FAT Type and Cluster Size Depends on Logical Drive Size
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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-DOSdisk, a 1-byte file is allocated 1 cluster of disk space, wasting theunused area of the cluster. A file that is 3.2 clusters large is given4 clusters. Overall, a smaller cluster size means less waste.
The cluster size for a drive is decided by FORMAT, depending on thesize of the logical drive (see table, below). "Logical drive" refersto an MS-DOS volume accessed by a drive letter (A:, B:, C:, D:, and soforth). Hard disk users may want to consider cluster size whenchoosing how to partition their drive(s).
The cluster size of a floppy drive cannot be changed. The cluster sizeof a hard drive can be changed only by changing the size of thelogical drive, which is done by repartitioning the hard drive.
CHKDSK displays the allocation unit size for a logical drive. FDISK'soption 4 displays the size(s) of logical drives on the hard drive(s).
NOTE: DoubleSpace-compressed drives appear to have 8K clusters, butinternally vary the sectors-per-cluster as necessary (this informationis recorded in the MDFAT). For example, a 10K file which compressesby a factor of 2:1 actually uses 5K, or 10 sectors, of drive space.
The following is a table of logical drive sizes, FAT (File AllocationTable) 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 32KNOTES:
- 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.
cluster and disk and FAT
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
Article ID: 67321 - Last Review: 12/04/2015 09:05:31 - Revision: 2.1
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
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