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MTIO(4) 										  MTIO(4)

       mtio - UNIX magtape interface

       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

       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 possible, 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 operations 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"

       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+.


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

       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)
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