 Versions through Windows NT 3.51 did not have an application programming interface for moving data clusters on hard disks.Executive Software, later renamed Diskeeper Corporation, released Diskeeper defragmentation software for Windows NT 3.51, which shipped with a customized version of the NT kernel and file system drivers that could move clusters.Microsoft included file system control (FSCTL) commands to move clusters in the Windows NT 4.0 kernel, which worked for both NTFS and FAT partitions.