### A good answer might be:

    #  main()
    #  {
    #    int a, b;                      // a: 0($fp),  b: 4($fp)
    #    write("enter an int:")
    #    read( a );
    #    b = fact( a );
    #    write("factorial is:")
    #    print( b );
    #  }
             .text
             .globl  main
    main:
                                      # prolog
             sub     $sp,$sp,4        #   1. Push return address
             sw      $ra,($sp)
             sub     $sp,$sp,4        #   2. Push caller's frame pointer
             sw      $fp,($sp)
                                      #   3. No S registers to push
             sub     $fp,$sp,8        #   4. $fp = $sp - space_for_variables
             move    $sp,$fp          #   5. $sp = $fp

# Guts of `main()`

The next part of `main()` is straightforward. The SPIM trap services for writing a string and reading an integer are used. The integer is returned in `$v0`. It is saved in the variable `a` (on the stack).

                                      # write("enter an int:")
             li     $v0,4             #   print string service
             la     $a0,prompt1       #   address of prompt
             syscall
                                      # read( a )
             li     $v0,5             #   read integer service
             syscall                  #   $v0 gets the integer
             sw     $v0,0($fp)        #   save in variable a

Next the code implements `b = fact( a )`. This is done by following the protocol for a subroutine call, then storing the returned value into the variable `b`:

                                      # subroutine call: b = fact( a )
                                      #   1. No T registers to push
             lw      $a0,_____($fp)   #   2. Put argument into $a0
             jal     _____            #   3. Jump and link to subroutine

                                      # return from subroutine
                                      #   1. No T registers to restore

             sw     $v0,_____($fp)    # b = fact( a )

### QUESTION 20:

Fill in the blanks.