A good answer might be:

Four levels deep: main calls doLines which calls convert which calls conCh.

main Subroutine

An advantage of modular programming is that each subroutine can be displayed and explained independently of the others. Here is the design of  main.

To simplify the discussion, branch delays and load delays have been turned OFF in SPIM. 

         .text
         .globl  main
main:
         ??????                   # what goes here?
         
         la      $a0,mainPr       # prompt the user
         li      $v0,4            # service 4
         syscall

         jal     doLines          # process lines of input
         
         ??????                   # what goes here?
         
         jr      $ra              # return to OS 

         .data
mainPr:  .ascii  "Type each line of text followed by ENTER.\n"
         .asciiz "Type Q at the start of a line to finish.\n"


QUESTION 13:

According to the Stack-based Linking convention does main need to push and later pop the return address?