Microsoft KB Archive/43262

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Patch for MS-DOS Version 4.01 and Files Larger Than 32 MB PSS ID Number: Q43262 Article last modified on 09-24-1993 PSS database name: O_MSDOS

4.00 4.01


The information in this article applies to:
- Microsoft MS-DOS operating system versions 4.0 and 4.01


There is a problem in MS-DOS versions 4.0 and 4.01 that causes file allocation table (FAT) corruption when files larger than 32 MB are appended to.

This problem does not occur in later versions of MS-DOS.


This problem occurs in MS-DOS versions 4.0 and 4.01 when ALL of the following occur:

  1. BUFFERS (CONFIG.SYS command) are allocated with secondary cache enabled.
  2. A disk partition larger than 32 MB is in effect.
  3. You are sequentially accessing a file and any part of that file falls within the range of 32 MB to 32 MB plus 10K on the disk, and the portion of the file being accessed falls between that same narrow range.
  4. Immediately after sequentially accessing this file, you perform an operation that causes the system to read a FAT sector.

If all of these conditions are true, the FAT is corrupted (and data is lost). The chance of all of the above occurring in the sequence described is remote.


Microsoft has provided a patch utility that corrects this problem. The name of the utility is PATCH.EXE. A description file, READ.ME, is included to provide instruction on how to use the PATCH.EXE program. These two files have been archived into the file DOSPATCH. This file can be found in the Software/Data Library by searching on the word DOSPATCH, the Q number of this article, or S12254. DOSPATCH was archived using the PKware file-compression utility.

Additional reference words: cserve 4.0 noupd softlib

Copyright Microsoft Corporation 1993.