Atari 400 Technical Reference Manual page 73

The Disk File Manager will then insert the name D and the Handler
vector table address in the device table.
NOTE: There is a discrepancv between the Disk File Manager's
numbering of diskette sectors (0-719) and the disk controller's
numbering of diskette sectors (1-720)i
as a result, onlv sectors
1- 719 are used bV the Disk File Manager.
The Disk File Manager
us~s
the Disk Handler to perform all
diskette reads and writes; the DFM's function is to support and
maintain the directory/file/bitmap structures as described in the
following pages:
OPERATING SYSTEM C016555 -- Section 5
88