the functions (procedures) in MIPS

Question

I\'m new in MIPS language and I don\'t understand how the functions (procedures) in the MIPS assembly language work. Here are but I will specify my problem :

1. <

Answer 1

1.the first two are instructions,the third it's kind of special register

• jal=jump and link (Address of following instruction put in $ra,and jump to target address)
• jr=jump to specify register
• $ra=return address

we often use the instruction like this ...

• jr $ra (Copy $ra to program counter)

it means return(jump) to the address saved in $ra .

2.

Here's an example function (procedure) in C

int main(){
   x=addthem(a,b);
}
int addthem(int a, int b){
   return a+b;
}

function in MIPS

.text
main:    #assume value a is already in $t0, b in $t1
    add $a0,$0,$t0   # it's the same function as move the value
    add $a1,$0,$t1 
    jal addthem      # call procedure
    add $t3,$0,$v0   # move the return value from $v0 to where we want
    syscall

addthem:
    addi $sp,$sp,-4     # Moving Stack pointer
    sw $t0, 0($sp)      # Store previous value

    add $t0,$a0,$a1     # Procedure Body
    add $v0,$0,$t0      # Result

    lw $t0, 0($sp)      # Load previous value
    addi $sp,$sp,4      # Moving Stack pointer 
    jr $ra              # return (Copy $ra to PC)