V7M/man/man4/rx.4

.TH RX 4 
.SH NAME
rx  \- RX02 floppy disk
.SH DESCRIPTION
.I RX?
refers to an entire disk as a single sequentially-addressed
file.
The physical disk sector size is 128 bytes
for single density and 256 bytes for double density,
the logical block size
is 512 bytes.
Each diskette has 500 logical blocks, single
density and 1001 logical blocks, double density.
The minor device numbers have the following
significance:
.br
.nf

	name	minor device	unit	density
	----	------------	----	-------
	rx0	     0		  0	single
	rx1	     1		  1	single
	rx2	     2		  0	double
	rx3	     3		  1	double
.fi
.PP
The
.I rx
files
discussed above access the disk via the system's normal
buffering mechanism
and may be read and written without regard to
physical disk records.
There is also a `raw' interface
which provides for direct transmission between the disk
and the user's read or write buffer.
A single read or write call results in exactly one I/O operation
and therefore raw I/O is considerably more efficient when
many words are transmitted.
The names of the raw RX files
begin with
.I rrx
and end with a number which selects the same disk
as the corresponding
.I rx
file.
.PP
In raw I/O the buffer must begin on a word boundary,
and counts should be a multiple of 512 bytes
(a disk block).
Likewise
.I seek
calls should specify a multiple of 512 bytes.
.SH FILES
/dev/rx?, /dev/rrx?
.SH BUGS
In raw I/O
.I read
and
.IR write (2)
truncate file offsets to 512-byte block boundaries,
and
.I write
scribbles on the tail of incomplete blocks.
Thus,
in programs that are likely to access raw devices,
.I read, write
and
.IR lseek (2)
should always deal in 512-byte multiples.