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     The minor code 40h (Set Device Parameters) function must be used
     before an attempt to write, read, format, or verify a track on a
     logical drive. In general, the following sequence applies to any
     of these operations:
 
        ■ Get the current parameters (minor code 60h). Examine and save
          them. Set the new parameters (minor code 40h).
        ■ Perform the task.
        ■ Retrieve the original parameters and restore them with minor
          code 40h.
 
     For minor codes 40h (Set Device Parameters) and 60h (Get Device
     Parameters), the parameter block is formatted as follows:
 
     Special-functions field: offset 00h, length = 1 byte
 
     Bits    Value     Meaning
 
     0       0         Device BPB field contains a new default BPB
             1         Use current BPB
     1       0         Use all fields in parameter block
             1         Use track layout field only
     2       0         Sectors in track may be different sizes
                       (should always be avoided)
             1         Sectors in track are all same size;
                       sector numbers range from 1 to the
                       total number of sectors in the track
                       (should always be used)
     3-7     0         Reserved
 
     Device type field: offset 01h, length 1 byte
 
     Value     Meaning
 
     0         320/360K, 5.25-inch disk
     1         1.2 megabyte, 5.25-inch disk
     2         720K, 3.5-inch disk
     3         Single-density, 8-inch disk
     4         Double-density, 8-inch disk
     5         Fixed disk
     6         Tape drive
     7         Other type of block device
 
     Device attributes field: offset 02h, length 1 word
 
     Bits     Value     Meaning
 
     0        0       Removable storage medium
              1       Nonremovable storage medium
     1        0       Door lock not supported
              1       Door lock supported
     2-15     0       Reserved
 
     Number of cylinders field: offset 04h, length 1 word
 
     Maximum number of cylinders supported on the block device.
 
     Media type field: offset 06h, length 1 byte
 
     Value     Meaning
 
     0         1.2 megabyte, 5.25-inch disk
     1         320/360K, 5.25-inch disk
 
     Device BPB field: offset 07h, length 31 bytes
 
     If bit 0 = 0 in special-functions field, this field contains the
     new default BPB for the device.
 
     If bit 0 = 1 in special-functions field, the BPB in this field is
     returned by the device driver in response to subsequent Build BPB
     requests.
 
     Track layout field: offset 26h, variable-length table
 
     Length     Meaning
 
     Word       Number of sectors in track
     Word       Number of first sector in track
     Word       Size of first sector in track
     .
     .
     .
     Word       Number of last sector in track
     Word       Size of last sector in track
 
     The device BPB field is a 31-byte data structure that describes
     the current drive and its control areas. The field is formatted
     as follows:
 
     Bytes       Meaning
 
     00h-01h     Bytes per sector
     02h         Sectors per cluster (allocation unit)
     03h-04h     Reserved sectors, beginning at sector 0
     05h         Number of file allocation tables (FATs)
     06h-07h     Maximum number of root-directory entries
     08h-09h     Number of sectors
     0Ah         Media descriptor
     0Bh-0Ch     Sectors per FAT
     0Dh-0Eh     Sectors per track
     0Fh-10h     Number of heads
     11h-14h     Number of hidden sectors
     15h-18h     Large number of sectors
                 (if bytes 08h-09h = 0)
     19h-1Eh     Reserved
 
     When minor code 40h (Set Device Parameters) is used, the number of
     cylinders should not be altered, or some or all of the volume may
     become inaccessible.
 
     For minor codes 41h (Write Track) and 61h (Read Track), the
     parameter block is formatted as follows:
 
     Bytes       Meaning
 
     00h         Special-functions field (must be 0)
     01h-02h     Head
     03h-04h     Cylinder
     05h-06h     Starting sector
     07h-08h     Sectors to transfer
     09h-0Ch     Transfer buffer address
 
     For minor codes 42h (Format and Verify Track) and 62h (Verify
     Track), the parameter block is formatted as follows:
 
     Bytes       Meaning
 
     00h         Special-functions field  (must be 0)
 
                 Bits     Significance
 
                 0        Format/Verify track
                 1        Format status call [Version 4.0+]
                 1-7      Reserved (0)
 
     01h-02h     Head
     03h-04h     Cylinder
 
     In MS-DOS 4.0, this function may be called with bit 0 of the
     special-functions field set after a minor code 40h call (Set
     Device Parameters) to determine whether the driver supports the
     specified number of tracks and sectors per track. A status is
     returned in the special-functions field which is interpreted as
     follows:
 
     Value    Meaning
 
     0        Specified number of tracks and sectors per track
              supported
     1        This function not supported by the ROM BIOS
     2        Specified number of tracks and sectors per track not
              supported
     3        No disk in drive
 
     For minor codes 46h (Set Media ID) and 66h (Get Media ID), the
     parameter block is formatted as follows:
 
     Bytes       Meaning
 
     00h-01h     Information level (must be 0)
     02h-05h     Volume serial number (a hash of the time and date
                 at format time)
     06h-10h     Volume label from boot record (space padded)
     11h-19h     File system type (space padded). The string for
                 current versions can be "FAT12   ", "FAT16   ",
                 or "NO NAME  "
 
     These minor functions work in MS-DOS 4.0 and higher
 
     For minor codes 47h (Set Access Flag) and 67h (Get Access Flag),
     the parameter block is formatted as follows:
 
     Byte     Meaning
 
     00h      Special-functions field (must be 0)
     01h      Disk access flag
 
     When the disk access flag is zero, access to the medium is blocked
     by the driver. The flag is set to zero when the driver detects an
     unformatted medium or a medium with an invalid boot record. When
     the access flag is nonzero, read/write operations to the medium
     are allowed by the driver. A formatting program must set the disk
     access flag with minor code 47h before it requests minor code 42h
     (Format and Verify Track).
                                    -♦-