Answer:

#  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 services four and five 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 )

Answer:

Guts of main()

QUESTION 20:

Guts of `main()`