Subroutine convert

After reading in a line, doLines calls convert to convert the entire line to upper case. convert calls conChar for each character in the line.

convert needs a register to hold a character pointer that moves through the line. It can't use a T register for this because it calls another subroutine. So it uses an S register. But it must restore the S register to its original state when it returns to its caller.

This is the situation that calls for pushing an S register on the stack, using it in the subroutine body, and then popping it from the stack before returning to the caller.

# convert -- convert a line to all capitals
#
# on entry:
#    $a0 -- address of input buffer
#    $a1 -- length of input buffer
#
# register use:
#    $s0 -- pointer into character buffer
#
# on exit:
#    no return values

         .text
         .globl  convert
convert:   
         sub     $sp,$sp,4        # push the return address
         sw      $ra,($sp)
                # What should be done HERE?? 
         

                                  # for ( p=buffer; *p!=0; p++ )
         move    $s0,$a0          # p=buffer

cloop:   lbu     $a0,($s0)        # get a char from the string
         beqz    $a0,endC         # exit if null byte
                                  # argument a0: char to convert
         jal     conChar          # convert character
         sb      $v0,($s0)        # put converted char into string                 
         addu    $s0,$s0,1        # p++
         b       cloop         
 
endC:   
                # And what should be done HERE??
         
         lw      $ra,($sp)        # pop return address
         add     $sp,$sp,4         
         jr      $ra              # return to caller