Explanation of Critical Error Codes Returned by ERDEV (57368)

SUMMARY

ERDEV is an integer function that returns an error code from the last device to declare a critical error. ERDEV is set by the critical error handler, Interrupt 24h, when MS-DOS detects a critical MS-DOS call error.

For block and character device errors, ERDEV will contain the error code from Interrupt 24h in the lower 8 bits. For block devices only, bit positions 9 to 16 contain device-attribute information which is found in the device-driver header of the device that the error is coming from.

ERDEV can also be set by a time-out error on the communications port and indicates which option in the OPEN COM statement (CD, CS, or DS) is experiencing the time-out.

When using Microsoft Basic PDS for MS-DOS, version 7.0 or 7.1, ERDEV can also be set by a time-out error on the communications port and indicates which option in the OPEN COM statement (CD, CS, or DS) is experiencing the time-out.

The information in this article is also included with the Help file provided with the Standard and Professional Editions of Microsoft Visual Basic for MS-DOS, version 1.0.

MORE INFORMATION

If ERDEV returns an error from a block device (such as a floppy disk drive or fixed disk) or from a character device (such as a terminal or printer), the low byte of ERDEV will return the MS-DOS error code, a value between 0 and 12. The following is a list of these errors:

0 write-protect error
1 unknown unit
2 drive not ready
3 unknown command
4 date error (CRC)
5 bad request structure length
6 seek error
7 unknown media type
8 sector not found
9 printer out of paper
10 write fault
11 read fault
12 general failure
13 reserved
14 reserved
15 invalid disk change (MS-DOS version 3.0 only)

For more information on MS-DOS error codes, see the section on Interrupt 24h on Page 481 of "Advanced MS-DOS Programming, 2nd Edition," by Ray Duncan (Microsoft Press, 1988).

If the device returning an error is a block device, the high byte of the integer returned by ERDEV will contain device attribute information. This device attribute information comes from the device-attribute word in the device header. The only bits from this word returned by ERDEV are bits 15, 14, 13, XX, 3, 2, 1, and 0, in that order. XX indicates that bits 12 through 4 of the device-attribute word will always return zero. The following is a description of the bits in the device-attribute word that are meaningful to ERDEV:

     Bit
   Position   Bit     Significance
   --------   ---     ------------

      8       15      0 if block device
      7       14      1 if IOCTL read and write supported
      6       13      1 if BIOS parameter block in boot sector
                        should be used to determine media
                        characteristics
                      0 if media ID byte should be used
      4        3      1 if current CLOCK$ device
      3        2      1 if current NUL device
      2        1      1 if driver supports 32-bit sector addressing
                        (MS-DOS version 4.0)
      1        0      1 if current standard input device (stdin)

For more information on device-attribute words, see Page 264 of the "Advanced MS-DOS Programming" Microsoft Press book.

ERDEV also returns information for COM time-out errors in the low byte. If there is a time-out, ERDEV is set to a value that indicates the signal line that timed out, according to the following table:

   ERDEV Value    Signal Line
   -----------    -----------

      128         Clear to Send (CTS) timeout
      129         Data Set Ready (DSR) timeout
      130         Data Carrier Detect (DCD) timeout

The following program lines generate the MS-DOS and COM time-out error codes (low byte) and device attribute information (high byte):

   x= ERDEV
   DosErrCode = x AND &HFF            ' Low byte of ERDEV.
   DevAttr = (x AND &HFF00) \256      ' High byte of ERDEV.

The following example prints the values of ERDEV after the program tries to OPEN a file on a floppy when the door is open:

Example

' Open floppy drive A: door open before running this example.
ON ERROR GOTO ErrorHandler
OPEN "a:\file.dat" FOR OUTPUT AS #1
END

ErrorHandler:
   PRINT "ERDEV$='"; ERDEV$; "'"
   PRINT "ERDEV=&H"; HEX$(ERDEV)
   DosErrCode% = ERDEV AND &HFF  ' low byte of ERDEV.

   RESUME NEXT