Complete main()

Back to the example program (you were probably hoping that I'd forget). Here is the complete main(). There is nothing new in it; its listed here so you can see all the parts in place.

#  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

                                  # 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
                        
                                  # subroutine call
                                  #   1. No T registers to push
         lw      $a0,0($fp)       #   2. Put argument into $a0
         jal     fact             #   3. Jump and link to subroutine
         
                                  # return from subroutine 
                                  #   1. No T registers to restore
                                  
         sw     $v0,4($fp)        # b = fact( a )
        
                                  # write("factorial is:")
         li     $v0,4             #   print string service
         la     $a0,prompt2       #   address of prompt
         syscall
                                  # print( b )
         lw     $a0,4($fp)        # load a into $a0
         li     $v0,1             # print integer service
         syscall                                  

                                  # end the print line
         li     $v0,4             #   print string service
         la     $a0,lf            #   address of line feed
         syscall
                                  # epilog
                                  #   1. No return value         
         add     $sp,$fp,8        #   2. $sp = $fp + space_for_variables       
                                  #   3. No S registers to pop      
         lw      $fp,($sp)        #   4. Pop $fp
         add     $sp,$sp,4        #           
         lw      $ra,($sp)        #   5. Pop $ra
         add     $sp,$sp,4        #                                    
         jr      $ra              # return to OS 

         .data
prompt1: .asciiz "enter an int:"
prompt2: .asciiz "factorial is:"
lf:      .asciiz "\n"