	$a0 (f)
5	$t0 (f)
6	$ra (b)
7	$s0 (b)
8	$s1 (b)
9	$v0 (b)
10	$a0 (b)
11	$a1 (b)
12	$t0 (b)
13	$t1 (b)
14	$t2(b)
15
16

The following function is used to write a function foo to   save the register to the stack

FunctionFoo:

# PROLOGUE

# begin by reserving space on the stack

addiu $sp, $sp, -FrameSize

# now, store needed registers

sw $ra, 0($sp)

sw $s0, 4($sp)

...

# BODY

...

# EPILOGUE

# restore registers

lw $s0 4($sp)

lw $ra 0($sp)

# release stack spaces

addiu $sp, $sp, FrameSize

# return to normal execution

jr $ra