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Unix Version 7 - man page for rk (v7 section 4)

RK(4)				     Kernel Interfaces Manual				    RK(4)

       rk  -  RK-11/RK03 or RK05 disk

       Rk?   refers  to  an  entire  disk  as a single sequentially-addressed file.  Its 256-word
       blocks are numbered 0 to 4871.  Minor device numbers are drive numbers on one controller.

       The rk 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 RK files begin with rrk and end with a number which selects the same disk
       as the corresponding rk file.

       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 seek calls should specify a multiple of 512 bytes.

       /dev/rk?, /dev/rrk?

       In raw I/O read and write(2) truncate file offsets to 512-byte block boundaries, and write
       scribbles on the tail of incomplete blocks.  Thus, in programs that are likely  to  access
       raw devices, read, write and lseek(2) should always deal in 512-byte multiples.


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