Int 24  C - Dos 1+ - Critical Error Handler                                [D]


Notes: invoked when a critical (usually hardware) error is encountered by DOS
     (see #2231); should never be called directly
   when DOS terminates a program, it copies the previous value of the
     INT 24 vector out of the PSP (see #1066) and into the interrupt
     vector table

See Also: INT 21/AH=95h

(Table 2231)
Values critical error handler is called with:
   AH = type and processing flags (see #2232)
   AL = drive number if AH bit 7 clear
   BP:SI -> device driver header (see #1332 at INT 21/AH=52h)
       (BP:[SI+4] bit 15 set if character device)
   DI low byte contains error code if AH bit 7 set (see #2233)
   STACK:  DWORD   return address for INT 24 call
       WORD    flags pushed by INT 24
       WORD    original AX on entry to INT 21
       WORD    BX
       WORD    CX
       WORD    DX
       WORD    SI
       WORD    DI
       WORD    BP
       WORD    DS
       WORD    ES
       DWORD   return address for INT 21 call
       WORD    flags pushed by INT 21

Return: AL = action code (see #2234)
   SS,SP,DS,ES,BX,CX,DX preserved

Notes: the only DOS calls the handler may make are INT 21/AH=01h-0Ch,30h,59h
   if the handler returns to the application by popping the stack, DOS
     will be in an unstable state until the first call with AH > 0Ch
   for DOS 3.1+, IGNORE (AL=00h) is turned into FAIL (AL=03h) on network
     critical errors
   if IGNORE specified but not allowed, it is turned into FAIL
   if RETRY specified but not allowed, it is turned into FAIL
   if FAIL specified but not allowed, it is turned into ABORT
   (DOS 3.0+) if a critical error occurs inside the critical error
     handler, the DOS call is automatically failed (AL set to 03h and
     the INT 24 call skipped)

Bitfields for critical error type and processing flags:
Bit(s) Description (Table 2232)
 7 clear = disk I/O error
   set   = -- if block device, bad FAT image in memory
       -- if char device, error code in DI
 6 unused
 5 Ignore allowed (DOS 3.0+)
 4 Retry allowed (DOS 3.0+)
 3 Fail allowed (DOS 3.0+)
 2-1   disk area of error
   00 = DOS area   01 = FAT
   10 = root dir   11 = data area
 0 set if write, clear if read

(Table 2233)
Values for critical error code:
 00h   write-protection violation attempted
 01h   unknown unit for driver
 02h   drive not ready
 03h   unknown command given to driver
 04h   data error (bad CRC)
 05h   bad device driver request structure length
 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
 0Dh   (DOS 3.0+) sharing violation
 0Eh   (DOS 3.0+) lock violation
 0Fh   invalid disk change
 10h   (DOS 3.0+) FCB unavailable
 11h   (DOS 3.0+) sharing buffer overflow
 12h   (DOS 4.0+) code page mismatch
 13h   (DOS 4.0+) out of input
 14h   (DOS 4.0+) insufficient disk space

(Table 2234)
Values for critical error handler action code:
 00h   ignore error and continue processing request
 01h   retry operation
 02h   terminate program as though INT 21/AH=4Ch called (INT 20h for DOS 1.x)
 03h   fail system call in progress (DOS 3+)