FAT Type and Cluster Size Depends on Logical Drive Size

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Article ID: 67321 - View products that this article applies to.
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SUMMARY

MS-DOS allocates disk space for files in units of one or more sectors; these units are called "clusters" or "allocation units." On any MS-DOS disk, a 1-byte file is allocated 1 cluster of disk space, wasting the unused area of the cluster. A file that is 3.2 clusters large is given 4 clusters. Overall, a smaller cluster size means less waste.

The cluster size for a drive is decided by FORMAT, depending on the size of the logical drive (see table, below). "Logical drive" refers to an MS-DOS volume accessed by a drive letter (A:, B:, C:, D:, and so forth). Hard disk users may want to consider cluster size when choosing how to partition their drive(s).

The cluster size of a floppy drive cannot be changed. The cluster size of a hard drive can be changed only by changing the size of the logical drive, which is done by repartitioning the hard drive.

CHKDSK displays the allocation unit size for a logical drive. FDISK's option 4 displays the size(s) of logical drives on the hard drive(s).

NOTE: DoubleSpace-compressed drives appear to have 8K clusters, but internally vary the sectors-per-cluster as necessary (this information is recorded in the MDFAT). For example, a 10K file which compresses by a factor of 2:1 actually uses 5K, or 10 sectors, of drive space.

MORE INFORMATION

The following is a table of logical drive sizes, FAT (File Allocation Table) types, and cluster sizes:
                  Drive Size      FAT Type     Sectors     Cluster
               (logical volume)              Per Cluster     Size
               ----------------   --------   -----------   -------
(Floppy Disks)      360K           12-bit         2         1K
                    720K           12-bit         2         1K
                   1.2 MB          12-bit         1       512 bytes
                   1.44 MB         12-bit         1       512 bytes
                   2.88 MB         12-bit         2         1K
(Hard Disks)     0 MB - 15 MB      12-bit         8         4K
                16 MB - 127 MB     16-bit         4         2K
               128 MB - 255 MB     16-bit         8         4K
               256 MB - 511 MB     16-bit        16         8K
               512 MB - 1023 MB    16-bit        32        16K
              1024 MB - 2047 MB    16-bit        64        32K
				
NOTES:
  • Sectors are 512 bytes in size, except on some RAM drives.
  • In the past, some OEMs have modified their versions of MS-DOS to support other sector and/or cluster sizes. The Microsoft MS-DOS 5 Upgrade Setup will, if possible, convert the logical drive to MS-DOS 5.0 compatible. This entails converting the sector size to 512 bytes while retaining the nonstandard cluster size.
  • MS-DOS determines the FAT size based on the number of clusters. If there are 4086 or fewer clusters, a 12-bit FAT is used. If there are 4087 or more clusters, a 16-bit FAT is used.
For more information on this topic, query on the following words:
cluster and disk and FAT

Properties

Article ID: 67321 - Last Review: November 16, 2006 - Revision: 2.1
APPLIES TO
  • Microsoft MS-DOS 3.1
  • Microsoft MS-DOS 3.2 Standard Edition
  • Microsoft MS-DOS 3.21 Standard Edition
  • Microsoft MS-DOS 3.3 Standard Edition
  • Microsoft MS-DOS 3.3a
  • Microsoft MS-DOS 4.0 Standard Edition
  • Microsoft MS-DOS 4.01 Standard Edition
  • Microsoft MS-DOS 5.0 Standard Edition
  • Microsoft MS-DOS 5.0a
  • Microsoft MS-DOS 6.0 Standard Edition
  • Microsoft MS-DOS 6.2 Standard Edition
  • Microsoft MS-DOS 6.21 Standard Edition
  • Microsoft MS-DOS 6.22 Standard Edition
  • Microsoft Windows 95
Keywords: 
KB67321
Retired KB Content Disclaimer
This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.

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