◄Summary►  ◄Notes►  ◄Example►  ◄Back►
──────────────────────────────────────────────────────────────────────────────
 
▀ On entry to the critical-error interrupt handler, bit 7 of register AH
  is clear (0) if the error was a disk I/O error; otherwise, it is set (1).
  BP:SI contains the address of a device-driver header from which addi-
  tional information can be obtained. Interrupts are disabled. The regis-
  ters will be set up for a retry operation, and an error code will be in
  the lower half of the DI register, with the upper half undefined.
 
  The lower byte of DI contains:
 
  00H  write-protect error
  01H  unknown unit
  02H  drive not ready
  03H  unknown command
  04H  data error (CRC)
  05H  bad 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  reserved
  0EH  reserved
  0FH  invalid disk change (MS-DOS 3.0 and later)
 
  Note that these are the same error codes returned by the device driver
  in the request header. Also, upon entry, the stack is set up as follows:
 
  ┌─────────┐
  │  Flags  │ ┐
  ├─────────┤ │
  │   CS    │ ├──Flags and CS:Ip pushed on stack
  ├─────────┤ │  by original Int 21H call
  │   IP    │ ┘
  ├─────────┤
  │   ES    │ ┐
  ├─────────┤ │
  │   DS    │ │
  ├─────────┤ │
  │   BP    │ │
  ├─────────┤ │
  │   DI    │ │
  ├─────────┤ │
  │   SI    │ ├──Registers at point of
  ├─────────┤ │  original Int 21H call
  │   DX    │ │
  ├─────────┤ │
  │   CX    │ │
  ├─────────┤ │
  │   BX    │ │
  ├─────────┤ │
  │   AX    │ ┘
  ├─────────┤
  │  Flags  │ ┐
  ├─────────┤ │
  │   CS    │ ├──Return address for Int 24H handler
  ├─────────┤ │
  │   IP    │ ┘
  └─────────┘  ←SS:SP on entry to Int 24H handler
 
▀ When a disk I/O error occurs, MS-DOS automatically retries the operation
  before issuing a critical- error Int 24H. The number of retries varies
  in different versions of MS-DOS but is typically in the range three to
  five.
 
▀ Int 24H handlers must preserve the SS, SP, DS, ES, BX, CX, and DX regis-
  ters. Only Int 21H Functions 01-0CH and 59H can be used by an Int 24H
  handler; other function calls will destroy the MS-DOS stack and its abil-
  ity to retry or ignore an error.
 
▀ When the Int 24H handler issues an IRET, it should return an action code
  in AL that will be interpreted by DOS as follows:
 
  0  ignore the error
  1  retry the operation
  2  terminate the program
  3  [3.0+] fail the function call in progress
 
▀ If the Int 24H handler returns control directly to the application pro-
  gram rather than to MS-DOS, it must restore the program's registers, re-
  moving all but the last three words from the stack, and issue an IRET.
  Control returns to the instruction immediately following the function
  call that caused the error. This option leaves MS-DOS in an unstable
  state until a call to an Int 21H function higher than Function 0CH is
  made.
                                    -♦-