                DIFFERENCES AND NEW ADDITIONS
                  TO THE INT 24H HARD ERROR
                   HANDLER(s) FOR MSDOS 2.0


1.) Additional Constraints:

        Under previous versions it was not  explicitly  stated
    that an  INT  24H  handler  must preserve the ES register.
    It is now required that  INT  24H  handlers  preserve  ES.

        When it is  desired  to  ignore  an  error,  the  same
    registers must  be  preserved  as  when  it  is desired to
    retry the operation (SS,SP,DS,BX,CX,DX).

        It was not clearly stated in  the  past,  but  it  was
    true, that  only  system  calls 1-12 can be made by an INT
    24H handler.  Making any  other  calls  will  destroy  the
    DOS stack  and  thus  its  ability  to  retry or ignore an
    error.

        INT 24H Handlers  should  always  return  to  the  DOS
    on a  retry,  ignore,  or abort.  Failure to return to the
    DOS will leave the DOS in an unstable state  until  a  non
    1-12 function call is made.

2.) Additional features:

        Character device  errors  are  now  handled by the INT
    24H mechanism.   Previously  only  Disk  I/O  errors  were
    handled by  the  INT  24H handler.  Additional information
    is now passed to the INT 24H handler  in  the  BP  and  SI
    registers (which need not be preserved).

        BP:SI is  a  DWORD  pointer  to  the  Device Header of
    the device causing the error.  Information can  be  gotten
    from this  header  as  to  whether  the  device is a block
    or character device, and if  the  device  is  a  character
    device the  name  of the device can also be obtained.  The
    DEVICE-DRIVERS document for 2.0  contains  the  definition
    of this header format.

    NOTE:  AL  (drive number for Disk errors) is indeterminate
        on character device errors.  Bit 7  of  AH  is  always
        1 for  character  device  errors, previously bit 7 was
        1 only in the case of a bad memory image of  the  FAT.

LIST OF INT 24H ERROR CODES PASSED IN DI

    0 Write Protect violation
    1 Unknown Unit              NEW
    2 Drive not ready
    3 Unknown command           NEW
    4 CRC error
    5 Bad Drive Request Structure length   NEW
    6 Seek error
    7 Unknown media             NEW
    8 Sector not found

    9 Printer out of paper      NEW
    A Write Fault
    B Read Fault                NEW
    C General Failure

As mentioned above BP:SI points to the device header:

BP:SI->
           +--------------------------------------+
           | DWORD Pointer to next device         |
           |  (-1 if last device)                 |
           +--------------------------------------+
           | WORD Attributes                      |
           |  Bit 15 = 1 if char device 0 if blk  |
           |  if bit 15 is 1                      |
           |      Bit 0 = 1 if Current sti device |
           |      Bit 1 = 1 if Current sto output |
           |      Bit 2 = 1 if Current NUL device |
           |      Bit 3 = 1 if Current CLOCK dev  |
           |  Bit 14 is the IOCTL bit (see below) |
           |  Bit 13 is the NON IBM FORMAT bit    |
           +--------------------------------------+
           | WORD Pointer to Device strategy      |
           |      entry point                     |
           +--------------------------------------+
           | WORD Pointer to Device interrupt     |
           |      entry point                     |
           +--------------------------------------+
           | 8-BYTE character device name field   |
           | Character devices set a device name  |
           | For block devices the first byte is  |
           | The number of units                  |
           +--------------------------------------+

    To tell if the error  occured  on  a  block  or  character
device you  must  look  at bit 15 in the attribute field (WORD
at BP:SI+4).

    If the name of the character device  is  desired  look  at
the eight bytes starting at BP:SI+10.
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