v2.0/bin/INT24.DOC

   1                 DIFFERENCES AND NEW ADDITIONS
   2                   TO THE INT 24H HARD ERROR
   3                    HANDLER(s) FOR MSDOS 2.0
   4
   5
   6 1.) Additional Constraints:
   7
   8         Under previous versions it was not  explicitly  stated
   9     that an  INT  24H  handler  must preserve the ES register.
  10     It is now required that  INT  24H  handlers  preserve  ES.
  11
  12         When it is  desired  to  ignore  an  error,  the  same
  13     registers must  be  preserved  as  when  it  is desired to
  14     retry the operation (SS,SP,DS,BX,CX,DX).
  15
  16         It was not clearly stated in  the  past,  but  it  was
  17     true, that  only  system  calls 1-12 can be made by an INT
  18     24H handler.  Making any  other  calls  will  destroy  the
  19     DOS stack  and  thus  its  ability  to  retry or ignore an
  20     error.
  21
  22         INT 24H Handlers  should  always  return  to  the  DOS
  23     on a  retry,  ignore,  or abort.  Failure to return to the
  24     DOS will leave the DOS in an unstable state  until  a  non
  25     1-12 function call is made.
  26
  27 2.) Additional features:
  28
  29         Character device  errors  are  now  handled by the INT
  30     24H mechanism.   Previously  only  Disk  I/O  errors  were
  31     handled by  the  INT  24H handler.  Additional information
  32     is now passed to the INT 24H handler  in  the  BP  and  SI
  33     registers (which need not be preserved).
  34
  35         BP:SI is  a  DWORD  pointer  to  the  Device Header of
  36     the device causing the error.  Information can  be  gotten
  37     from this  header  as  to  whether  the  device is a block
  38     or character device, and if  the  device  is  a  character
  39     device the  name  of the device can also be obtained.  The
  40     DEVICE-DRIVERS document for 2.0  contains  the  definition
  41     of this header format.
  42
  43     NOTE:  AL  (drive number for Disk errors) is indeterminate
  44         on character device errors.  Bit 7  of  AH  is  always
  45         1 for  character  device  errors, previously bit 7 was
  46         1 only in the case of a bad memory image of  the  FAT.
  47
  48 LIST OF INT 24H ERROR CODES PASSED IN DI
  49
  50     0 Write Protect violation
  51     1 Unknown Unit              NEW
  52     2 Drive not ready
  53     3 Unknown command           NEW
  54     4 CRC error
  55     5 Bad Drive Request Structure length   NEW
  56     6 Seek error
  57     7 Unknown media             NEW
  58     8 Sector not found
  59
  60     9 Printer out of paper      NEW
  61     A Write Fault
  62     B Read Fault                NEW
  63     C General Failure
  64
  65 As mentioned above BP:SI points to the device header:
  66
  67 BP:SI->
  68            +--------------------------------------+
  69            | DWORD Pointer to next device         |
  70            |  (-1 if last device)                 |
  71            +--------------------------------------+
  72            | WORD Attributes                      |
  73            |  Bit 15 = 1 if char device 0 if blk  |
  74            |  if bit 15 is 1                      |
  75            |      Bit 0 = 1 if Current sti device |
  76            |      Bit 1 = 1 if Current sto output |
  77            |      Bit 2 = 1 if Current NUL device |
  78            |      Bit 3 = 1 if Current CLOCK dev  |
  79            |  Bit 14 is the IOCTL bit (see below) |
  80            |  Bit 13 is the NON IBM FORMAT bit    |
  81            +--------------------------------------+
  82            | WORD Pointer to Device strategy      |
  83            |      entry point                     |
  84            +--------------------------------------+
  85            | WORD Pointer to Device interrupt     |
  86            |      entry point                     |
  87            +--------------------------------------+
  88            | 8-BYTE character device name field   |
  89            | Character devices set a device name  |
  90            | For block devices the first byte is  |
  91            | The number of units                  |
  92            +--------------------------------------+
  93
  94     To tell if the error  occured  on  a  block  or  character
  95 device you  must  look  at bit 15 in the attribute field (WORD
  96 at BP:SI+4).
  97
  98     If the name of the character device  is  desired  look  at
  99 the eight bytes starting at BP:SI+10.
 100