MTIO(4) 						     Kernel Interfaces Manual							   MTIO(4)

NAME

       mtio - UNIX magtape interface

DESCRIPTION

       The  files  mt0,  ...,  mt15 refer to the UNIX magtape drives, which may be on the MASSBUS using the TM03 formatter ht(4), or on the UNIBUS
       using either the TM11 or TS11 formatters tm(4) or ts(4).  The following description applies to any of the transport/controller pairs.

       The files mt0, ..., mt7 are 800bpi (or the transport's lowest density), mt8, ..., mt15 are 1600bpi (or the transport's second density), and
       mt16,  ..., mt23 are 6250bpi (or the transport's third density).  (But note that only 1600 bpi is available with the TS11.)  The files mt0,
       ..., mt3, mt8, ..., mt11, and mt16, ..., mt19 are rewound when closed; the others are not.  When a file open for  writing  is  closed,  two
       end-of-files are written.  If the tape is not to be rewound it is positioned with the head between the two tapemarks.

       A standard tape consists of a series of 1024 byte records terminated by an end-of-file.	To the extent possible, the system makes it possi-
       ble, if inefficient, to treat the tape like any other file.  Seeks have their usual meaning and it is possible to read or write a byte at a
       time.  Writing in very small units is inadvisable, however, because it uses most of the tape in record gaps.

       The  mt files discussed above are useful when it is desired to access the tape in a way compatible with ordinary files.	When foreign tapes
       are to be dealt with, and especially when long records are to be read or written, the `raw' interface is appropriate.  The associated files
       are  named rmt0, ..., rmt23, but the same minor-device considerations as for the regular files still apply.  A number of other ioctl opera-
       tions are available on raw magnetic tape.  The following definitions are from <sys/mtio.h>:

       /*
	* Structures and definitions for mag tape io control commands
	*/

       /* structure for MTIOCTOP - mag tape op command */
       struct	 mtop {
	    short     mt_op;	     /* operations defined below */
	    daddr_t   mt_count; /* how many of them */
       };

       /* operations */
       #define MTWEOF 0    /* write an end-of-file record */
       #define MTFSF  1    /* forward space file */
       #define MTBSF  2    /* backward space file */
       #define MTFSR  3    /* forward space record */
       #define MTBSR  4    /* backward space record */
       #define MTREW  5    /* rewind */
       #define MTOFFL 6    /* rewind and put the drive offline */
       #define MTNOP  7    /* no operation, sets status only */
       #define MTCACHE	   8	/* enable controller cache */
       #define MTNOCACHE   9	/* disable controller cache */

       /* structure for MTIOCGET - mag tape get status command */

       struct	 mtget	   {
	    short     mt_type;	/* type of magtape device */
       /* the following two registers are grossly device dependent */
	    short     mt_dsreg; /* ``drive status'' register */
	    short     mt_erreg; /* ``error'' register */
       /* end device-dependent registers */
	    short     mt_resid; /* residual count */
       /* the following two are not yet implemented */
	    daddr_t   mt_fileno;     /* file number of current position */
	    daddr_t   mt_blkno; /* block number of current position */
       /* end not yet implemented */
       };

       /*
	* Constants for mt_type byte.  These are the same
	* for other controllers compatible with the types listed.
	*/
       #define	 MT_ISTS	0x01	  /* TS-11 */
       #define	 MT_ISHT	0x02	  /* TM03 Massbus: TE16, TU45, TU77 */
       #define	 MT_ISTM	0x03	  /* TM11/TE10 Unibus */
       #define	 MT_ISMT	0x04	  /* TM78/TU78 Massbus */
       #define	 MT_ISUT	0x05	  /* SI TU-45 emulation on Unibus */
       #define	 MT_ISCPC  0x06      /* SUN */
       #define	 MT_ISAR	0x07	  /* SUN */
       #define	 MT_ISTMSCP	0x08	  /* DEC TMSCP protocol (TU81, TK50) */

       /* mag tape io control commands */
       #define	 MTIOCTOP  _IOW(m, 1, struct mtop)	 /* do a mag tape op */
       #define	 MTIOCGET  _IOR(m, 2, struct mtget) /* get tape status */
       #define MTIOCIEOT   _IO(m, 3)	       /* ignore EOT error */
       #define MTIOCEEOT   _IO(m, 4)	       /* enable EOT error */

       #ifndef KERNEL
       #define	 DEFTAPE   "/dev/rmt12"
       #endif

       Each read or write call reads or writes the next record on the tape.  In the write case the record has the same length as the buffer given.
       During a read, the record size is passed back as the number of bytes read, provided it is no greater than the buffer size; if the record is
       long, an error is indicated.  In raw tape I/O seeks are ignored.  A zero byte count is returned when a tape mark is read, but another  read
       will fetch the first record of the new tape file.

       The  MTCACHE  and  MTNOCACHE  commands do not apply to all tape drivers.  At present only the TMSCP driver supports those commands and then
       only for drives such as the TU81+.

FILES

       /dev/mt?
       /dev/rmt?

SEE ALSO

       mt(1), tar(1), tp(1), ht(4), tm(4), ts(4)

BUGS

       The status should be returned in a device independent format.

       The special file naming should be redone in a more consistent and understandable manner.

3rd Berkeley Distribution					 December 22, 1995							   MTIO(4)