Open DOS File

From CometWiki

Latest revision as of 09:51, 13 June 2009

Open DOS File

Syntax:

DOSFC(AX-value, CX-value, file-name) EXCP=statement-label  

Entry:

 AX-value = "@3D00@"         (read only)
 CX-value = "@0000@"

(or)

 AX-value = "@3D42@"         (read/write)
 CX-value = "@0000@"

Return:
If the function call is successful, the AX field will contain the file's handle.

If an exception occurs, byte 2 of the AX field will contain the DOS error code (in hex).

Discussion:
The DOSFC function call can be used to open a DOS file. For read-only purposes, the function requires that the AX-value be set to "@3D00@" and the CX-value be set to null.

For read/write purposes, the function requires that the AX-value be set to "@3D42@" and the CX-value be set to null.

After the function is executed, the file will be open and the AX register will contain the file's handle. As noted earlier, it is the responsibility of the programmer to ensure that any file handles opened are closed upon completion of the desired function. Failure to comply will result in degradation of system performance and possible system hangs.

Example:

 ! S SFOPEN,DSK
 ! O FOPEN,DSK
 ! L T00,E
 ! R QMONITOR
 !
 !==========  OPEN DOS FILE HANDLE  =============================
 !
 LENGTH 2 & LOCAL AX$,BX$,CX$,DX$        ! Define registers
            LOCAL FILEHANDLE$            ! Define file handle
 LENGTH 64 & LOCAL FILENAME$             ! Define file name
 !
 LENGTH  3 & LOCAL DOSCODE$              ! Define DOS error code
 LENGTH 37 & LOCAL DOSMESSAGE$           ! Define DOS message
 !
 100 FORMAT (ET)                         ! Screen format
 !
 1000 FORMAT DOSMESSAGE$                 ! File input format
 !
 CLEAR                                   ! Initialize variables
 PRINT (0,100)                           ! Set typewriter mode
 !
 PRINT (0) "ENTER FILENAME TO OPEN:"     ! Display prompt
 INPUT (0) FILENAME$                     ! Enter file name
 IF FILENAME$ = "" THEN RUN "QMONITOR"   ! If null, then stop
 FILENAME$ = FILENAME$ + "@00@"          ! Add null byte to name
 !
 AX$ = "@3D42@"                          ! Set AX to "OPEN FILE"
 CX$ = "@0000@"                          ! Set CX to null
 !
 DOSFC(AX$,CX$,FILENAME$) EXCP=BRANCH1   ! Perform DOSFC call
 !
 FILEHANDLE$ = AX$                       ! Save file handle
 !
 PRINT (0) FILENAME$;" OPENED SUCCESSFULLY."
 PRINT (0) "PRESS ENTER TO CLOSE THE FILE."
 INPUT (0) ""
 !
 AX$ = "@3E00@"                          ! Set AX to "CLOSE FILE"
 BX$ = FILEHANDLE$                       ! Set BX to file handle
 CX$ = "@0000@"                          ! Set CX to null
 DX$ = "@0000@"                          ! Set DX to null
 !
 DOSMS(AX$,BX$,CX$,DX$) EXCP=BRANCH2     ! Perform DOSMS call
 !
 PRINT (0) FILENAME$;" CLOSED SUCCESSFULLY."
 PRINT (0) "PRESS ENTER TO STOP."
 INPUT (0) ""
 RUN "QMONITOR"
 !
 BRANCH1:                                ! Exception routine 1
 PRINT (0) "FILE NOT OPENED."            ! Display message
 OPEN (1) "QERCOMET"                     ! Open error file
 DOSCODE$ = "D" + HEXASC(SUB(AX$,2,1))   ! Construct key to file
 READ (1,1000) KEY=DOSCODE$              ! Read error record
 PRINT (0) "DOS error code: ";DOSCODE$   ! Display DOS error code
 PRINT (0) DOSMESSAGE$                   ! Display error message
 INPUT (0) ""                            ! Hold
 CLOSE (1)                               ! Close error file
 RUN "QMONITOR"                          ! Exit
 !
 BRANCH2:                                ! Exception routine 2
 PRINT (0) "FILE NOT CLOSED."            ! Display message
 OPEN (1) "QERCOMET"                     ! Open error file
 DOSCODE$ = "D" + HEXASC(SUB(AX$,2,1))   ! Construct key to file
 READ (1,1000) KEY=DOSCODE$              ! Read error record
 PRINT (0) "DOS error code: ";DOSCODE$   ! Display DOS error code
 PRINT (0) DOSMESSAGE$                   ! Display error message
 INPUT (0) ""                            ! Hold
 CLOSE (1) & RUN "QMONITOR"              ! Close error file/exit
 END