### A good answer might be:

A downward arc corresponds to a push of one stack frame. An upward arc corresponds to a pop of one stack frame.

# Prolog of `main()`

Here is the main routine's pseudocode and prolog. Notice that there are two variables.

    #  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      $__,($sp)
             sub     $sp,$sp,4        #   2. Push caller's frame pointer
             sw      $__,($sp)
                                      #   3. No S registers to push
             sub     $fp,$sp,_____    #   4. $fp = $sp - space_for_variables
             move    $sp,$fp          #   5. $sp = $fp

### QUESTION 19:

Fill in the blanks.