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 Explanation of MS-DOS Critical Error Codes Returned by ERDEV - Q57368
 
 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.
 
 The information in this article applies to:
 
  - The Standard and Professional Editions of Microsoft Visual Basic
    version 1.0 for MS-DOS
  - Microsoft GW-BASIC versions 3.2, 3.22 and 3.23 for MS-DOS
  - Microsoft QuickBasic versions 2.0, 2.01, 3.0 4.0, 4.0b, and 4.5 for
    MS-DOS
  - Microsoft Basic Professional Development System (PDS) versions 7.0
    and 7.1 for MS-DOS
  - Microsoft Basic Compiler versions 6.0 and 6.0b for MS-DOS
 
 When using Microsoft Visual Basic versions 1.0 for MS-DOS, or Microsoft
 Basic PDS version 7.0 or 7.1 for MS-DOS, 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.
 
 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 a MS-DOS error code which
 is 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 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 ROM 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.
 
 When using Microsoft Visual Basic version 1.0 for MS-DOS, or
 Microsoft Basic PDS version 7.0 or 7.1 for MS-DOS, 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 code fragment generates 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) \56      ' High byte of ERDEV.
 
 The following example prints the values of ERDEV after the program
 tries to OPEN a read only file for OUTPUT:
 
 Example
 -------
 
 ' To try this example in VBDOS.EXE:
 ' 1. From the File menu, choose New Project.
 ' 2. Copy the code example to the Code window.
 ' 3. Press F5 to run the program.
 
 DEFINT A-Z
 ON ERROR GOTO ErrorHandler
 OPEN "C:\heFile.DAT" FOR OUTPUT A #1   ' TheFile is a read-only
                                        ' file.
 END
 
 ErrorHandler:
    x = ERDEV
    DosErrCode = x AND &HFF              ' Low byte of ERDEV.
    PRINT "The MS-DOS error code returned by ERDEV => "; x
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