Subroutines part 3

We need to talk about how subroutines communicate with their callers in the MIPS world.

When a subroutine is called, it is typically sent some actual parameters by its caller. The values of these parameters will affect the execution of the subroutine. A function will also return a value to its caller. This page is about how that communication is done.

• Registers $a0, $a1, $a2, and $a3 are reserved for carrying parameters to the subroutine.
• This means that, before a subroutine can be called, the correct values must be placed into these registers.
• If a subroutine has one parameter, it goes in $a0.
• If it has two parameters, the first goes in $a0, and the second goes in $a1.
• Similarly for three and four parameters.
• If there are more than four parameters, the stack is used for the others, but don't worry about that. It won't happen in this course.
• Registers $v0 and $v1 are reserved for carrying a function's returned value back to the caller.
• If the returned value is 32 bits, it goes in $v0.
• If the returned value is 64 bits, e.g. a double, it is split between $v0 and $v1.

A typical scenario for the whole thing would therefore be like this:

• copy the parameters into the $a registers; this would likely use move and lw instructions
• call the subroutine; this will always be "jal subroutine-name"
• copy the returned value from $v0 to its proper place; this would usually be a move or a sw

Here's an example: we need to call doubleit, a function that returns twice the value in its parameter. You can paste this code into MARS and run it. You can also single-step through it and watch the values in the registers and program counter change.

# call doubleit to double the value of x and put the doubled value into two-x; in C, it would look like this:
# two-x = doubleit(x);

.include    "macros.asm"      # we need this because quit is a macro

.data

x:      .word 15
two-x:  .word 0

.text

main:   lw   $a0  x           # parameter
        jal  doubleit
        sw   $v0  two-x       # returned value
        quit

# this function must put twice its parameter (in $a0) into its returned-value register ($v0)
doubleit:    add  $v0  $a0  $a0      # $v0 = 2 * $a0
             jr   $ra