Print Struct Subroutine

Data that is contained in a struct is treated as a whole. In OO terms, it is treated like an object. It would be nice to have a subroutine that takes one of our structs as a parameter and prints it out. Let us write a subroutine that uses the Stack-based Calling Convention of Chapter 27. (It might not hurt you to review that chapter.)

Here is a small subroutine. For now, it only prints out the age field of the argument struct. It uses register $s0 so it must first push the value in that register on the stack. It does not call any other subroutine so it does not need to push $ra. (The SPIM service requests do not change $ra.)

The argument is the address of the struct. A large struct can be passed as an argument to a subroutine by giving the subroutine the address of the struct. The various fields of the struct are accessed using displacements off the address.

# Subroutine PStruct: print a struct
#
# Registers on entry: $a0 --- address of the struct
#                     $ra --- return address
#
# Registers:          $s0 --- address of the struct
#
        .text

PStruct:
        sub     $sp,$sp,4         # push $s0
        sw      $s0,($sp)         # onto the stack
        
        move    $s0,$a0           # make a safe copy 
                                  # of struct address
        la      $a0,agest         # print "age:"
        li      $v0,4
        syscall
        lw      $a0,0($s0)        # print age
        li      $v0,1
        syscall

        add     $sp,$sp,4         # restore $s0 of caller 
        lw      $s0,($sp)
        jr      $ra               # return to caller        

        .data
agest:  .asciiz "age:   "

For example, the age field of our struct is a displacement of zero off of the struct's base address. So this code gets the integer age using the statement lw $a0,0($s0).