SCSI Device Command  Operation
-------------------  ---------
hd_scsicmd           issue a scsi-direct command to a scsi unit.


Trackdisk Device Commands Supported by the SCSI Device
------------------------------------------------------
td_changestate       return the disk present/not-present status of a drive.
td_format            initialize one or more tracks with a data buffer.
td_protstatus        return the write-protect status of a disk.
td_seek              move the head to a specific track.


Exec Commands Supported by SCSI Device
--------------------------------------
CMD_READ             Read one or more sectors from a disk.
CMD_START            Restart a SCSI unit that was previously stopped with
                     CMD_STOP.
CMD_STOP             Stop a SCSI unit.
CMD_WRITE            Write one or more sectors to a disk.


Exec Functions as Used in This Chapter
--------------------------------------
abortio()            abort an i/o request to the scsi device.
allocmem()           allocate a block of memory.
allocsignal()        allocate a signal bit.
checkio()            return the status of an i/o request.
closedevice()        relinquish use of the scsi device. all requests must
                     be complete before closing.

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

freemem()            free a block of previously allocated memory.
freesignal()         free a previously allocated signal.
opendevice()         obtain use of the scsi device.  you specify the type
                     of unit and its characteristics in the call to
                     OpenDevice().

waitio()             wait for completion of an i/o request and remove it
                     from the reply port.


Exec Support Functions as Used in This Chapter
----------------------------------------------
createextio()        create an extended iorequest structure for use in
                     communicating with the SCSI device.

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

deleteextio()        delete the extended iorequest structure created by
                     createextio().

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