MIPS Calling Convention

MIPS Calling Convention

Chapter 2.7

Appendix A.6

Procedure CallsMain

Procedure

Call Procedure

Call Procedure

Procedure Calls

• Procedure must _____ _______ from any call

• Procedure uses _____ that main was using

• We need a convention to– – –

Name Reg Number Usage Preserved across call?

$zero 0 The constant 0 Yes

$v0-$v1 2-3 Function results No

$a0-$a3 4-7 Function Arguments No

$t0-$t7 8-15 Temporaries No

$s0-$s7 16-23 Saved Yes

$t8-$t9 24-25 More temporaries No

$gp 28 Global pointer Yes

$sp 29 Stack pointer Yes

$fp 30 Frame pointer Yes

$ra 31 Return address Yes

Page 140, Figure 3.13

MIPS-specific info

MIPS-specific info – who cares?• Preserved – Value is same after call

– Caller

– Procedure

• Not preserved – No guarantees– Caller

– Procedure

Steps for caller

1. Store away any temporary registers we want

2. Pass function parameters to procedure

3. Transfer control to procedure

4. (then procedure executes)

5. Get return value

6. Restore any temp regs we saved away

Steps for procedure

1. Allocate stack space

2. Store preserved regs we may use

3. Perform task

4. Place result in proper location for caller

5. Restore preserved regs we may have used

6. Transfer control back to caller

Write the caller & procedure code for the following function:

Caller: Callee:

Assume: g,h are in $s2,$s3. We want return value in $s0.Caller wants to preserve $t0 across function call.

int MyFunc(int g, int h){ return (g + h);}

Steps for procedure

1. Allocate stack space

2. Store preserved regs we may use

3. Perform task

4. Place result in proper location for caller

5. Restore preserved regs we may have used

6. Transfer control back to caller

How do we know what we’ll use until we write task code?

How do we know how much space until we know how many regs to store?

Definitions

• Leaf function– Makes no function calls

• Non-leaf function– Contains a function call

Allocating stack space

$sp

$sp$sp Stack space

for this function

Only allocate once per function!!!!

$sp contains address of bottom of stack. What operation on

$sp allocates space?

Minimum allocation:

24 bytes

How much stack space?

• Minimum allocation 24 bytes– $ra , $fp even if unused.– 4 words for $a0-$a3 in case we need it

• Preserved registers that we destroy (i.e. $s0)

• Local variables declared in our function

• Any other outgoing arguments (non-leaf)

• Total bytes must be divisible by 8 (aligned for floating-point numbers)

What actually goes in stack

$ra

Extra Arguments

Extra outgoing arguments

$spbefore call

$spduring call

padding

local dataL*4 bytes for local data

P*4 bytes for preserved regs ($s0-$s7)

A*4 bytes for outgoing args

$fp

preserved registers(and $a0-$a3)

$fpduring call

Example

int foo(int arg1, int arg2){

int myarray[64];myarray[3] = 5;…bar(a1, a2, a3, a4, a5);…return (myarray[3]);

}

Local 256-byte array

Non-leaf function, 5 outgoing args

Assume it needs 2 saved

registers

Caller’s Stackaddi $sp, $sp, - (1+64+1+2+6)*4

sw $ra, 73*4($sp)sw $fp, 72*4($sp)sw $s1, 67*4($sp)sw $s0, 66*4($sp)addi $t0, $zero,5 # $t0 = 5sw $t0, (1+3)*4 ($sp)# myarray[3] = 5…lw $ra, 73*4($sp)lw $fp, 72*4($sp)lw $s1, 67*4($sp)lw $s0, 66*4($sp)addi $sp, $sp, (1+64+1+2+6)*4jr $ra

$a3

myarray

$spbefore call

$spduring call

$a2

$s1$s0

padding

outgoing arg 5

$ra$fp$a0$a1

Recursive call: SumToN

int SumToN(int N){ if (N < 2) return 1; else return (SumToN(N-1) + N);}

Both the caller and the callee!!!

MIPS Stack SumToN(2)$sp

SumToN: addi $sp, $sp, -24 sw $ra, 20 ($sp) sw $fp, 16 ($sp) addi $fp, $sp, 20 slti $t0, $a0, 2 beq $t0, $0, result addi $v0, $zero,1 j end result: sw $a0, 0 ($sp) addi $a0, $a0, -1 jal SumToN lw $a0, 0 ($sp) add $v0, $v0, $a0end: lw $ra, 20 ($sp) lw $fp, 16 ($sp) addi $sp, $sp, 24 jr $ra


$sp



$fp

$sp

$ra



$a0 (2)

$fp

$sp

$ra



$a0 (2)

$fp

$sp

$ra


$fp

$sp

$ra


$a0 (2)

$fp

$sp

$ra


$fp

$sp

$ra

$v0 = 1


$a0 (2)

$fp

$sp

$ra


$fp$ra

$v0 = 1


$a0 (2)

$fp

$sp

$ra


$fp$ra

$v0 = 3

MIPS Stack SumToN(2)

$a0 (2)

$fp

$sp$ra


$fp$ra

$v0 = 3

MIPS Stack SumToN(2)

$a0 (2)

$fp

$sp$ra


$fp$ra

$v0 = 3

What about the garbage in the stack?

MIPS instructions

• Rule: Destination register always comes first

• Exception:

• Rule: Any instruction involving constants has “i” at the end of instruction(add vs addi)

• Exception:

MIPS Calling Convention

Documents

sp addi

preserved regs

following function

preserved value

temp regs

ramips stack sumton2sumton

return g h

return value