Question

I am trying to have the function vbsme call another function called sad… is the following procedure correct about saving the registers and return address?? the caller is supposed to save register $t0-$t7, but where and how should I do that?

vbsme: li $v0, 0   # reset $v0 
     li $v1, 0   # reset $v1
     li  $t0, 1   # i(row) = 1 
     li  $t1, 1   # j(col) = 1
     lw  $t2, 0($a0)  # row size
     lw  $t3, 4($a0)  # col size
     mul  $t4, $t2, $t3  # row * col
     li  $t5, 0   # element = 0
    loop:   bgeq  $t5, $t4, exit  # if element >= row * col then exit

     addi $sp, $sp, -16  # create space on the stack pointer
     sw $ra, -12($sp)  # save return address
     sw $s6, -8($sp)  # save return address
     sw $s7, -4($sp)  # save return address
     subi $s7, $t0, 1  # 1st parameter: i-1
     subi $s6, $t1, 1  # 2nd parameter: j-1
     jal  sad   # calculate the sum of absolute difference using the frame starting from row a0 and col a1
     lw $ra, -12($sp)  # restore return address
     lw $s6, -8($sp)  
     lw $s7, -4($sp)
     addi $sp, $sp, 16  # restore stack pointer
     jr $ra

Answer 1

$sx registers are guaranteed to be unchanged accross function calls, so its the callee (sum function) the responsible of saving them, only if its going to change their value.

$tx registers, on the other hand, are not guaranteed to be unchanged over function calls, so its the responsability of the caller (vbsme) to save them.

You should save $sx in the callee stack.

So when you start coding the sum function, you should save space in the stack
If you want to save n registers, then save n*4.

Space in the stack is saved by subtracting on the $sp register, which points to the base of the stack. Before your function code, you should create the stack for that function, saving all caller-saved registers, return address and global pointer registers when neccesary

sum:
       #stack frame creation. Caller registers saved, 
       # return address and frame pointer

       subu $sp,$sp,36 #Save space in the stack for registers $s0, $s7 + $ra
       sw   $ra,32($sp)
       sw   $s0,0($sp)
       sw   $s1,4($sp)
       #and so on. Note that also you should save the $ra register only if you are
       # going to call another function

       #do something with $sx  

       #stack frame destruction
       #restore $sx and $ra registers
       lw  $ra,32($sp)
       lw  $s0,0($sp)
       lw  $s1,4($sp)
       ...
       lw  $s7,28($sp)

        jr $ra

By the way, by convention, registers $a0, $a3 should keep the arguments to the function you are calling. Also, note that because you are using the $s0, $s7 registers, you have to do some extra work. Convention says that if you don’t use them, then you shouldn’t save them, so maybe you could use the $tx (temporary) registers instead.