This article explains the actions that MS-DOS takes when a critical error
happens during a disk access. MS-DOS takes specific actions for each of the
four possible Critical Error (Interrupt 24h) responses: Ignore, Abort,
Retry, or Fail.
According to the "MS-DOS Encyclopedia," MS-DOS takes the following actions
for each possible critical error response.
Ignore The MS-DOS function call returns to the application
program as if the function had completed successfully.
For example, if the program is trying to read from the
disk, MS-DOS tells the program that all is well. This
can result in corrupted data in your disk file or
in data returned to your application.
Retry The critical error handler signals MS-DOS to reissue the
last function call made to the device that caused the
Abort The current application is terminated immediately.
Unwritten disk buffers are NOT written to disk, the
directory information is not updated, and modified
interrupt vectors are not reset. Choosing this option can
leave the system in a very unstable state.
Fail This causes MS-DOS to return to the application software>
with an error condition. In programming terms, the carry
flag is set upon exit from the INT 21h DOS function call.
This allows the application program to handle the error
itself and take appropriate action, rather than letting
MS-DOS do it.
The MS-DOS kernel and device drivers do not mark a FAT entry as being bad
when a critical error occurs, or at any other time. This task must be done
by a separate utility. The RECOVER utility provided with MS-DOS can do
this, but only if the bad sector exists within a file's allocated clusters
-- free bad clusters are not checked by RECOVER. The FORMAT utility is
typically the best solution, since it scans all areas of the disk for bad
sectors. The disadvantage is that FORMAT also destroys any data on the
Some third-party disk utilities provide disk-scanning features that examine
the entire disk data area for bad sectors, and then mark them as bad in the
FAT (without disturbing other data on the disk).