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  Constant:  _DISK_FORMAT, _DISK_READ, _DISK_RESET, _DISK_STATUS,
             _DISK_VERIFY, _DISK_WRITE
 
  Context:   _bios_disk
 
  Synopsis:  The <service> argument of _bios_disk selects the disk
             function desired.
 
     The argument is set with one of the following constants:
 
     Constant         Action
 
     _DISK_FORMAT     Formats the track specified by <diskinfo>. The
                      head and track fields indicate the track to be
                      formatted. Only one track can be formatted in a
                      single call. The buffer field points to a set of
                      sector markers. The format of the markers depends
                      on the type of disk drive; see the "IBM PC
                      Technical Reference" to determine the marker
                      format. There is no return value.
 
     _DISK_READ       Reads one or more disk sectors into memory. This
                      service uses all fields of the structure pointed
                      to by <diskinfo>. If no error occurs, the
                      function returns 0 in the high-order byte and
                      returns the number of sectors read in the low-
                      order byte. If there is an error, the high-order
                      byte will contain a set of status flags, as
                      under _DISK_STATUS (below).
 
     _DISK_RESET      Forces the disk controller to do a hard reset,
                      preparing for floppy-disk I/O. This is useful
                      after an error occurs in another operation, such
                      as a read. If this service is specified, the
                      <diskinfo> argument is ignored.
 
     _DISK_STATUS     Obtains the status of the last disk operation. If
                      this service is specified, the <diskinfo>
                      argument is ignored. Status is returned in the
                      high-order bits of the return value, as listed
                      below:
 
                      Bits       Meaning
 
                      0x01**     Invalid request or bad command
                      0x02**     Address mark not found
                      0x04**     Sector not found
                      0x05**     Reset failed
                      0x07**     Drive parameter activity failed
                      0x09**     DMA overrun
                      0x0A**     Bad sector flag detected
                      0x10**     Data read (ECC) error
                      0x11**     Corrected data read (ECC) error
                      0x20**     Controller failure
                      0x40**     Seek error
                      0x80**     Disk timed out or failed to respond
                      0xAA**     Drive not ready
                      0xBB**     Undefined error
                      0xCC**     Write fault on drive
                      0xE0**     Status error
 
     _DISK_VERIFY     Checks the disk to be sure the specified sectors
                      exist and can be read. It also runs a CRC (cyclic
                      redundancy check) test. This service uses all
                      fields (except buffer) of the structure pointed
                      to by <diskinfo>. If no error occurs, the
                      function returns 0 in the high-order byte and
                      returns the number of sectors compared in the
                      low-order byte. If there is an error, the high-
                      order byte will contain a set of status flags, as
                      defined under _DISK_STATUS (above).
 
     _DISK_WRITE      Writes data from memory to one or more disk
                      sectors. This service uses all fields of the
                      structure pointed to by <diskinfo>. If no error
                      occurs, the function returns 0 in the high-order
                      byte and returns the number of sectors written in
                      the low-order byte. If there is an error, the
                      high-order byte will contain a set of status
                      flags, as defined under _DISK_STATUS (above).