Topic       : TOS - The Operating System
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Version     : tos.hyp (December 19, 2008)
Subject     : Programmieren/Atari
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Compiled on : Atari
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View Ref-File11.5.16.34  The Drive Medium Descriptor (DMD)                      TOS

The DMD is created by the kernel (!) for each opened drive and also 
released again if necessary. The XFS stores all data here that it has 
to memorize for the drive. The following fields are the ones that the 
kernel requires (the ancestor object from which the XFS derives its 
XFS-DMD):

d_xfs:      DS.L   1    /* 0x00: The filesystem driver            */
d_drive:    DS.W   1    /* 0x04: Drive number 0..25               */
d_root:     DS.L   1    /* 0x06: Pointer to DD of the root        */
d_biosdev:  DS.W   1    /* 0x0a: BIOS drive, or -1                */
d_driver:   DS.L   1    /* 0x0c: With devcode, defines the medium */
d_devcode:  DS.L   1    /* 0x10: e.g. SCSI target and drive       */
d_dfs:      DS.L   1    /* 0x14: DOS-specific filesystem driver   */

Other XFSs or DFSs enter further data into the DMD. One can envisage 
the DMD as an object class. A DMD derived for the DOS_XFS only has the 
additional entry d_dfs. The FAT enters extra data such as the cluster 
size and the number of sectors.

See also: MagiC's XFS-concept   DD