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