%
% The Lion's Commentary, file ch16.tex, version 1.3, 16 May 1994
%
\se{The RK Disk Driver}

The RK disk storage system employs a
removable disk cartridge containing a
single disk, which is mounted inside a
drive with moving read/write heads.

The device designated RK11-D consists
of a disk controller together with a
single drive. Additional drives, designated RK05, up to a total of seven, may
be added to a single RK11-D.

A requirement for more than eight
drives would require an additional controller with a different set of UNIBUS
addresses. Also the code in the file
``rk.c'' would have to be modified to
handle the case of two or more controllers. This case is most unlikely
because requirements for large amounts
of on-line disk storage will be more
economically provided otherwise e.g.
by the RP04 disk system.


\begin{center}
\begin{tabbing}
Cartridge \= capacity: \= 1,228,800 words\\
\> (4800 512 byte records)\\
Surfaces/cartridge: \> \> 2\\
Tracks/surface: \> \>200 (plus 3 spare)\\
Sectors/Track: \> \> 12\\
Words/Sector: \> \> 256\\
Recording density: \> \> 2040 bpi maximum\\
Rotation speed: \> \> 1500 rpm\\
Half revolution: \> \> 20 msecs\\
Track positioning:\\
\> 10 msecs (one track)\\
\> 50 msecs (average)\\
\> 85 msecs (worst case)\\
Interrupt Vector Address: 220\\
Priority Level: \> \> 5\\
\end{tabbing}
\end{center}

\begin{tabular}{lll}
\multicolumn{3}{c}{\bf Unibus Register Addresses}\\
Drive Status        & RKDS & 777400 \\
Error               & RKER & 777402 \\
Control Status      & RKCS & 777404 \\
Word Count          & RKWC & 777406 \\
Current bus address & RKBA & 777410 \\
Disk address        & RKDA & 777412 \\
Data Buffer         & RKDB & 777416 \\
\end{tabular}

\begin{center}
 Table 16.1 RK Vital Statistics
\end{center}

The average total access time is 70
milllseconds. With multi-drive subsystems,
seeking by one drive may be overlapped with reading or writing by
another drive. However this feature is
not used by UNIX because of bugs which
existed at one time in the hardware
controller.


In initiating a data transfer, RKDA,
RRBA and RKC are set, and then RKCS is
set. Upon completion, status information is available in RKCS, RRER and
RKDS. When an error occurs, UNIX simply
calls ``deverror'' (2447) to display RKER
and RKDS on the system console, without
any attempt at analysis. An operation
is repeated up to ten times before an
error is reported by the device driver.


The register formats which are
described fully in the ``PDP11 Peripherals Handbook'' are reflected in the
program code at several points. The
following summaries suffice to describe
the features used by UNIX:

\begin{tabular}{ll}
\multicolumn{2}{c}{\bf Control Status Register (RKCS)}\\ \hline
\multicolumn{1}{c}{bit} & \multicolumn{1}{c}{description}
\\
15 & Set when any bit of RKER (the\\
   & Error Register) is set;\\
\\
7  & Set when the control is no\\
   & longer engaged in actively\\
   & executing a function and is ready\\
   & to accept a command;\\
\\
6  & When set, the control will issue\\
   & an interrupt to vector address\\
   & 220 upon operation completion or\\
   & error;\\
\\
5--4 & Memory Extension. The two most\\
   & significant bits of the 13 bit\\
   & physical bus address. (The other\\
   & 16 bits are recorded in RKBA.);\\
\\
3--1 & Function to be performed:\\
\\
   & CONTROL RESET: 000\\
   & WRITE: 001\\
   & READ: 010\\
   & etc.,\\
\\
0 & Initiate the function designated\\
 & by bits 1 to 3 when set. (write\\
 & only);\\
\\
\multicolumn{2}{c}{\bf Word Count Register (RKWC)}\\ \hline
\end{tabular}

Contains the twos complement of the
number of words to be transferred.

\bigskip

\begin{tabular}{ll}
\multicolumn{2}{c}{\bf Disk Address Register (RKDA)}\\ \hline
\multicolumn{1}{c}{bit} & \multicolumn{1}{c}{description}
\\
15--13 & Drive number (0 to 7)\\
12--5 & Cylinder number (0 to 199)\\
 4 & Surface number (0,1)\\
 3-0 & Sector address (0 to 11)\\
\end{tabular}
 

\sbs{The file `rk.c'}

This file contains the code which is
specific to the RK disk system, i.e.
which is the RK ``device driver''.


\sbs{rkstrategy (5389)}

The strategy routine is called, e.g.
from ``swap'' (5212), to handle both read
and write requests.

\bd
\item[5397:] The test and call on ``mapalloc''
 here is a ``no-op'' except on the
 PDP11/70 system;

\item[5399:] The code from here to line 5402
 appears to be unnecessarily devious! See the discussion of
 ``rkaddr'' below. If the block
 number is too large, set the
 ``B\_ERROR'' flag and report ``completion'';

\item[5407:] Link the buffer into a FIFO list
 for the controller. The list is
singly linked, uses the ``av\_forw''
pointer of the ``buf'' structures,
and has head and tail pointers in
``rktab''. Interrupts from disk
devices may not be allowed after
the first step;

\item[5414:] If the RK controller is not
 currently active, wake it up via
 a call on ``rkstart'' (5440), which
 checks that there is something to
 do (5444), flags the controller
 as busy (5446) and calls
 ``devstart'' (5447), passing as
 parameters:

\bi
\item a pointer to the first enqueued
buffer header;

\item the address of the RKDA disk
address register. (The value
passed is in effect 0177412. See
lines 5363, 5382.);

\item a ``disk address'' computed by
``rkaddr'';

\item zero (not really important in our
discussion, and may be ignored).

\ei
\ed


\sbs{rkaddr (5420)}

The code in this procedure incorporates
a special feature for files which
extend over more than one disk drive.
This feature is described in the UPM
Section ``RK(IV)''. Its usefulness seems
to be restricted.

The value returned by ``rkaddr'' is formatted for direct transmission to the
control register, RKDA.


\sbs{devstart (5096)}

This procedure when called for the RK
disk loads appropriate values into the
registers RKDA, RKBA, RKWC and RKCS in
succession. Only the last value needs
to be computed at this stage.

The calculation, though messy in
appearance, is straight forward. Note
that ``hbcom'' is zero and ``rbp-$>$b\_xmem''
contains the two high order bits of the
physical core address. The loading of
RKCS initialises the disk controller
i.e. the operation is now entirely
under the control of the hardware.

``devstart'' returns to ``rkstart'' (5448),
which returns to ``rkstrategy'' (5416),
which resets the processor priority and
returns to ``swap'' (5213), which ...

\sbs{rkintr (5451)}

This procedure is invoked to handle the
interrupts which occur when RK disk
operations are completed.

\bd
\item[5455:] Check for a false alarm!

\item[5459:] Inspect the error bit; if set ...

\item[5460:] Call ``deverror'' (2447) to display
a message on the system console
terminal;

\item[5461:] Clear the internal registers of
 the disk controller and ...

\item[5462:] Wait till this is completed (usually a few microseconds);

\item[5463:] If the operation has been retried
 less than ten times, call
 ``rkstart'' to try again. Otherwise
 give up and report an error;

\item[5469:] Set the ``retry'' (!) count back to
 zero, remove the current operation from the ``actf'' list, and
 complete the operation by calling
 ``iodone'';

\item[5472:] ``rkstart'' is called unconditionally here. If the call is not
 necessary (because the ``actf''
 list is empty) ``rkstart'' will
 return immediately (5444).
\ed


\sbs{iodone (5018)}

This routine is primarily concerned
with the return of resources when a
block i/o operation has completed. It:

\bi
\item frees up the Unibus map (for 11/70's,
 if appropriate);

\item sets the ``B\_DONE'' flag;

\item releases the buffer if the i/o was
 asynchronous, or else resets the
 ``B\_WANTED'' flag and wakes up any
 process waiting for the i/o
 operation to complete.
\ei