help/info/variant.html

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  13 <title>variant</title>
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  32 <h1>variant</h1>
  33 <hr align="left" width="75%">
  34 <p>The attribute is used to refine the currently specified encoding scheme instead of its replacement.
  35 <p>Currently, two encoding schemes are supported in TITAN: RAW and TEXT.
  36 <p>The RAW coder encodes and decodes TTCN-3 values to/from bitstreams. The TEXT coder can be utilized for text-based protocols.
  37 <p>There are presently twenty-two arguments for the RAW coder specified as follows
  38 <ul>
  39   <li>FIELDLENGTH specifies the length of the encoded type;</li>
  40   <li>COMP specifies how negative integer numbers are encoded ;</li>
  41   <li>ALIGN specifies whether the actual value will be padded from left or right;</li>
  42   <li>BITORDERINFIELD specifies whether the most or the less significant bit will be sent first on the port;</li>
  43   <li>BYTEORDER specifies whether the first or the last byte will be sent first on the port;</li>
  44   <li>BITORDER specifies the sequence of the bits within an octet;</li>
  45   <li>HEXORDER specifies the sequencer of the hexadecimal digits in the encoded data;</li>
  46   <li>FIELDORDER specifies how consecutive fields of a structured type is framed into octets;</li>
  47   <li>EXTENSION_BIT manipulates the bit used in some protocols to code data spanning several octets;</li>
  48   <li>EXTENSION_BIT_GROUP&nbsp; limits the extension bit mechanism to a group of the fields in a record;</li>
  49   <li>PADDING&nbsp;is used when the encoded type shall end at a multiple of padding units;</li>
  50   <li>PREPADDING is used when the encoded type shall start at a multiple of padding units;</li>
  51   <li>PADDING_PATTERN specifies padding pattern used by padding mechanism;</li>
  52   <li>PADDALL means that the padding parameter specified for a whole&nbsp; record or set will be valid for every field of the record/set;</li>
  53   <li>LENGTHTO specifies which fields are considered when calculating the length parameter;</li>
  54   <li>LENGTHINDEX extends the LENGTHTO attribute;</li>
  55   <li>UNIT attribute is used in conjunction with the LENGTHTO or POINTERTO attribute;</li>
  56   <li>POINTERTO specifies to which field a pointer points;</li>
  57   <li>PTROFFSET specifies where the pointed field area starts and the base field of pointer calculating;</li>
  58   <li>TAG identifies specific values in certain fields of the set, record elements or union choices;</li>
  59   <li>CROSSTAG is used when one field of a record specifies the union member of an other field of a record;</li>
  60   <li>PRESENCE indicates the presence of an other optional field within records.</li>
  61 </ul>
  62 <p>The above arguments are described in detail in the Appendix B of the TITAN user documentation.</p>
  63 <p>Related keywords:</p>
  64 <ul>
  65   <li><a href="encode.html"><b><font face="Courier New" color="#003258" size="4">encode</font></b></a></li>
  66   <li><a href="with.html"><b><font face="Courier New" color="#003258" size="4">with</font></b></a></li>
  67   <li><a href="override.html"><b><font face="Courier New" color="#003258" size="4">override</font></b></a></li>
  68 </ul>
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  73   <tr>
  74     <td width="100%">
  75     <h3 align="center"><font face="Courier New" color="#003258" size="5"><b>variant</b></font> [ <font face="Courier New" color="#003258" size="5"><b>override</b></font> ] [ <font
  76       face="Courier New" color="#003258" size="5"><b>(</b></font><i>qualifier</i><font face="Courier New" color="#003258" size="5"><b>)</b></font> ] <font face="Courier New" color="#003258" size="5"><b>&quot;</b></font><i>argument</i><b><font
  77       face="Courier New" color="#003258" size="5">&quot;</font></b></h3>
  78     </td>
  79   </tr>
  80 </table>
  81 </center>
  82 </div>
  83 <ul>
  84   <li>
  85   <p>the optional <b><font face="Courier New">override</font></b> directive forces all contained types at all lower scopes to be forced to the specified attribute.</p>
  86   </li>
  87   <li>
  88   <p>the optional <i>qualifier</i> points out the entity for which the argument is valid.</p>
  89   </li>
  90   <li>
  91   <p><i>argument</i> is a free-text string specifying the actual attribute.</p>
  92   </li>
  93   <li>When using the variant attribute, it is very important to specify the desired codec in the with attributes of the module definition using the <code>encode</code> directive.
  94 </ul>
  95 <hr align="left" width="50%">
  96 <p>Example:
  97 <p><code> module m {<br>
  98 type integer INT2 with { variant &quot;COMP(2scompl)&quot;, &quot;FIELDLENGTH(8)&quot; }&nbsp;</font><br>
  99 } with { encode "RAW" } </code>
 100 <p>The encoding of the integer value -1 results in the bit string 11111111.</p>
 101 <hr align="left" width="25%">
 102 <hr align="left" width="25%">
 103 <p><a HREF="BNF.html#attribkeyword">BNF definition</a> of <font face="Courier New"> variant</font></p>
 104 </body>
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