How DoubleSpace Uses Sector Allocation (98407)
The information in this article applies to:
• Microsoft MS-DOS operating system 6.0
• Microsoft MS-DOS operating system 6.2
• Microsoft MS-DOS operating system 6.22
• Microsoft Plus! for Windows 95
• Microsoft Windows 95
This article was previously published under Q98407
This information applies to both Microsoft DoubleSpace and Microsoft
DriveSpace. For MS-DOS 6.00 and 6.20, use DBLSPACE in place of DRVSPACE
for commands and filenames.
DriveSpace uses sector allocation granularity even though you cannot see
evidence of this in the Bytes Available On Disk value reported by CHKDSK.
Since the file allocation table (FAT) system is cluster based, DriveSpace
uses its own FAT (which is called the MDFAT [Microsoft DriveSpace FAT]) to
achieve sector allocation. Each entry in the FAT corresponds to an entry in
the MDFAT, as well as a logical cluster of 8 kilobytes (K) for MS-DOS and
Windows 95 DriveSpace, or 32 K for Plus! DriveSpace 3. Each MDFAT entry
also corresponds to a sequence of sectors less than or equal to 8K (MS-DOS
and Windows 95 DriveSpace) or 32 K (Plus! DriveSpace 3).
NOTE: Plus! DriveSpace 3 supports fragmented MDFAT clusters, so a 32 K
DriveSpace 3 MDFAT cluster need not necessarily be contiguous.
Since disk size reporting is accomplished by adding up the total number of
clusters and then multiplying by the cluster size, there is no way to get a
non-cluster size value for Total Disk Space or Bytes Available On Disk
values from CHKDSK.
Estimated Compression Ratio (ECR)
When you run CHKDSK on a DriveSpace drive, it lists the number of used
clusters in terms of uncompressed data. When it lists the number of unused
clusters, it must specify some multiple of clusters based on how many
compressed clusters it thinks can fit inside the actual free space. It does
this by taking the ECR and multiplying it by the actual free space on the
drive (in sectors) and then dividing that by the number of sectors in a
As a result, free disk space can only be specified as a integral number of
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