.TH SCSI 4
.CT 2 sa
.SH NAME
scsi \- SCSI interface
.SH SYNOPSIS
.B "#include <scsi.h>"
.SH DESCRIPTION
The special file
.F /dev/scsi
provides a low level interface to a SCSI bus.
Commands are transmitted to the bus by
.IR write ;
the response to each command is received with
.IR read (2).
The format of a command is
.IP
.EX
.ta \w'unsigned char scsistatus;  'u
unsigned long tran_id;	/* transaction id */
unsigned char target;	/* SCSI id of target device */
unsigned char flags;	/* flags for this transaction */
char cmd[10];	/* SCSI command */
char data[];	/* optional data */
.EE
.LP
Thus, to send 
.I n
bytes of data, the byte count for
.I write
should be
.IR n +16.
Possible flags are
.B SCSI_WR
(data goes from host to SCSI),
.B SCSI_RD
(data goes from SCSI to host),
.B SCSI_BRESET
(reset the SCSI bus),
.B SCSI_RESET
(reset the controller),
and
.B SCSI_SENSE
(return extended sense data on error).
For most controllers, 
.B SCSI_BRESET
implies
.BR SCSI_RESET .
Flags are OR'ed together, and there must be exactly one of
.B SCSI_WR
and
.BR SCSI_RD .
.LP
The data read is structured as
.IP
.EX
unsigned long tran_id;	/* transaction id */
unsigned char status;	/* scsi status byte */
unsigned char message;	/* scsi message byte */
unsigned char flags;	/* flags for this transaction */
unsigned char c_type;	/* 1=td 2=us */
unsigned short c_reg1;	/* td=sa, us=csr */
unsigned short c_reg2;	/* td=mscp, us=per */
unsigned char sense[22];	/* extended sense data */
char data[];	/* any data */
.EE
.LP
Thus, to read 
.I n
bytes of data. the byte count to
.I read
should be
.IR n +34.
If
.I flags
has the
.B SCSI_CERR
bit set,
there was a controller error, which is described by the
.B c_
fields.
The values of
.B csr
(or
.BR sa )
and
.B per
(or
.BR mscp )
are documented in the interface manual for the U.S. Design 1158
Unibus controller (or T.D. Systems Viking controller).
If the
.B SCSI_SENSE
bit was set in the 
.I write,
and the status byte shows a check condition,
an attempt is made to get extended sense information.
If the attempt succeeds the
.B SCSI_SENSE
is set in
.I flags.
Otherwise, the status and message bytes for the failed
attempt are placed in
.B sense[0]
and
.B sense[1]
respectively.
.PP
The transaction id identifies which
.I write
caused the results for this
.IR read .
This will become more important when multiple simultaneous transactions
are allowed.
.SH FILES
.F /dev/scsi
.SH "SEE ALSO"
.IR scsish (8)