.po 1i
.TH MT 4 Zilog
.V 08/15/84 17:31:11
.\" @[$]mt.4	1.19	08/15/84 17:31:11 - Zilog Inc.
.ad
.fi
.SH NAME
mt \- Zilog streaming magnetic tape interface
.SH DESCRIPTION
The files
.BR mt0 ", " ... ", " mt3
refer to the 1600 bpi streaming magnetic-tape controller.
The files
.BR mt0 ", " ... ", " mt3
are normal-rewind on close, and the files
.BR nmt0 ", " ... ", " nmt3
are no-rewind on close.
When opened for reading or writing, the tape is assumed to
be in the desired position.
When a file is closed, a double end-of-file (double tape mark)
is written if the file was opened for writing.
If the file is normal-rewind, the tape is rewound.
If the file is no-rewind and was opened for writing,
the tape is positioned before the second of the two
.SM EOF
marks just written.
If the file is no-rewind and was opened read-only, the tape is
positioned after the        
.SM EOF
following the data just read.
Once opened, reading is restricted to tape 
between the read-head position at open and either
the last   
.SM EOF
or
the last write.
Encountering the
.SM EOF
returns a zero-length read.
By judiciously choosing
.B mt\^
files, it is possible to read and write multi-file tapes.
.PP
A standard tape consists of
several 512-byte records terminated by an \s-1EOF\s+1.
In a limited fashion, the tape may be treated like any other file.
Seeks have their usual meaning and it is possible
to read or write a byte at a time.
.PP
The
.B mt
files discussed above are useful
for accessing the tape as an ordinary file. 
When unknown tapes are to be dealt with, and especially
when long records are to be read or written, the
"raw" interface is appropriate.
The raw special files are named
.BR rmt0 ", " ... ", " rmt3.
Each
.B read\^
or
.B write\^
call reads or writes the next record on the tape.
In the case of a write, 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,
up to the buffer size specified.
In raw tape I/O, seeks are ignored.
Encountering an 
.SM EOF
returns a zero-length read, with the tape positioned
after the \s-1EOF\s+1; the next read returns the next
record.
.PP
.BR smt0 ", " ... ", " smt3
produce files with short inter-record gaps:  
(.6in) rather than the default (1.2in) gap.
This applies only to writing;
either length gap can be read.
.BR fmt0 ", " ... ", " fmt3
produce the high speed mode of the 
drives: 100ips rather than the
default 25 ips.
.PP
Any combination of these options is 
available through one of the /dev/*mt?
files.
.PP
The minor device number of the device 
special files determines which combination
of the above options applies to them.
The minor device number is formed by ANDing the bits of the desired 
options.
Each bit position in the minor device number represents an option.
.IP
The rightmost two bits, Bits 0 and 1 respectively, represent one of 
four controllers.
.sp
The next three bits, Bits 2, 3, and 4 respectively, represent one of eight tape 
drives for each controller.
.sp
The next position, Bit 5, if on, represents high speed mode.
.sp
The next position, Bit 6, if on,
indicates that the tape is to be rewound on close.
.sp
The next position, Bit 7, denotes long gap if on, short gap if off.
.PP
These bits are only relevant when using the 
.BR mknod (M)
command to create new
device files for nine-track magnetic tape drives.
The device files for all combinations of options for four tape drives 
are automaticaly created by using the \fBmkmt\fR(M) command.
.PP
Streaming occurs automatically
when there are several reads or writes and the 
data is not read in from the disk.
If the tape drives do not stream, 
then the drive must reposition the tape
after each block read or written.
.PP
There are several 
.BR ioctl (2)
system calls that apply to the nine-track tape drive.
The calls reference an open /dev/*mt* special file.
The syntax of the calls is: 
.sp
.nf
                \fBioctl(\fIfd, REQUEST, count\fR);
.fi
.sp
\fIfd\fR is a valid file descriptor of a /dev/*mt* file.
.br
\fIREQUEST\fR is one of the options below.
.br
\fIcount\fR applies to certain of the requests below.
.sp
.B MTIOBF 
\- Skips forward the number of tape blocks specified 
in the count field.
This command will not skip over file marks or past the end-of-tape mark.
The tape blocks can be any size.
.sp
.B MTIOBR 
\- Skips backward the specified number of tape blocks.
This command will not skip back over file marks or before the 
beginning-of-tape mark.
The tape blocks can be of any size.
.bp
.B MTIOFF 
\- Skips forward the number of files (determined 
by file marks) specified in the count field.
The tape is positioned at the beginning of the next file.
The tape will not skip past the end-of-tape mark.
.sp
.B MTIOFR
\- Skips backward the number of files (determined
by file marks) specified in the count field.
The tape is positioned at the beginning of the appropriate file.
The tape will not skip past the beginning-of-tape mark.
.sp
.B MTIOWFM
\- Writes a file mark at the current position.
If the short-gap device is used, the file mark has a short 
gap before it.
This command does not take a count.
.sp
.B MTIOSE
\- Security erase.  The tape is erased at high speed
from the current position to one meter past the end-of-tape mark.
This command does not take a count.
.sp
.B MTIORW
\- Rewind the tape.  Leave the tape positioned at the 
load point.
This command does not take a count.
.sp
.B MTIORWUL
\- Rewind and unload the tape; leave the drive offline.
The tape is completely rewound from the current position.
This command does not take a count.
.sp
.B MTIOOL
\- Put the drive online.
This command does not take a count.
.SH FILES
/dev/mt0		/dev/nmt0
.br
/dev/rmt0		/dev/nrmt0
.br
/dev/smt0		/dev/nsmt0
.br
/dev/fmt0		/dev/nfmt0
.br
/dev/srmt0	/dev/nsrmt0
.br
/dev/frmt0	/dev/nfrmt0
.br
/dev/fsmt0	/dev/nfsmt0