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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.
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 drive.
Error Action ---------------- 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 critical error. 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.
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).
Article ID: 67586 - Last Review: October 26, 2013 - Revision: 3.0
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