.TITLE    LetterWriter
       
       ; Demonstrates:  RMS, macros

       ;  Purpose - Write a letter to all customers who have a B rating.
       
       ;  Before running, the user must assign input and optionally 
       ;  assign output which is assumed to go to SYS$PRINT.  Example:
       ;    $  ASSIGN  Customers.Dat   InputFile
       ;    $  ASSIGN  LPA0:   SYS$PRINT
       ;    $  RUN Letter
       
       ;  Declare Files and Records .................
       InFAB:    $FAB   FNM=<InputFile:>   ;  FAB for input
       OutFAB:   $FAB   FNM=<SYS$PRINT>, - ;  FAB for output
                        RAT=CR             ;    use carriage return
       InRAB:    $RAB   FAB=InFAB, -       ;  Associate input file
                        UBF=InBuf, -       ;            buffer
                        USZ=InLength       ;            buffer size
       OutRAB:   $RAB   FAB=OutFAB         ;  Associate output file
                                           ;  (Other parameters assigned
                                           ;  during run.)
       
       ;  Declare Buffers ...........................
       InLength = 26
       InBuf:    .BLKB  InLength           ;  Input buffer
       DearBuf:  .ASCII "Dear (name goes here)         "
                                           ;  An output buffer for     
       DearLength = . - DearBuf
       
       ;  Other data and constants ..................
       BRating = ^A/B/
       Comma = ^A/,/
       
       ;  Macro for checing R0 for error codes
                 
                 .MACRO CheckError    ?Continue
                   BLBS   R0, Continue     ;  If R0 is even
                   BRW    ErrorStop        ;    branch to ErrorStop
       Continue:
                 .ENDM  CheckError
       
       ; ............................................
       ;  Begin program .............................
       ; ............................................
       
                 .ENTRY Letter, 0
       
       ;  Initialize files and records ..............
                 $OPEN    FAB=InFAB        ;  Open input file
                 CheckError
                 $CREATE  FAB=OutFAB       ;  Create output file
                 CheckError
                 $CONNECT RAB=InRAB        ;  Connect input record
                 CheckError
                 $CONNECT RAB=OutRAB       ;  Connect output record
       
       ;  Begin main loop ...........................
       
       ;  Read input record .........................
       Loop:     $GET     RAB=InRAB        ;  Read record
                 CMPL     R0, #RMS$_EOF    ;  If end of file
                 BNEQ     Cont             ;    then
                 BRW      Done             ;    terminate program
       Cont:     CheckError                ;  Check for errors
       ;  Check rating ..............................-
                 CMPB     InBuf, #BRating  ;  If rating is not a B
                 BNEQ     Loop             ;     Repeat at Loop
       ;  Process customers who have a B rating .....
                 BSBW     WriteDearCustomer;  Output "Dear ...,"
       ;         BSBW     WriteBodyOfLetter;  Future routine to write
                                           ;      letter
                 BRW      Loop             ;  Next customer
       
       ;  Done - terminate run ......................
       
       Done:     $CLOSE   FAB=InFAB        ;  Close input file
                 CheckError
                 $CLOSE   FAB=OutFab       ;  Close output file
                 CheckError
                 $EXIT_S                   ;  Normal exit
       
       ;  Error exit ................................
       
       ErrorStop:
                 $EXIT_S  R0               ; Terminate with message
       
       ;  ...........................................
       ;  Subroutines ............................... 
       ;  ...........................................
       
       ;  Subroutine to write "Dear CustomerName," ..
       
       ;  Assumes customer name begins in 3rd .......
       ;  character of Inbuf ........................
       
       WriteDearCustomer:
                 SUBW3    #2, InRAB+RAB$W_RSZ, R6
                                           ;  Calculate length of name
                 MOVC3    R6, -            ;  Length of customer name
                          InBuf+2, -       ;  Location of customer name
                          DearBuf+5        ;  Address of name in DearBuf
                 MOVB     #Comma, (R3)     ;  Put comma at end of name
                 ADDW3    R6, #6, OutRAB+RAB$W_RSZ
                                           ;  Store length of DearBuf
                 MOVAB    DearBuf, OutRAB+RAB$L_RBF
                                           ;  Store address of DearBuf
                 $PUT     RAB=OutRAB       ;  Output "Dear name,"
                 CheckError
                 RSB
       
                 .END     Letter
       ;