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ERDEV returns an error code from the last device to generate an error.
ERDEV$ returns the name of the device that generated the error.
 
ERDEV
ERDEV$
    ■ ERDEV$ contains the 8-byte character device name if the error is
      on a character device, such as a printer, or the 2-byte block
      device name (A:, B:, etc.) if the device is not a character device.
      It contains the 3-byte block name (COM) if a communications port
      experiences a timeout.
 
    ■ ERDEV is set by the critical error handler (interrupt 24H) when
      DOS detects a critical DOS call error from a character or block
      device. It also is set by a timeout error on a communications
      port.
 
    ■ ERDEV returns an integer value that contains information
      about the error.  The format of the information depends upon
      the source of the error.
 
    ■ For block device errors, bits 0-7 (the low byte) of ERDEV contain the
      DOS error code. Bits 8-15 (the high byte) contain device-attribute
      information:
 
      Bit(s)    Value     Meaning
      ══════    ══════    ═══════════════════════════════════════════════
      0-7       xxH       Contains DOS error code xxH
        8        0        Read operation
                 1        Write operation
      9-10                Indicate the affected disk area:
                00            MS-DOS
                01            File allocation table
                10            Root directory
                11            Files area
        11*      0        Fail response not allowed
                 1        Fail response allowed
        12*      0        Retry response not allowed
                 1        Retry response allowed
        13*      0        Ignore response not allowed
                 1        Ignore response allowed
        14                Not used
        15       0        Always zero
 
          *bits 11-13 apply only to MS-DOS versions 3.1 and later.
 
    ■ For character device errors, bits 0-7 contain the DOS error code.
      The upper bits are not significant.
 
    ■ For COM timeout errors, ERDEV returns a value indicating which option
      in the OPEN COM statement is experiencing the timeout. Therefore, it's
      not necessary to break ERDEV into low and high bytes.
 
      ERDEV Value        OPEN COM Option Timeout
      ══════════════     ══════════════════════════════════════════════
      80H                CS (CTS timeout)
      81H                DS (DSR timeout)
      82H                CD (RLSD timeout)
 
Usage Notes
    ■ Because ERDEV and ERDEV$ return meaningful information only after
      an error, they usually are used in error handlers specified by an
      ON ERROR statement.
 
    ■ ERDEV and ERDEV$ cannot be used on the left side of an assignment.
 
    ■ Use ERDEV and ERDEV$ only in DOS.
 
    ■ The following example program lines extract the low and high
      bytes from the ERDEV value.  DosErrCode stores the block
      or character device error code. DevAttr stores the block
      device attribute information.
 
        DosErrCode = ERDEV AND &HFF          'low byte of ERDEV
        DevAttr = (ERDEV AND &HFF00) \ 256   'high byte of ERDEV
 
    ■ For more information about device attributes, see the Microsoft MS-DOS
      Programmer's Reference, or books such as The Peter Norton Guide to the
      IBM PC or Advanced MS-DOS.