222 lines
9.5 KiB
HTML
222 lines
9.5 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta name="Copyright" content="Copyright (c) 2006 by IBM Corporation">
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<title>Using Low-Level Screen I/O Services APIs</title>
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<!-- Begin Header Records ========================================= -->
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<!-- US Government Users Restricted Rights -->
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<!-- Use, duplication or disclosure restricted by -->
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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<!-- Change History: -->
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<!-- YYMMDD USERID Change description -->
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<!-- DSMMST1 SCRIPT A converted by B2H R4.1 (346) (CMS) by V2KEA304 -->
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<!-- at RCHVMW2 on 29 Jan 1999 at 16:05:09 -->
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<!-- File Edited November 2001 -->
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<!-- End Header Records -->
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<link rel="stylesheet" type="text/css" href="../rzahg/ic.css">
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</head>
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<body>
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<!-- Java sync-link -->
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<script language="Javascript" src="../rzahg/synch.js" type="text/javascript">
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</script>
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<h2>Using Low-Level Screen I/O Services APIs</h2>
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<h3><a name="HDR5250ERR">DSM Error Handling</a></h3>
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<p>Calls to most of the interfaces can result in a direct I/O operation, or in
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the storing of commands in a command buffer. The command buffer provides a way
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of saving the commands so that multiple operations can be specified and
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performed in a single I/O operation. DSM performs error handling as much as
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possible prior to issuing an I/O operation. For example, if a request is made
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to place the screen in wide mode, and the display does not support this mode,
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DSM detects and reports the error condition before performing an I/O operation.
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This way of handling errors is particularly useful in the case where multiple
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commands have been saved in a buffer. Otherwise, there is no obvious way to
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determine which command was in error when the I/O operation fails.</p>
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<p>The errors that can occur for each operation are listed with the operation.
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If an error message indicates that the error is issued for a negative response
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code, this means that the error was not detected by DSM and occurred on the I/O
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operation.</p>
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<p><strong>Note:</strong> When you are using the i5/OS<SUP>(TM)</SUP> TELNET display station
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emulation, an unsuccessful I/O operation may be undetected initially, but will
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be reported on the next operation.</p>
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<br>
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<h3><a name="HDRDEVSUPP">Device Support</a></h3>
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<p>The 5250 Query command is used to determine the valid commands for a
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particular device. This command is issued for the current device at the start
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of each DSM session and the information is saved for subsequent queries. If the
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5250 Query command is not supported, the base data stream support as documented
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in the 5250 Functions Reference is assumed, with color and wide support being
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determined by the device type.</p>
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<br>
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<h3><a name="HDRLGLSCR">Operating Environments</a></h3>
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<p>The low-level interfaces operate within an environment that can be defined
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using the Create Low-Level Environment (QsnCrtEnv) API. The low-level
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environment defines the operating modes, such as DBCS support and the window
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mode. The environment is passed as a parameter to most of the low-level
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services APIs. There is no need to define a low-level environment unless you
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need a specific operating environment that is different from the default. The
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default low-level environment is indicated on the low-level service APIs by
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specifying the environment handle as zero.</p>
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<br>
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<h3><a name="HDRDIRECT">Direct and Indirect Operations</a></h3>
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<p>Many of the low-level APIs accept an optional command buffer as a parameter.
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For such APIs, the command buffer can be used to store and accumulate a group
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of requested operations. The accumulated operations can then be written to the
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screen in a single I/O operation. Better performance can be achieved because a
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group of repetitive operations can be issued to the screen without having to
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recall the sequence of individual APIs for each repeated operation.</p>
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<p>A <strong>direct operation</strong> is one that omits the command buffer.
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The requested operation takes place immediately. For most APIs, specifying the
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command buffer results in an <strong>indirect operation</strong>. No I/O
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operation takes place and the operation is simply stored in the command buffer.
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Several of the screen input APIs, however, do perform a direct operation when
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the command buffer is specified. This semantic is discussed in
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<a href="QsnReadInp.htm">Read Input Fields (QsnReadInp) API</a>.</p>
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<br>
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<h3><a name="HDRDBCS">DBCS Considerations</a></h3>
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<p>You can write DBCS data enclosed with SO/SI to the screen, and when the
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underlying display supports it, graphic DBCS using the Write Data (QsnWrtDta)
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API to specify the data stream Write Extended Attribute order. (See the 5250
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data stream documentation for further details.) You can define fields as being
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DBCS through the Set Field (QsnSetFld) API using the appropriate field control
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word. DBCS data can be written to such fields as described above. If you
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specify DBCS support on the low-level environment description,
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(see <a href="QsnCrtEnv.htm#HDRLLDESC">Format of the Low-Level Environment Description</a>),
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the APIs will handle the parsing of DBCS data and fields appropriately.</p>
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<p>The APIs do not provide any special processing of DBCS data, such as adding
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SO/SI to DBCS graphic data when graphic data is not supported by the display.
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For example, if you want to define a field as graphic DBCS and write graphic
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DBCS data to it, code the QsnSetFld API specifying a control word of
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QSN_FCS_DBCS_PURE (x'8220') and then write the graphic data to a command buffer
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using the QsnWrtDta API. Precede and follow this data with Write Extended
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Attribute orders to add the extended NLS SO/SI attributes. If you want to write
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a graphic data value to a non-graphic DBCS field, you must enclose the graphic
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DBCS data with SO/SI prior to calling the QsnWrtDta API.</p>
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<br>
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<h3><a name="HDRLLPERF">Performance Considerations</a></h3>
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<p>The following operations can incur overhead and adversely affect the
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performance of your application:</p>
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<ul>
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<li><strong>QsnCrtEnv</strong><br>
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<br>
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<p>Specifying translation of x'3f' to x'1f' can incur overhead because all
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outgoing data must be checked for this value. This option should be specified
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only if CDRA is on and translation between the code pages will result in a
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x'3f' occurrence in data to be displayed.</p>
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</li>
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<li><strong>QsnSavScr</strong><br>
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<br>
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<p>This operation results in the entire contents of the screen being read,
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which can adversely affect response time. This is typically about 3KB, but
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could be up to 28KB.</p>
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</li>
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<li><strong>QsnRstScr</strong><br>
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<br>
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<p>This operation writes the result of a save screen back to the device, which
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can adversely affect response time.</p>
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</li>
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</ul>
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<p>If you have GUI support, you can put additional commands after the QsnSavScr
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or QsnRstScr APIs to reduce I/O operations and improve performance.</p>
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<p>Deleting structures associated with handles, such as command buffers, prior
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to exiting a program will improve performance for the programs that use APIs
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that create handles.</p>
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<br>
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<h3><a name="HDRLLLIMIT">Limitations and Restrictions</a></h3>
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<p>The following limitations or restrictions apply to the low-level
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interfaces:</p>
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<ul>
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<li>Certain functions are supported by control units that do not support the
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5250 Query command. If the Query command is not supported, it is assumed that
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the particular function is not supported either. These functions are
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transparent data support and move cursor order support. Device attributes such
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as wide mode and color support are determined based on the device type if the
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5250 Query command is not supported.<br>
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<br>
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</li>
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<li>For the Retrieve Display Mode (QsnRtvMod) operation to correctly report the
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current state of the display, all commands that affect this state (such as a
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Clear Unit or Clear Unit Alternate) must occur as the first command in any
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command stream written to the display. This is because the work station control
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unit inspects the first command in the stream to determine if a state change is
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taking place. Most iSeries<SUP>(TM)</SUP> programs, including the DSM APIs, send these
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commands only at the beginning of a stream. If you write a stream in which such
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commands do not appear at the beginning of the stream, the results of the
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Retrieve Display Mode (QsnRtvMod) operation may not be accurate.<br>
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<br>
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</li>
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<li>When conversions are performed, they are performed only after a Read Input
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Fields (QsnReadInp), Read Modified Fields (QsnReadMDT), Read Modified Alternate
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(QsnReadMDTAlt), Read Immediate (QsnReadImm), or Read Modified Immediate
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Alternate (QsnReadMDTImmAlt) operation. They are performed on all incoming
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field data, including transparent and numeric data. You must turn conversion on
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and off. To prevent certain data from being converted, you explicitly set the
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conversion options on the QsnCrtEnv and QsnChgEnv APIs. The conversions that
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are affected by this are CDRA conversion based on the job CCSID and conversions
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of X'1F' in the incoming data stream to X'3F'.</li>
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</ul>
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<hr>
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<center>
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<table cellpadding="2" cellspacing="2">
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<tr align="center">
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<td valign="middle" align="center"><a href="#Top_Of_Page">Top</a> | <a href=
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"dsm.htm">Dynamic Screen Manager APIs</a> | <a href="aplist.htm">APIs by
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category</a></td>
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</tr>
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</table>
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</center>
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</body>
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</html>
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