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Int 24h                 Critical-Error Handler

    Critical-Error Handler (Interrupt 24h) carries out program-specific
    actions in response to critical errors during read and write
    operations. DOS issues this interrupt if a critical error occurs
    while a system function is attempting to read from or write to a
    device or file. The handler carries out its actions then returns
    to the system to retry the function, terminate the function, or
    terminate the current program.

    Programs must not issue Interrupt 24h.

    ------------------------------------------------------------------

    Before issuing Int 24h, DOS does the following:

        .   Sets the AX, DI, BP, and SI registers with information
            about the error, such as the error value, location of the
            error, type of device, and type of operation.

        .   Sets the program's stack to be the current stack. When the
            handler receives control, the stack has the following
            contents (from the top of the stack):

                The return address (CS:IP) and the flags needed for
                the IRET instruction back to the system.

                The contents of the program's registers at the time
                the system function that caused the error was called.
                The registers are preserved in the following order:
                AX,BX,CX,DX,SI,DI,BP,DS, and ES.

                The return address (CS:IP) and the flags needed for
                the IRET instruction back to the program.

        .   Sets internal system variables, such as those for InDOS
            and ErrorMode. InDOS is set to zero to permit the handler
            to call system functions. ErrorMode is set to 1 to prevent
            the system from issuing Int 24h again before the handler
            returns; DOS issues only one Int 24h at a time.

    ------------------------------------------------------------------

    DOS passes information about the error to the handler by using the
    following registers:

      Register  Description
        AH      Specifies information about when and where the error
                occurred, as well as how the critical-error handler
                can respond to the error.
                Bit     Meaning
                0       0 = Error during read operation
                        1 = Error during write operation
                2,1     Location of error (only if the error
                        occurred on a block device)
                        00 = reserved sector (DOS area)
                        01 = file allocation table (FAT)
                        10 = directory
                        11 = data area
                3       0|1 = Handler cannot|can Fail the function  (*)
                4       0|1 = Handler cannot|can Retry the function (*)
                5       0|1 = Handler cannot|can Ignore the error   (*)
                              (*) = (DOS 3.0+)
                6       Reserved
                7       0 = Error occurred on a block device
                        1 = Error occurred on a character device
                            if bit 15 in the dhAttributes field
                            (offset 04h) of the DeviceHeader structure
                            is set, otherwise the error occurred in
                            the memory image of the FAT.
        AL      Specifies the drive number (0 = A, 1 = B, 2 = C, etc.)
                if the error occurred on a block device. This register
                is not used for errors that occur on character
                devices.
        DI      Specifies the error value:
                (Low-order byte of DI only, upper byte is undefined)
                00h     Attempt to write on write-protected disk
                01h     Unknown unit
                02h     Drive not ready
                03h     Unknown command
                04h     CRC error in data
                05h     Incorrect length of drive-request structure
                06h     Seek error
                07h     Unknown media type
                08h     Sector not found
                09h     Printer out of paper
                0Ah     Write fault
                0Bh     Read fault
                0Ch     General failure
        BP:SI   Points to the DeviceHeader structure that contains
                information about the device on which the error
                occurred.
                The Int 24h handler must not change the contents of
                the DeviceHeader struture.

    ------------------------------------------------------------------

    The critical-error handler must determine what action to take in
    response to the error. For example, the default handler displays
    information about the error and prompts the user for input on how
    to proceed.

    The Int 24h handler can call the following Interrupt 21h
    functions:
                Character I/O           Functions 01h through 0Ch
                Get Version Number      Function 30h
                Get Ctrl-C Check Flag   Function 3300h
                Set Ctrl-C Check Flag   Function 3301h
                Get Startup Drive       Function 3305h
                Get MS-DOS Version      Function 3306h
                Set PSP Address         Function 50h
                Get PSP Address         Functions 51h and 62h
                Get Extended Error      Function 59h
                No other system functions are permitted

    Get Extended Error (function 59h) retrieves detailed information
    about the error and is useful for handlers that display as much
    information as possible about the error.


    The Int 24h handler must preserve the BX,CX,DX,DS,ES,SS, and SP
    registers and restore the preserved values before returning to the
    system. The critical-error handler returns to the system by using
    the IRET instruction. Before returning, it also must set the AL
    register to a value specifying the action the system should take.
    Depending on what actions are allowed (as specified by bits 3, 4,
    and 5 in the AH register), the AL register can contain one of the
    following values:

        Value   Meaning
        00h     Ignore the error. The system permits the system
                functions to return to the program as if it had
                completed successfully.
        01h     Retry the operation. The system calls the system
                function again. In this case, the system expects the
                handler to have preserved and restored registers
                before returning.
        02h     Terminate the program. The system sets the termination
                type to be exit_critical_error (02h) and carries out
                the same actions as End Program (Int 21h function 4ch).
        03h     Terminate the function ("fail"). The system permits
                the system function to return to the program with an
                error value.
        
    DOS checks the value to ensure that it is allowed. If values 00h
    and 01h are not allowed, the system terminates (fails) the
    function. If value 03h is not allowed, the system terminates the
    program.

    ------------------------------------------------------------------

    COMMAND.COM provides the default critical-error handler, which
    displays a message about the error and, after displaying a
    question such as "Abort, Retry, Fail, or Ignore?", prompts the
    user for a response.