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ST(4)			    BSD Programmer's Manual			 ST(4)

NAME
     st - machine-independent SCSI tape driver

SYNOPSIS
     st0 at tg? unit 0
     st* at tg? unit ?

DESCRIPTION
     The st driver for tape drives on the Small Computer System Interface (AN-
     SI X3.131-1992) bus is a machine-independent generic device which employs
     machine-dependent drivers for individual host bus adapters (see hba(4))
     to send commands to manipulate tapes.  The st driver takes care of open-
     ing the tape device, locking out other users and serializing I/O; gener-
     ating the correct read and write Command Descriptor Blocks, and handling
     specialized tape commands such as space forward, write filemarks or
     rewind; and detecting errors and reporting them in a standard way to the
     user's terminal.  The driver supports the standard mt(1) tape motion com-
     mands.

     The clone entry st* should be listed last; this will automatically match
     any otherwise unmatched SCSI tapes.  Additional instances of st devices
     can be ``wired'' to particular targets as described in tg(4).

FIXED BLOCK SIZE
     The st driver defaults to 1024 byte records.  Some DAT and 8mm drives use
     jumpers to select the length of fixed length records. If the drive is
     jumpered for 512 byte records then the drive may not operate successfully
     with 1024 byte records.  The mt(1) command can be used to select 512 byte
     operation by the st driver. Refer to the mt(1) man page for details.

SUBUNIT ENCODING
     There are 10 bits in the subunit field. The low order bit is the no-
     rewind bit. The next 8 bits are the SCSI density bits.  These can be used
     to force selection of a particular density, given that the drive supports
     the density and supports density selection.  A value of zero means use
     the default density.  The high bit of the subunit filed is used to force
     fixed length records for DAT and 8mm drives. Refer to the mknod(8) man
     page for details on creating device special files.

RESIDUAL BYTE COUNT
     An application program is allowed to do read(2) system calls with a
     length greater than the expected tape record. The read(2) system call
     should return the number of bytes actually transfered.  Some, hopefully
     all older, tape drives do not report back the number of bytes actually
     transfered when the tape record was shorter than the length requested in
     the SCSI read operation.  To work around this, by default, the residual
     byte count returned from the SCSI HBA is used for SCSI 1 and CCS tape
     drives. Some SCSI 2 tape drives use the SCSI disconnect option in a man-
     ner which makes it impossible for the HBA to correctly determine the
     residual byte count. For this reason, by default, only the residual byte
     count returned from the target is used with SCSI 2. Therefore, using the
     default settings, a SCSI 2 tape drive which does not return the residual
     byte count correctly will not operate correctly. It is also conceivable,
     but highly unlikely, that SCSI 1/CCS drives exist that will have problems
     using the default settings.  It is possible with boot(8) to force the non
     default action for both SCSI 1/CCS and SCSI 2 tape drives.	 The -param
     command is used to set the ``usehbaresid'' parameter. The values that
     ``usehbaresid'' can be set to are ``yes'' or ``no.'' The following are
     examples of lines which might be added to boot.default: the command
	   -parm  st0 usehbaresid=yes
	   -parm  st1 usehbaresid=no

BUS RESET and REWIND
     If the HBA driver detects that the SCSI bus is hung in will reset the bus
     and notify all the unit drivers of which the st is one.  If this occurs
     st will not allow read or write operations until a user application com-
     mands a rewind or unload. This is to prevent possibly over writing infor-
     mation at the beginning of the media because the tape drive rewound when
     the SCSI bus was reset.

FILES
     /dev/rst[0-n]	   auto-rewind tape devices
     /dev/nrst[0-n]	   no-rewind tape devices
     /dev/rst[0-n]_q11	   force QIC-11 tape format
     /dev/rst[0-n]_q150	   force QIC-150 tape format
     /dev/rst[0-n]_fixed   force fixed-length records on 8mm or DAT drives
     /dev/rst[0-n]_dds	   force DAT dds 1 density selection
     /dev/rst[0-n]_ddsII   force DAT dds 2 density selection
     /dev/rst[0-n]_fdds	   force both fixed-length records and DAT dds 1 den-
			   sity selection
     /dev/rst[0-n]_fddsII  force both fixed-length records and DAT dds 2 den-
			   sity selection

SEE ALSO
     hba(4),  mt(1),  mknod(8)

HISTORY
     Written by Donn Seeley of BSDI for BSD/OS 0.3.

BUGS
     Many SCSI-1 tapes don't support the SCSI `mode select' command, which the
     driver uses to select tape density.  The driver currently assumes that
     most tapes do support this feature; this assumption may not be wise, and
     it will break any tape that does not support it and that isn't listed
     specifically in the driver.  Currently all Sankyo tapes and some Wangtek
     tapes are assumed not to support mode selection.

     Most SCSI-1 tapes can't report tape position.  The Archive Viper 150 sup-
     ports a read physical record number command which gives somewhat differ-
     ent results from the SCSI-2 read position command.

     Cooked tape doesn't work.

BSDI BSD/OS			 June 4, 1996				     2
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