man/man4/rf.4

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.he 'RF (IV)'3/15/72'RF (IV)'
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NAME            rf  --  RF11-RS11 fixed-head disk file
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DESCRIPTION     This file refers to the
concatenation of both RS-11 disks.
It may be either read or written, although writing is inherently
very dangerous, since
a file system resides there.

The disk contains 2048 256-word blocks,
numbered 0 to 2047.
Like the other block-structured devices (TC, RK)
this file is addressed in blocks, not bytes.
This has two consequences:
seek\b\b\b\b____ calls refer to block numbers, not byte numbers;
and sequential reading or writing always advance the read
or write pointer by at least one block.
Thus successive reads of 10 characters from this file
actually read the first 10 characters from successive
blocks.
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FILES           /dev/rf0
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SEE ALSO        tc(IV), rk(IV)
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BUGS            The fact that this device
is addressed in terms of blocks, not bytes, is extremely
unfortunate.  It is due entirely to the fact that
read and write pointers (and consequently the arguments
to seek\b\b\b\b____) are single-precision
numbers.