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.\" Copyright (c) 1990 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to Berkeley by
.\" the Systems Programming Group of the University of Utah Computer
.\" Science Department.
.\"
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.\"	@(#)st.4	5.1 (Berkeley) 6/29/90
.\"
.TH ST 4 "June 29, 1990"
.UC 7
.SH NAME
st \- CCS SCSI tape driver
.SH SYNOPSIS
.B "tape st0 at scsi? slave ?"
.SH DESCRIPTION
The
.I 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
.I st
tape interface provides a standard tape drive interface 
as described in
.IR mtio (4)
with the following exceptions:
.TP 3
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.
.TP 3
2.
Only the ``raw'' interface is supported.
.PP
Special Exabyte Support:

The MTIOCGET ioctl call on an Exabyte returns this structure:

.nf
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 */
};
.fi

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
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 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
.I unit#
to each of the device names.

.(b M
.nf
       norewind    high density    short filemarks    fixed 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

.fi
.)b
.SH BUGS

The HP 98268 SCSI controller hardware can not do odd length DMA
transfers.  If odd length DMA I/O is requested the driver will use the
"Program Transfer Mode" of the Fujitsu MB87030 chip. Read requests are
normally 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.

.(q
.in +4
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.
Say you have a tape with 3 tar files on it, want to save the first
file, and want to begin writing over the 2nd file.

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 reading
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 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 "mt fsf too_many".
.in -4
.q)

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.
.SH "SEE ALSO"
mt(1),
tar(1),
mtio(4),

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

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