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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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>Writing New Devices - target</TITLE
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TITLE="eCos Synthetic Target"
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>eCos Reference Manual</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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><A
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HREF="synth-syscalls.html"
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>Prev</A
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HREF="synth-new-host.html"
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>Next</A
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></TD
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><H1
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><A
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NAME="SYNTH-NEW-TARGET">Writing New Devices - target</H1
83 
><DIV
84 
CLASS="REFNAMEDIV"
85 
><A
86 
NAME="AEN18180"
87 
></A
88 
><H2
89 
>Name</H2
90 
>Writing New Devices&nbsp;--&nbsp;extending the synthetic target, target-side</DIV
91 
><DIV
92 
CLASS="REFSYNOPSISDIV"
93 
><A
94 
NAME="AEN18183"><H2
95 
>Synopsis</H2
96 
><DIV
97 
CLASS="FUNCSYNOPSIS"
98 
><A
99 
NAME="AEN18184"><P
100 
></P
101 
><TABLE
102 
BORDER="5"
103 
BGCOLOR="#E0E0F0"
104 
WIDTH="70%"
105 
><TR
106 
><TD
107 
><PRE
108 
CLASS="FUNCSYNOPSISINFO"
109 
>#include &lt;cyg/hal/hal_io.h&gt;
110 
      </PRE
111 
></TD
112 
></TR
113 
></TABLE
114 
><P
115 
><CODE
116 
><CODE
117 
CLASS="FUNCDEF"
118 
>int synth_auxiliary_instantiate</CODE
119 
>(const char* package, const char* version, const char* device, const char* instance, const char* data);</CODE
120 
></P
121 
><P
122 
><CODE
123 
><CODE
124 
CLASS="FUNCDEF"
125 
>void synth_auxiliary_xchgmsg</CODE
126 
>(int device_id, int request, int arg1, int arg2, const unsigned char* txdata, int txlen, int* reply, unsigned char* rxdata, int* rxlen, int max_rxlen);</CODE
127 
></P
128 
><P
129 
></P
130 
></DIV
131 
></DIV
132 
><DIV
133 
CLASS="REFSECT1"
134 
><A
135 
NAME="SYNTH-NEW-TARGET-DESCRIPTION"
136 
></A
137 
><H2
138 
>Description</H2
139 
><P
140 
>In some ways writing a device driver for the synthetic target is very
141 
similar to writing one for a real target. Obviously it has to provide
142 
the standard interface for that class of device, so for example an
143 
ethernet device has to provide <TT
144 
CLASS="FUNCTION"
145 
>can_send</TT
146 
>,
147 
<TT
148 
CLASS="FUNCTION"
149 
>send</TT
150 
>, <TT
151 
CLASS="FUNCTION"
152 
>recv</TT
153 
> and similar
154 
functions. Many devices will involve interrupts, so the driver
155 
contains ISR and DSR functions and will call
156 
<TT
157 
CLASS="FUNCTION"
158 
>cyg_drv_interrupt_create</TT
159 
>,
160 
<TT
161 
CLASS="FUNCTION"
162 
>cyg_drv_interrupt_acknowledge</TT
163 
>, and related
164 
functions.
165 
    </P
166 
><P
167 
>In other ways writing a device driver for the synthetic target is very
168 
different. Usually the driver will not have any direct access to the
169 
underlying hardware. In fact for some devices the I/O may not involve
170 
real hardware, instead everything is emulated by widgets on the
171 
graphical display. Therefore the driver cannot just peek and poke
172 
device registers, instead it must interact with host-side code by
173 
exchanging message. The synthetic target HAL provides a function
174 
<TT
175 
CLASS="FUNCTION"
176 
>synth_auxiliary_xchgmsg</TT
177 
> for this purpose.
178 
    </P
179 
><P
180 
>Initialization of a synthetic target device driver is also very
181 
different. On real targets the device hardware already exists when the
182 
driver's initialization routine runs. On the synthetic target it is
183 
first necessary to instantiate the device inside the I/O auxiliary, by
184 
a call to <TT
185 
CLASS="FUNCTION"
186 
>synth_auxiliary_instantiate</TT
187 
>. That
188 
function performs a special message exchange with the I/O auxiliary,
189 
causing it to load a Tcl script for the desired type of device and run
190 
an instantiation procedure within that script.
191 
    </P
192 
><P
193 
>Use of the I/O auxiliary is optional: if the user does not specify
194 
<TT
195 
CLASS="OPTION"
196 
>--io</TT
197 
> on the command line then the auxiliary will not
198 
be started and hence most I/O operations will not be possible. Device
199 
drivers should allow for this possibility, for example by just
200 
discarding any data that gets written. The HAL exports a flag
201 
<TT
202 
CLASS="VARNAME"
203 
>synth_auxiliary_running</TT
204 
> which should be checked.
205 
    </P
206 
></DIV
207 
><DIV
208 
CLASS="REFSECT1"
209 
><A
210 
NAME="SYNTH-NEW-TARGET-INSTANTIATE"
211 
></A
212 
><H2
213 
>Instantiating a Device</H2
214 
><P
215 
>Device instantiation should happen during the C++ prioritized static
216 
constructor phase of system initialization, before control switches to
217 
<TT
218 
CLASS="FUNCTION"
219 
>cyg_user_start</TT
220 
> and general application code. This
221 
ensures that there is a clearly defined point at which the I/O
222 
auxiliary knows that all required devices have been loaded. It can
223 
then perform various consistency checks and clean-ups, run the user's
224 
<TT
225 
CLASS="FILENAME"
226 
>mainrc.tcl</TT
227 
> script, and make the main window
228 
visible.
229 
    </P
230 
><P
231 
>For standard devices generic eCos I/O code will call the device
232 
initialization routines at the right time, iterating through the
233 
<TT
234 
CLASS="VARNAME"
235 
>DEVTAB</TT
236 
> table in a static constructor. The same
237 
holds for network devices and file systems. For more custom devices
238 
code like the following can be used:
239 
    </P
240 
><TABLE
241 
BORDER="5"
242 
BGCOLOR="#E0E0F0"
243 
WIDTH="70%"
244 
><TR
245 
><TD
246 
><PRE
247 
CLASS="PROGRAMLISTING"
248 
>#include &lt;cyg/infra/cyg_type.h&gt;
249 
class mydev_init {
250 
  public:
251 
    mydev_init() {
252 
        &#8230;
253 
    }
254 
};
255 
static mydev_init mydev_init_object CYGBLD_ATTRIB_INIT_PRI(CYG_INIT_IO);</PRE
256 
></TD
257 
></TR
258 
></TABLE
259 
><P
260 
>Some care has to be taken because the object
261 
<TT
262 
CLASS="VARNAME"
263 
>mydev_init_object</TT
264 
> will typically not be referenced
265 
by other code, and hence may get eliminated at link-time. If the code
266 
is part of an eCos package then problems can be avoided by putting the
267 
relevant file in <TT
268 
CLASS="FILENAME"
269 
>libextras.a</TT
270 
>:
271 
    </P
272 
><TABLE
273 
BORDER="5"
274 
BGCOLOR="#E0E0F0"
275 
WIDTH="70%"
276 
><TR
277 
><TD
278 
><PRE
279 
CLASS="PROGRAMLISTING"
280 
>cdl_package CYGPKG_DEVS_MINE {
281 
    &#8230;
282 
    compile -library=libextras.a init.cxx
283 
}</PRE
284 
></TD
285 
></TR
286 
></TABLE
287 
><P
288 
>For devices inside application code the same can be achieved by
289 
linking the relevant module as a <TT
290 
CLASS="FILENAME"
291 
>.o</TT
292 
> file rather
293 
than putting it in a <TT
294 
CLASS="FILENAME"
295 
>.a</TT
296 
> library.
297 
    </P
298 
><P
299 
>In the device initialization routine the main operation is a call to
300 
<TT
301 
CLASS="FUNCTION"
302 
>synth_auxiliary_instantiate</TT
303 
>. This takes five
304 
arguments, all of which should be strings:
305 
    </P
306 
><P
307 
></P
308 
><DIV
309 
CLASS="VARIABLELIST"
310 
><DL
311 
><DT
312 
><TT
313 
CLASS="VARNAME"
314 
>package</TT
315 
></DT
316 
><DD
317 
><P
318 
>For device drivers which are eCos packages this should be a directory
319 
path relative to the eCos repository, for example
320 
<TT
321 
CLASS="LITERAL"
322 
>devs/eth/synth/ecosynth</TT
323 
>. This will allow the I/O
324 
auxiliary to find the various host-side support files for this package
325 
within the install tree. If the device is application-specific and not
326 
part of an eCos package then a NULL pointer can be used, causing the
327 
I/O auxiliary to search for the support files in the current directory
328 
and then in <TT
329 
CLASS="FILENAME"
330 
>~/.ecos/synth</TT
331 
>
332 
instead.
333 
        </P
334 
></DD
335 
><DT
336 
><TT
337 
CLASS="VARNAME"
338 
>version</TT
339 
></DT
340 
><DD
341 
><P
342 
>For eCos packages this argument should be the version of the package
343 
that is being used, for example <TT
344 
CLASS="LITERAL"
345 
>current</TT
346 
>. A simple
347 
way to get this version is to use the
348 
<TT
349 
CLASS="FUNCTION"
350 
>SYNTH_MAKESTRING</TT
351 
> macro on the package name.
352 
If the device is application-specific then a NULL pointer should be
353 
used.
354 
        </P
355 
></DD
356 
><DT
357 
><TT
358 
CLASS="VARNAME"
359 
>device</TT
360 
></DT
361 
><DD
362 
><P
363 
>This argument specifies the type of device being instantiated, for
364 
example <TT
365 
CLASS="LITERAL"
366 
>ethernet</TT
367 
>. More specifically the I/O
368 
auxiliary will append a <TT
369 
CLASS="FILENAME"
370 
>.tcl</TT
371 
> suffix, giving
372 
the name of a Tcl script that will handle all I/O requests for the
373 
device. If the application requires several instances of a type
374 
of device then the script will only be loaded once, but the script
375 
will contain an instantiation procedure that will be called for each
376 
device instance.
377 
        </P
378 
></DD
379 
><DT
380 
><TT
381 
CLASS="VARNAME"
382 
>instance</TT
383 
></DT
384 
><DD
385 
><P
386 
>If it is possible to have multiple instances of a device then this
387 
argument identifies the particular instance, for example
388 
<TT
389 
CLASS="LITERAL"
390 
>eth0</TT
391 
> or <TT
392 
CLASS="LITERAL"
393 
>eth1</TT
394 
>. Otherwise a NULL
395 
pointer can be used.
396 
        </P
397 
></DD
398 
><DT
399 
><TT
400 
CLASS="VARNAME"
401 
>data</TT
402 
></DT
403 
><DD
404 
><P
405 
>This argument can be used to pass additional initialization data from
406 
eCos to the host-side support. This is useful for devices where eCos
407 
configury must control certain aspects of the device, rather than
408 
host-side configury such as the target definition file, because eCos
409 
has compile-time dependencies on some or all of the relevant options.
410 
An example might be an emulated frame buffer where eCos has been
411 
statically configured for a particular screen size, orientation and
412 
depth. There is no fixed format for this string, it will be
413 
interpreted only by the device-specific host-side Tcl script. However
414 
the string length should be limited to a couple of hundred bytes to
415 
avoid possible buffer overflow problems.
416 
        </P
417 
></DD
418 
></DL
419 
></DIV
420 
><P
421 
>Typical usage would look like:
422 
    </P
423 
><TABLE
424 
BORDER="5"
425 
BGCOLOR="#E0E0F0"
426 
WIDTH="70%"
427 
><TR
428 
><TD
429 
><PRE
430 
CLASS="PROGRAMLISTING"
431 
>    if (!synth_auxiliary_running) {
432 
      return;
433 
    }
434 
    id = synth_auxiliary_instantiate("devs/eth/synth/ecosynth",
435 
             SYNTH_MAKESTRING(CYGPKG_DEVS_ETH_ECOSYNTH),
436 
             "ethernet",
437 
             "eth0",
438 
             (const char*) 0);</PRE
439 
></TD
440 
></TR
441 
></TABLE
442 
><P
443 
>The return value will be a device identifier which can be used for
444 
subsequent calls to <TT
445 
CLASS="FUNCTION"
446 
>synth_auxiliary_xchgmsg</TT
447 
>. If
448 
the device could not be instantiated then <TT
449 
CLASS="LITERAL"
450 
>-1</TT
451 
> will
452 
be returned. It is the responsibility of the host-side software to
453 
issue suitable diagnostics explaining what went wrong, so normally the
454 
target-side code should fail silently.
455 
    </P
456 
><P
457 
>Once the desired device has been instantiated, often it will be
458 
necessary to do some additional initialization by a message exchange.
459 
For example an ethernet device might need information from the
460 
host-side about the MAC address, the <A
461 
HREF="synth-new-target.html#SYNTH-NEW-TARGET-INTERRUPTS"
462 
>interrupt vector</A
463 
>, and
464 
whether or not multicasting is supported.
465 
    </P
466 
></DIV
467 
><DIV
468 
CLASS="REFSECT1"
469 
><A
470 
NAME="SYNTH-NEW-TARGET-XCHGMSG"
471 
></A
472 
><H2
473 
>Communicating with a Device</H2
474 
><P
475 
>Once a device has been instantiated it is possible to perform I/O by
476 
sending messages to the appropriate Tcl script running inside the
477 
auxiliary, and optionally getting back replies. I/O operations are
478 
always initiated by the eCos target-side, it is not possible for the
479 
host-side software to initiate data transfers. However the host-side
480 
can raise interrupts, and the interrupt handler inside the target can
481 
then exchange one or more messages with the host.
482 
    </P
483 
><P
484 
>There is a single function to perform I/O operations,
485 
<TT
486 
CLASS="FUNCTION"
487 
>synth_auxiliary_xchgmsg</TT
488 
>. This takes the following
489 
arguments:
490 
    </P
491 
><P
492 
></P
493 
><DIV
494 
CLASS="VARIABLELIST"
495 
><DL
496 
><DT
497 
><TT
498 
CLASS="VARNAME"
499 
>device_id</TT
500 
></DT
501 
><DD
502 
><P
503 
>This should be one of the identifiers returned by a previous
504 
call to <TT
505 
CLASS="FUNCTION"
506 
>synth_auxiliary_instantiate</TT
507 
>, specifying the
508 
particular device which should perform some I/O.
509 
         </P
510 
></DD
511 
><DT
512 
><TT
513 
CLASS="VARNAME"
514 
>request</TT
515 
></DT
516 
><DD
517 
><P
518 
>Request are just signed 32-bit integers that identify the particular
519 
I/O operation being requested. There is no fixed set of codes, instead
520 
each type of device can define its own.
521 
         </P
522 
></DD
523 
><DT
524 
><TT
525 
CLASS="VARNAME"
526 
>arg1</TT
527 
>, <TT
528 
CLASS="VARNAME"
529 
>arg2</TT
530 
></DT
531 
><DD
532 
><P
533 
>For some requests it is convenient to pass one or two additional
534 
parameters alongside the request code. For example an ethernet device
535 
could define a multicast-all request, with <TT
536 
CLASS="VARNAME"
537 
>arg1</TT
538 
>
539 
controlling whether this mode should be enabled or disabled. Both
540 
<TT
541 
CLASS="VARNAME"
542 
>arg1</TT
543 
> and <TT
544 
CLASS="VARNAME"
545 
>arg2</TT
546 
> should be signed
547 
32-bit integers, and their values are interpreted only by the
548 
device-specific Tcl script.
549 
         </P
550 
></DD
551 
><DT
552 
><TT
553 
CLASS="VARNAME"
554 
>txdata</TT
555 
>, <TT
556 
CLASS="VARNAME"
557 
>txlen</TT
558 
></DT
559 
><DD
560 
><P
561 
>Some I/O operations may involve sending additional data, for example
562 
an ethernet packet. Alternatively a control operation may require many
563 
more parameters than can easily be encoded in <TT
564 
CLASS="VARNAME"
565 
>arg1</TT
566 
>
567 
and <TT
568 
CLASS="VARNAME"
569 
>arg2</TT
570 
>, so those parameters have to be placed in
571 
a suitable buffer and extracted at the other end.
572 
<TT
573 
CLASS="VARNAME"
574 
>txdata</TT
575 
> is an arbitrary buffer of
576 
<TT
577 
CLASS="VARNAME"
578 
>txlen</TT
579 
> bytes that should be sent to the host-side.
580 
There is no specific upper bound on the number of bytes that can be
581 
sent, but usually it is a good idea to allocate the transmit buffer
582 
statically and keep transfers down to at most several kilobytes.
583 
         </P
584 
></DD
585 
><DT
586 
><TT
587 
CLASS="VARNAME"
588 
>reply</TT
589 
></DT
590 
><DD
591 
><P
592 
>If the host-side is expected to send a reply message then
593 
<TT
594 
CLASS="VARNAME"
595 
>reply</TT
596 
> should be a pointer to an integer variable
597 
and will be updated with a reply code, a simple 32-bit integer. The
598 
synthetic target HAL code assumes that the host-side and target-side
599 
agree on the protocol being used: if the host-side will not send a
600 
reply to this message then the <TT
601 
CLASS="VARNAME"
602 
>reply</TT
603 
> argument
604 
should be a NULL pointer; otherwise the host-side must always send
605 
a reply code and the <TT
606 
CLASS="VARNAME"
607 
>reply</TT
608 
> argument must be valid.
609 
         </P
610 
></DD
611 
><DT
612 
><TT
613 
CLASS="VARNAME"
614 
>rxdata</TT
615 
>, <TT
616 
CLASS="VARNAME"
617 
>rxlen</TT
618 
></DT
619 
><DD
620 
><P
621 
>Some operations may involve additional data coming from the host-side,
622 
for example an incoming ethernet packet. <TT
623 
CLASS="VARNAME"
624 
>rxdata</TT
625 
>
626 
should be a suitably-sized buffer, and <TT
627 
CLASS="VARNAME"
628 
>rxlen</TT
629 
> a
630 
pointer to an integer variable that will end up containing the number
631 
of bytes that were actually received. These arguments will only be
632 
used if the host-side is expected to send a reply and hence the
633 
<TT
634 
CLASS="VARNAME"
635 
>reply</TT
636 
> argument was not NULL.
637 
         </P
638 
></DD
639 
><DT
640 
><TT
641 
CLASS="VARNAME"
642 
>max_rxlen</TT
643 
></DT
644 
><DD
645 
><P
646 
>If a reply to this message is expected and that reply may involve
647 
additional data, <TT
648 
CLASS="VARNAME"
649 
>max_rxlen</TT
650 
> limits the size of that
651 
reply. In other words, it corresponds to the size of the
652 
<TT
653 
CLASS="VARNAME"
654 
>rxdata</TT
655 
> buffer.
656 
         </P
657 
></DD
658 
></DL
659 
></DIV
660 
><P
661 
>Most I/O operations involve only some of the arguments. For example
662 
transmitting an ethernet packet would use the
663 
<TT
664 
CLASS="VARNAME"
665 
>request</TT
666 
>, <TT
667 
CLASS="VARNAME"
668 
>txdata</TT
669 
> and
670 
<TT
671 
CLASS="VARNAME"
672 
>txlen</TT
673 
> fields (in addition to
674 
<TT
675 
CLASS="VARNAME"
676 
>device_id</TT
677 
> which is always required), but would not
678 
involve <TT
679 
CLASS="VARNAME"
680 
>arg1</TT
681 
> or <TT
682 
CLASS="VARNAME"
683 
>arg2</TT
684 
> and no
685 
reply would be expected. Receiving an ethernet packet would involve
686 
<TT
687 
CLASS="VARNAME"
688 
>request</TT
689 
>, <TT
690 
CLASS="VARNAME"
691 
>rxdata</TT
692 
>,
693 
<TT
694 
CLASS="VARNAME"
695 
>rxlen</TT
696 
> and <TT
697 
CLASS="VARNAME"
698 
>max_rxlen</TT
699 
>; in addition
700 
<TT
701 
CLASS="VARNAME"
702 
>reply</TT
703 
> is needed to get any reply from the host-side
704 
at all, and could be used to indicate whether or not any more packets
705 
are buffered up. A control operation such as enabling multicast mode
706 
would involve <TT
707 
CLASS="VARNAME"
708 
>request</TT
709 
> and <TT
710 
CLASS="VARNAME"
711 
>arg1</TT
712 
>,
713 
but none of the remaining arguments.
714 
    </P
715 
></DIV
716 
><DIV
717 
CLASS="REFSECT1"
718 
><A
719 
NAME="SYNTH-NEW-TARGET-INTERRUPTS"
720 
></A
721 
><H2
722 
>Interrupt Handling</H2
723 
><P
724 
>Interrupt handling in the synthetic target is much the same as on a
725 
real target. An interrupt object is created using
726 
<TT
727 
CLASS="FUNCTION"
728 
>cyg_drv_interrupt_create</TT
729 
>, attached, and unmasked.
730 
The emulated device - in other words the Tcl script running inside the
731 
I/O auxiliary - can raise an interrupt. Subject to interrupts being
732 
disabled and the appropriate vector being masked, the system will
733 
invoke the specified ISR function. The synthetic target HAL
734 
implementation does have some limitations: there is no support for
735 
nested interrupts, interrupt priorities, or a separate interrupt
736 
stack. Supporting those might be appropriate when targetting a
737 
simulator that attempts to model real hardware accurately, but not for
738 
the simple emulation provided by the synthetic target.
739 
    </P
740 
><P
741 
>Of course the actual implementation of the ISR and DSR functions will
742 
be rather different for a synthetic target device driver. For real
743 
hardware the device driver will interact with the device by reading
744 
and writing device registers, managing DMA engines, and the like. A
745 
synthetic target driver will instead call
746 
<TT
747 
CLASS="FUNCTION"
748 
>synth_auxiliary_xchgmsg</TT
749 
> to perform the I/O
750 
operations.
751 
    </P
752 
><P
753 
>There is one other significant difference between interrupt handling
754 
on the synthetic target and on real hardware. Usually the eCos code
755 
will know which interrupt vectors are used for which devices. That
756 
information is fixed when the target hardware is designed. With the
757 
synthetic target interrupt vectors are assigned to devices on the host
758 
side, either via the target definition file or dynamically when the
759 
device is instantiated. Therefore the initialization code for a
760 
target-side device driver will need to request interrupt vector
761 
information from the host-side, via a message exchange. Such interrupt
762 
vectors will be in the range 1 to 31 inclusive, with interrupt 0 being
763 
reserved for the real-time clock.
764 
    </P
765 
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