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)