loop:                             # get a line
         la      $a0,line         # argument: address of buffer
         li      $a1,132          # argument: length of buffer
         jal     getline          # get line from user
         
         la      $a0,line         # if "Q"
         jal     testEnd          # return to caller
         beqz    $v0,endloop          

                                  # convert to capitals
         la      $a0,line         # argument: address of buffer
         li      $a1,132          # argument: length of buffer
         jal     convert          # convert

Subroutine getLine

# getLine -- read in a line of user input
#
# on entry:
#    $a0 -- address of input buffer
#    $a1 -- length of buffer
#
# on exit:
#    no return values

         .text
         .globl  getLine
getLine:   
         move    $t0,$a0          # save buffer address          
         la      $a0,prompt       # prompt the user
         li      $v0,4            # service 4
         syscall

         move    $a0,$t0          # restore buffer address
         li      $v0,8            # service 8
         syscall                  # read in a line to the buffer
         
         jr      $ra              # return to caller 

         .data
prompt:
         .asciiz ">"

Notice how getLine reads data into an input buffer defined externally to itself. The parameters in $a0 and $a1 specify this buffer. It would be a design mistake to have getLine read into its own buffer or to hard-code the symbolic address of a buffer in another subroutine.

The buffer address parameter and the length parameter are similar to the parameters used in many "C" functions. Study this example to help in your future (or present) understanding of "C" pointer variables.