ST

Section: Devices and Network Interfaces (4)
Updated: June 30, 1989
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NAME

st - 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?

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.

This document was created by man2html, using the manual pages.
Time: 00:59:30 GMT, September 26, 2024