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.\" This code is derived from software contributed to Berkeley by
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.\"     from: @(#)st.4	8.2 (Berkeley) 11/30/93
.\"
.Dd November 30, 1993
.Dt ST 4 hp300
.Os
.Sh NAME
.Nm st
.Nd
.Tn CCS SCSI
tape driver
.Sh SYNOPSIS
.Cd "st* at oscsi? target ? lun ?"
.Sh DESCRIPTION
The
.Nm
driver was written especially to support the Exabyte
.Tn 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
.Tn ANSI SCSI-I
specification.
Besides extensive use with an Exabyte, the driver has been tested with an
Archive
.Tn QIC-24
tape drive.
The
.Nm
tape interface provides a standard tape drive interface
as described in
.Xr mtio 4
with the following exceptions:
.Bl -enum
.It
Density is dependent on device type.
Current Exabyte hardware has only one density.
The
.Tn EXB-8500
drive, when released, will have a high
density format of
.Tn 5.6GB .
On an Archive
.Tn QIC-24
drive the driver reads both
.Tn QIC-11
and
.Tn QIC-24
formats
but writes only
.Tn QIC-24 .
.It
Only the ``raw'' interface is supported.
.El
.Pp
Special Exabyte Support:
.Pp
The
.Dv MTIOCGET
.Xr ioctl 2
call on an Exabyte returns this structure:
.Bd -literal
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 */
};
.Ed
.Pp
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
.Tn BOT
side of a long filemark allowing
data to be appended with a write operation. 
Since the short filemark does not contain 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
.Tn EOT
side.
.Pp
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.
.Pp
For unit 0 here are the effects of minor device bits 2,3,4,5.
For other units add the
.Em unit#
to each of the device names.
.Bl -column norewind density filemarks -offset indent
.Em norewind	high	short	fixed
.Em 	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			X	X
nrst48	X		X	X
rst56		X	X	X
nrst56	X	X	X	X
.El
.Sh SEE ALSO
.Xr mt 1 ,
.Xr ct 4 ,
.Xr hd 4 ,
.Xr intro 4 ,
.Xr mtio 4 ,
.Xr sd 4
.Rs
.%T EXB-8200 8MM Cartridge Tape Subsystem Interface User Manual.
.Re
.Sh BUGS
The
.Tn HP
98268
.Tn SCSI
controller hardware can not do odd length
.Tn DMA
transfers.
If odd length
.Tn DMA I/O
is requested the driver will use the
"Program Transfer Mode" of the Fujitsu
.Tn MB87030
chip.
Read requests are normally even length for which a
.Tn 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
.Dv 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.
.Pp
The following only applies when using long filemarks.
Short filemarks can not be overwritten.
.Bd -filled -offset 4n
Due to the helical scan and the erase mechanism, there is a writing
limitation on Exabyte drives.
.Dq Li tar r
or
.Dq Li tar u
will not work
.Pf ( Dq Li 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:
.Pp
.Li "mt fsf 1; tar cf /dev/nrst0 ..."
.Pp
but for an Exabyte you need to do:
.Pp
.Li "mt fsf 1; mt bsf 1; mt weof 1; tar cf /dev/nrst0 ..."
.Pp
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
.Em after
it, clearing the
way to write more on the tape.
The erase head is not helical.
.Pp
One can position a tape to the end of what was last written by reading
until a
.Tn \*qBLANK CHECK\*q
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
.Tn \*qBLANK CHECK\*q
error, but the Exabyte stays positioned at the beginning 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
.Dq Li mt fsf too_many .
.Ed
.Pp
Archive support gets confused if asked to moved more filemarks than there are
on the tape.
.Pp
This man page needs some work.
Some of these are not really bugs, just unavoidable consequences of the
hardware.