Article: Q67321
Product(s): Microsoft Disk Operating System
Version(s): MS-DOS:3.x,4.x,5.x,6.0,6.2,6.21,6.22; Win2000:95
Operating System(s):
Keyword(s): msdos
Last Modified: 17-DEC-2000
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The information in this article applies to:
- Microsoft MS-DOS operating system versions 3.1, 3.2, 3.21, 3.3, 3.3a, 4.0, 4.01, 5.0, 5.0a, 6.0, 6.2, 6.21, 6.22
- Microsoft Windows 95
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SUMMARY
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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
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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
4085 or fewer clusters, a 12-bit FAT is used. If there are 4086 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
Additional query words: dblspace 3.20 3.21 3.30 3.30a 4.00 4.01 5.00 5.00a partition allocation unit 6.00 6.20 cluster granularity 6.21 6.22 win95 msdos
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Keywords : msdos
Technology : kbWin95search kbZNotKeyword3 kbMSDOSSearch kbMSDOS321 kbMSDOS400 kbMSDOS320 kbMSDOS330a kbMSDOS621 kbMSDOS622 kbMSDOS620 kbMSDOS600 kbMSDOS310 kbMSDOS500 kbMSDOS330 kbMSDOS401 kbMSDOS500a
Version : MS-DOS:3.x,4.x,5.x,6.0,6.2,6.21,6.22; Win2000:95
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