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Answer:

         .text
         .globl  main

main:
         jal     pread            # read first integer
         nop                      #  
         move    $s0,$v0          # save it in $s0
         jal     pread            # read second integer
         nop                      # 
         move    $s1,$v0          # save it in $s1
         jal     pread            # read third integer
         nop                      #  
         move    $s2,$v0          # save it in $s2
         
         addu    $s0,$s0,$s1      # compute the sum
         addu    $s3,$s0,$s2      # result in $s3
 
         li      $v0,4            # print a heading
         la      $a0,heading         
         syscall

         move    $a0,$s3          # move sum into parameter
         li      $v0,1            # print the sum
         syscall
        
         li      $v0,10           # exit
         syscall
          
         .data
heading:
         .asciiz "The sum is: "   

Global Symbols

Recall that modules (for us, subroutines) should not know about each other's symbolic addresses. It would violate the idea of modularity for main to do something to pread's prompt, for example.

But some symbolic addresses need to be used between modules. For example, pread is a symbolic address, and main must know about it and use it in the jal instruction.

A symbol that a subroutine makes visible to other subroutines is a global symbol. Global symbols often label entry points. Symbols that are not global are called local symbols. A symbol is made global by placing it in a list of symbols following the .globl directive. Some languages use the word external for what we are calling global.

         .globl  main

In the language C, a symbol that is visible to another module is called an external symbol. For example, the names of functions in C are external symbols.

Source programs for PC-SPIM (the older version of SPIM) are contained in a single file, which includes all subroutines. However, in professional software development each subroutine might be placed in a separate source file. Each file must say which of its symbolic addesses are global and might be referenced by other source files.

With QtSpim (the most recent version of SPIM) you can create separate source files and load them separately. For this example program:

  1. Create separate source files addthree.asm and pread.asm (see next page)
  2. Start QtSPIM. Check the settings.
  3. Click "File"
  4. Select the "Reinitalize and Load File" menu, then pick addThree.asm
  5. Click "File"
  6. Select the "Load File" menu, then pick pread.asm
  7. You will now have the two files linked together in memory, with one text section and one data section
  8. Click "Run" (as always)

QUESTION 14:

What global symbols are in the subroutine pread?