Interrupt 24h   Critical Error Handler  
(0:0090h) 
 
 When a critical error occurs within DOS, control is transferred to an error  
handler with an int 24h. This may be the standard DOS error handler (ABORT,  
RETRY, IGNORE) or a user-written routine. cri\tter.asm
 On entry to the error handler, AH will have its bit 7=0 (high order bit)  
if the error was a disk error (probably the most common error), bit 7=1 if 
not. 
 BP:SI contains the address of a Device Header Control Block from which  
additional information can be retrieved (see below). 
 The register is set up for a retry operation and an error code is in the  
lower half of the DI register with the upper half undefined. These are the  
error codes: 
 

 The user stack is in effect and contains the following from top to bottom: 
 
	IP      DOS registers from issuing int 24h 
	CS      int 24h 
	flags 
	AX      user registers at time of signal 
        BX      Int\21 request
	CX 
	DX 
	SI 
	DI 
	BP 
	DS 
	ES 
        IP      from original Int\21
	CS 
	flags 
  
 To reroute the critical error handler to a user-writen critical error handler, 
the following should be done: 
 
Before an int 24h occurs: 
1) The user application initialization code should save the int 24h vector and  
   replace the vector with one pointing to the user error routine. 
 
When the int 24h occurs: 
2) When the user error routine received control it should push the flag  
   registers onto the stack and execute a far call to the original int 24h 
   vector saved in step 1. 
3) DOS gives the appropriate prompt, and waits for user input (Abort, Retry, 
   Ignore, Fail). After the user input, DOS returns control to the user error  
   routine instruction following the far call. 
4) The user error routine can now do any tasks nescessary. To return to the  
   original application at the point the error occurred, the error routine needs 
   to execute an IRET instruction. Otherwise, the user error routine should  
   remove the IP, CS, and flag registers from the stack. Control can then be  
   passed to the desired point. 
	
 Provides the following values in registers on entry to the interrupt handler: 
 
entry   AH      status byte (bits) 
		7       0       disk I/O hard error 
			1       other error - if block device, bad FAT 
			        - if char device, code in DI 
		6       unused 
		5       0       if IGNORE is not allowed 
			1       if IGNORE is allowed 
		4       0       if RETRY  is not allowed 
			1       if RETRY  is allowed 
		3       0       if FAIL   is not allowed 
			1       if FAIL   is allowed 
		2 \\     disk area of error  00 = DOS area  01 = FAT 
		1 /                         10 = root dir  11 = data area 
		0       0       if read operation 
			1       if write operation 
	AL      drive number if AH bit 7 = 1, otherwise undefined 
		If it is as hard error on disk (AH bit 7=0), register AL  
		contains the failing drive number (0=A:, 1=B:, etc.). 
	BP:SI   address of a Device Header Control Block for which error 
		occurred block device if high bit of BP:SI+4 = 1 

 Provides the following values in registers on entry to the interrupt handler: 
 low byte of DI: error code (note: high byte is undefined) 
	       error code      description 
		00h             attempt to write on write-protected diskette 
		01h             unknown unit 
		02h             drive not ready 
		03h             unknown command 
		04h             data error (bad 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 
		0Fh             invalid disk change (DOS 3.x+) 
		10h     (DOS 3.x) FCB unavailable 
		11h     (DOS 3.x) sharing buffer overflow 
 

 The handler must return this information: 
 
 The registers are set such that if an IRET is executed, DOS responds according 
to (AL) as follows: 
 AL     00h  ignore the error 
	01h  retry the operation 
        02h  terminate via Int\22
	03h  fail the system call that is in progress (DOS 3.x+) 

note 1) Be careful when choosing to ignore a response because this causes DOS to 
	beleive that an operation has completed successfully when it may not  
	have. 
     2) If the error was a character device, the contents of AL are invalid. 
 
 
 








Interrupt 24h   Critical Error Handler  
 Other errors
 
 If AH bit 7=1, the error occurred on a character device, or was the result 
of a bad memory image of the Struct -FAT. The device header passed in BP:SI 
can be examined to determine which case exists. If the attribute byte high-order
bit indicates a block device, then the error was a bad FAT. Otherwise, the 
error is on a  character device. 
 If a character device is involved, the contents of AL are unpredictable, the 
error code is in DI as above. 
 
Notes: 
1.  Before giving this routine control for disk errors, DOS performs several 
    retries. The number of retries varies according to the DOS version. 
2.  For disk errors, this exit is taken only for errors occurring during an  
    Int\21 function call. Is not used for errors during Int\25 or Int\26.
3.  This routine is entered in a disabled state. 
4.  All registers must be preserved. 
5.  This interrupt handler should refrain from using DOS function calls. If 
    necessary, it may use calls 01h through 12h. Use of any other call destroys  
    the DOS stack and leaves DOS in an unpredictable state. 
6.  The interrupt handler must not change the contents of the device header. 


Interrupt 24h   Critical Error Handler  
 Other errors
7.  If the interrupt handler handles errors itself rather than returning to DOS, 
    it should restore the application program's registers from the stack,  
    remove all but the last three words on the stack, then issue an IRET. This  
    will return to the program immediately after the Int\21 that experienced
    the error. Note that if this is done DOS will be in an unstable state until 
    a function call higher than 12h is issued, therefore not recommended. 
8.  For DOS 3.x, IGNORE requests (AL=0) are converted to FAIL for critical  
    errors that occur on FAT or DIR sectors. 
9.  For DOS 3.10 up, IGNORE requests (AL=0) are converted to FAIL requests 
    for network critical errors (50-79). 
 











Interrupt 24h   Critical Error Handler  
 The device header pointed to by BP:SI is as follows: 
 
DWORD Pointer to next device (0FFFFh if last device) 
 
WORD Attributes: 
 
Bit     15      1       if character device. 
			If bit 15 is 1: 
			Bit 0 = 1 if current standard input 
			Bit 1 = 1 if current standard output 
			Bit 2 = 1 if current NULL device 
			Bit 3 = 1 if current CLOCK device 
		0       if block device 
Bit 14 is the Device\IOCTLs bit
WORD pointer to Device\drivers -strategy entry point
WORD pointer to Device\drivers -interrupt entry point
8-BYTE character device named field for block devices. The first byte is the 
number of units. 
 To tell if the error occurred on a block or character device, 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. 
 
Interrupt 24h   Critical Error Handler  
 HANDLING OF INVALID RESPONSES (DOS 3.x) 
 
	A) If IGNORE (AL=0) is specified by the user and IGNORE is not allowed 
	   (bit 5=0), make the response FAIL (AL=3). 
	B) If RETRY (AL=1) is specified by the user and RETRY is not allowed 
	   (bit 4=0), make the response FAIL (AL=3). 
	C) If FAIL (AL=3) is specified by the user and FAIL is not allowed (bit 
	   3=0), make the response ABORT. (AL=2) 


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