Command          Operation
--------         ----------
cmd_clear        mark track buffer as invalid. forces the track to be
ETD_CLEAR        re-read.  ETD_CLEAR also checks for a diskchange.

cmd_read         read one or more sectors from a disk.  etd_read also
etd_read         reads the sector label area and checks for a diskchange.

cmd_update       write out track buffer if it has been changed. etd_update
ETD_UPDATE       also checks for a diskchange.

cmd_write        write one or more sectors to a disk.  etd_write also
etd_write        writes the sector label area and checks for a diskchange.

td_addchangeint  add an interrupt handler to be activated on a diskchange.

td_changenum     return the current value of the diskchange counter used
                 by the ETD commands to determine if a diskchange has
                 occurred.

td_changestate   return the disk present/not-present status of a drive.

td_eject         eject a disk from a drive.  this command will only work
                 on drives that support an eject command (V36).

td_format        initialize one or more tracks with a data buffer.
ETD_FORMAT       ETD_FORMAT also initializes the sector label area.

td_getdrivetype  return the type of disk drive in use by the unit.

td_getgeometry   return the disk geometry table (v36).

td_getnumtracks  return the number of tracks usable with the unit.

td_motor         turn the motor on or off.  etd_motor also checks for a
ETD_MOTOR        diskchange.

td_protstatus    return the write-protect status of a disk.

td_rawread       read raw sector data from disk (unencoded mfm).
etd_rawread      etd_rawread also checks for a diskchange.

td_rawwrite      write raw sector data to disk.  etd_rawwrite also checks
etd_rawwrite     for a diskchange.

td_remchangeint  remove a diskchange interrupt handler.

td_seek          move the head to a specific track.  etd_seek also checks
                 for a

etd_seek         diskchange.


Exec Functions as Used in This Chapter
--------------------------------------
abortio()        abort a command to the trackdisk device.

BeginIO()        Initiate a command and return immediately (asynchronous
                 request).

closedevice()    relinquish use of a disk unit.

doio()           initiate a command and wait for completion (synchronous
                 request).

opendevice()     obtain exclusive use of a particular disk unit.


Exec Support Functions as Used in This Chapter
----------------------------------------------
createextio()    create an extended i/o request structure of type ioexttd.
                 This structure will be used to communicate commands to
                 the trackdisk device.

createport()     create a signal message port for reply messages from the
                 trackdisk device.  Exec will signal a task when a message
                 arrives at the reply port.

deleteextio()    delete an i/o request structure created by createextio().

deleteport()     delete the message port created by createport().