CS 300 – Lecture 10

Intro to Computer Architecture

/ Assembly Language

Strings and Characters and More

Function Calling

Basic ideas:

* Parameters passed into function

* Return address

* Return values

* Registers saved / destroyed during the call

* Saving other data during the call

The GORY DetailsSo what does a function look like?Initially:* Frame allocation* Save values to the frame (including return address). Usually need to move args there. Save $s0 - $s7 if used later.Finally:* Put return values in proper registers.* Resture $s0 - $s7* Deallocate the frame* Branch to the return address ("jr")Calling other functions:* Push needed information that is not in the stack frame or a saved register* Perform "jal"* Pop the stack

Aside: MIPS Register Convention

Name Register Number

Usage Preserve on call?

$zero 0 constant 0 (hardware) n.a.

$at 1 reserved for assembler n.a.

$v0 - $v1 2-3 returned values no

$a0 - $a3 4-7 arguments yes

$t0 - $t7 8-15 temporaries no

$s0 - $s7 16-23 saved values yes

$t8 - $t9 24-25 temporaries no

$gp 28 global pointer yes

$sp 29 stack pointer yes

$fp 30 frame pointer yes

$ra 31 return addr (hardware) yes

Example

Factorial: f(x) = if x <= 0 then 1 else x*f(x-1)fact: addi $sp, $sp, -8 # Make frame sw $ra, 4($sp) # save ra, a1 sw $a0, 0($sp) slti $t0, $a0, 1 # a0 < 1? beq $t0, $zero, L1 addi $v0, $zero, 1 # return 1 addi $sp, $sp, 8 jr $ra

Factorial

L1: addi $a0, $a0, -1

jal fact

lw $a0, 0($sp)

lw $ra, 4($sp)

addi $sp, $sp, 8

mul $v0, $a0, $v0

jr $ra

And Now for the Pentium …

_fact:pushl %ebpmovl %esp, %ebpsubl $8, %espcmpl $0, 8(%ebp)jne L2movl $1, -4(%ebp)jmp L1

%ebp = $fp esp = $sp

From "gcc –S"

See 2.16 for more

And Now for the Pentium …

L2: movl 8(%ebp), %eaxdecl %eaxmovl %eax, (%esp)call _factimull 8(%ebp), %eaxmovl %eax, -4(%ebp)

L1: movl -4(%ebp), %eaxleaveret

More About $fp

Why have both $sp and $fp?* Stack use varies within a function (why?). Using $fp, we always know where locals are* $fp allows for the creation of chained environments. This is needed in Java for nested objects.* $fp can accept incoming parameters from the stack top

Note that you often don't need $fp at all.