man/man4/rk.4

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.he 'RK (IV)'3/15/72'RK (IV)'
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NAME            rk  --  RK-11/RK03 (or RK05) disk
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DESCRIPTION     rk?\b\b\b___
refers to an entire RK03 disk as a single sequentially-addressed
file.
Its 256-word blocks are numbered 0 to 4871.
Like the RF disk and the tape files, its
addressing is block-oriented.
Consult the rf(IV) section.
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FILES           /dev/rk0        user available drive
.br
/dev/rk1        /usr file system
.br
/dev/rk2        /sys file system
.br
/dev/rk3        /crp file system
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SEE ALSO        rf(IV), tc(IV)
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BUGS            See rf(IV)