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ST(4)		      BSD/hp300 Kernel Interfaces Manual		 ST(4)

NAME
     st — CCS SCSI tape driver

SYNOPSIS
     tape st0 at scsi? slave ?

DESCRIPTION
     The st driver was written especially to support the Exabyte EXB-8200 8MM
     Cartridge Tape Subsystem.	It has several extensions specific to the
     Exabyte, but should support other tape drives as long has they follow the
     ANSI SCSI-I specification.	 Besides extensive use with an Exabyte, the
     driver has been tested with an Archive QIC-24 tape drive.	The st tape
     interface provides a standard tape drive interface as described in
     mtio(4) with the following exceptions:

     1.	  Density is dependent on device type.	Current Exabyte hardware has
	  only one density. The EXB-8500 drive, when released, will have a
	  high density format of 5.6GB.	 On an Archive QIC-24 drive the driver
	  reads both QIC-11 and QIC-24 formats but writes only QIC-24.

     2.	  Only the ``raw'' interface is supported.

     Special Exabyte Support:

     The MTIOCGET ioctl(2) call on an Exabyte returns this structure:

     struct  mtget {
	     short mt_type;   /* type of magtape device */
	     short mt_dsreg;  /* sc_flags */
	     short mt_erreg;  /* high 8 bytes error status */
	     /* low  8 bytes percentage of Rewrites
	     if writing, ECC errors if reading */
	     short mt_resid;  /* Mbyte until end of tape */
     };

     Bit 4 in the minor device number is used to select long filemarks or
     short filemarks. A long filemark occupies 2.12 MBytes of space on the
     tape, while a short filemark occupies 488 KBytes.	A long filemark
     includes an erase gap while the short filemark does not.  The tape can be
     positioned on the BOT side of a long filemark allowing data to be
     appended with a write operation.  Since the short filemark does not con‐
     tain an erase gap which would allow writing it is considered to be non-
     erasable.	If either type of filemark is followed by blank tape, data may
     be appended on its EOT side.

     Bit 5 in the minor device number selects fixed block mode with a block
     size of 1K.  Variable length records are the default if bit 5 is not set.

     For unit 0 here are the effects of minor device bits 2,3,4,5. For other
     units add the unit# to each of the device names.

	   norewind    high	  short	       fixed
		       density	  filemarks    block mode
	   rst0
	   nrst0       X
	   rst8			  X
	   nrst8       X	  X
	   rst16			       X
	   nrst16      X		       X
	   rst24		  X	       X
	   nrst24      X	  X	       X
	   rst32			       X
	   nrst32      X		       X
	   rst40		  X	       X
	   nrst40      X	  X	       X
	   rst48			       XX
	   nrst48      X		       XX
	   rst56		  X	       XX
	   nrst56      X	  X	       XX

SEE ALSO
     mt(1), tar(1), mtio(4),

     EXB-8200 8MM Cartridge Tape Subsystem Interface User Manual..

BUGS
     The HP 98268 SCSI controller hardware can not do odd length DMA trans‐
     fers.  If odd length DMA I/O is requested the driver will use the "Pro‐
     gram Transfer Mode" of the Fujitsu MB87030 chip. Read requests are nor‐
     mally even length for which a DMA transfer is used. If, however, the
     driver detects that a odd length read has happened (when a even length
     was requested) it will issue the EIO error and the last byte of the read
     data will be 0x00. Odd length read requests must match the size of the
     requested data block on tape.

     The following only applies when using long filemarks. Short filemarks can
     not be overwritten.

	 Due to the helical scan and the erase mechanism, there is  a  writing
	 limitation on Exabyte drives.	“tar r” or “tar u” will not work (“tar
	 c” is ok).  One can only start writing at  1) beginning of  tape,  2)
	 on  the  end  of  what was last written, 3) "front" side of a regular
	 (long) filemark.  For example, you have a tape with 3 tar  files.  If
	 you  want  to save the first file, but overwrite the second two files
	 with new data, on a normal 1/4" or 1/2" drive you would do:

	 mt fsf 1; tar cf /dev/nrst0 ...

	 but for an Exabyte you need to do:

	 mt fsf 1; mt bsf 1; mt weof 1; tar cf /dev/nrst0 ...

	 The regular long filemark  consists  of  an  erased  zone  3.8"  long
	 (needed  to  begin  a	write).	  In  this case, the first filemark is
	 rewritten in place, which creates an erased zone after	 it,  clearing
	 the way to write more on the tape.  The erase head is not helical.

	 One  can position a tape to the end of what was last written by read‐
	 ing until a "BLANK CHECK" error is returned.  Writing can be  started
	 at this point.	 (This applies to both long and short filemarks.)  The
	 tape does not become positioned somewhere down the "erased"  area  as
	 does  a  conventional	magtape.   One can issue multiple reads at the
	 "BLANK CHECK" error, but the Exabyte stays positioned at  the	begin‐
	 ning  of  the	blank area, ready to accept write commands.  File skip
	 operations do not stop at blank tape and will run into	 old  data  or
	 run  to the end of the tape, so you have to be careful not to “mt fsf
	 too_many”.

     Archive support gets confused if asked to moved more filemarks than there
     are on the tape.

     This man page needs some work.  Some of these are not really bugs, just
     unavoidable consequences of the hardware.

BSD			       November 30, 1993			   BSD
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