ST(4)ST(4)NAMEst - SCSI Tape Device Driver
SYNOPSIS
device_name as st0/1 at sc0 target ? lun ?
DESCRIPTION
This driver allows access to a number of tape drives with standard SCSI
interfaces. Four different devices in /dev access this driver:
/dev/rst? Generic SCSI tape, rewind on close
/dev/nrst? Generic SCSI tape, no rewind on close
/dev/rxt? Exabyte SCSI tape, rewind on close
/dev/nrxt? Exabyte SCSI tape, no rewind on close
All four of these devices are character devices. There is no block I/O
to tape on the NeXT machine. The Exabyte has its own device entry
because it has unusual initialization requirements that are performed
implicitly by the driver when /dev/rxt? or /dev/nrxt? is opened.
Tape operation occurs in two basic modes - fixed block size and
variable block size. In this context, a "block" refers to one record
on the tape. Some tape drives can write blocks of arbitrary length;
others require a constant block size. Ioctl() calls are provided to
set the appropriate mode. In the case of fixed block mode, if a
write() occurs with a byte count which is not an integral multiple of
the block size, the last partial block written will be padded with
zeroes to make a complete block. The default mode for the st driver is
variable block mode.
The following operations are allowed by the SCSI tape driver:
open() See open(2).
close()
Closing a SCSI tape devices has some implied effects. If the
last operation on the device was a write(), a tape mark is
written during the close operation. If the tape device is
/dev/rst? or /dev/rxt?, the tape will be rewound to beginning of
tape (BOT) during the close operation.
read() See read(2).
write()
See write(2). This operation is not legal at all times; for
example, after reading a portion of a tape, a write command is
illegal if more data exists on the tape. One can only write to
a tape if the tape is at BOT or at the end of recorded data.
ioctl()
The general form of an ioctl() call is
ioctl(int d, u_int request, char *argp)
The following ioctl requests are currently defined (see
/NextDeveloper/Headers/bsd/dev/scsireg.h and
/NextDeveloper/Headers/bsd/sys/mtio.h):
MTIOCFIXBLK
Sets the driver to fixed block mode. argp points to an int
containing the desired block size.
MTIOCVARBLK
Set variable block mode. *argp is undefined. This mode is the
default mode.
MTIOCMODSEL
Executes a mode select command. argp points to struct
modesel_parms (see /NextDeveloper/Headers/bsd/dev/scsireg.h).
Mode select and Mode Sense command are largely vendor unique
SCSI implementations.
MTIOCMODSEN
Executes a mode sense command. argp points to struct
modesel_parms (see /NextDeveloper/Headers/bsd/dev/scsireg.h).
MTIOCINILL
Inhibit illegal length. This causes the "suppress Illegal
length" bit to be set on all SCSI read commands. This allows
reading variable block sizes without knowing the block size (and
without receiving error status from the drive when doing so).
This is the default mode for the Exabyte device (/dev/rxt? and
/dev/nrxt?).
MTIOCALILL
Allow illegal length. This causes error status (EIO) to result
when the number of bytes requested in a read operation does not
equal the number of bytes actually in the block to be read.
This is the default mode for the non-Exabyte device (/dev/rst?
and /dev/nrst?).
MTIOCSRQ
Execute an arbitrary SCSI command using the scsi_req mechanism.
argp points to a struct scsi_req, which is defined in
/NextDeveloper/Headers/bsd/dev/scsireg.h as are all other
constants used with this mechanism. This struct contains both
the input to the st driver required for execution of a command
(SCSI CDB bytes, address at which DMA will occur, etc.) as well
as the status block upon return.
For MTIOCSRQ, the following status is provided:
srp->sr_io_status
Indicates success, I/O timeout, SCSI check status, DMA overrun,
etc.
srp->sr_scsi_status
The actual SCSI status byte returned by the target.
srp->sr_dma_xfr
Number of data bytes actually transferred to/from device.
srp->sr_exec_time
Command execution time, with microsecond resolution.
FILES
/NextDeveloper/Headers/bsd/dev/scsireg.h
/NextDeveloper/Headers/bsd/sys/mtio.h
/dev/rst?
/dev/nrst?
/dev/rxt?
/dev/nrxt?
SEE ALSOopen(2), read(2), write(2), ioctl(2), close(2), dump(8)BUGS
The default of variable block mode does not work for some older tape
drives. Before these drives can be used, the driver must be
reconfigured for the appropriate fixed block size using the MTIOCFIXBLK
ioctl.
NeXT Computer, Inc. June 30, 1989 ST(4)
