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<a href="00434.html" target="topic">30.8 Reading data</a>, <a href="01004.html" target="topic">主题</a></div>
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30.8.4 Reading an object</div>
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The sections above have outlined: </p>
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— How to select an object for access, in other words, marking this object as a target for access. This is </p>
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where the design navigation VPI is used. </p>
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— How to call vpi_load_init() as a hint on the areas to be accessed, and/or optionally load an object </p>
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into memory after obtaining a handle and then either loading objects individually or as a group using </p>
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the object collection. </p>
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— How to optionally iterate the design scope and the object collection to find the loaded objects if needed. </p>
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In this section reading data is discussed. Reading an object’s data means obtaining its value changes. VPI, before this extension, had allowed a user to query a value at a specific point in time—namely the current time, and its access does not require the extra step of giving a load hint or actually loading the object data. This step is added here because VPI is extended with a temporal access component: The user can ask about all the values in time (regardless of whether that value is available to a particular tool, or found in memory or a database, the mechanism is provided) since accessing this value horizon involves a larger memory expense, and possibly a considerable access time. Let’s see now how to access and traverse this value timeline of an object. </p>
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To access the value changes of an object over time, a traverse object is used, as introduced earlier in Section 30.4.1. Several VPI routines are also added to traverse the value changes (using this new handle) back and forth. This mechanism is very different from the “iteration” notion of VPI that returns objects related to a given object, the traversal here can walk or jump back and forth on the value change timeline of an object. To create a value change traverse handle the routine vpi_handle() must be called in the following manner: </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code01046"><pre class="Element12">vpiHandle trvsHndl = vpi_handle(vpiTrvsObj, object_handle);</pre></div></div>
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Note that the user (or tool) application can create more than one value change traverse handle for the same object, thus providing different views of the value changes. Each value change traverse handle shall have a means to have an internal index, which is used to point to its “current” time and value change of the place it points. In fact, the value change traversal can be done by increasing or decreasing this internal index. What this index is, and how its function is performed is left up to tools’ implementation; It is only used as a concept for explanation here. </p>
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Once created the traverse handle can point anywhere along the timeline; its initial location is left for tool implementation. However, if the traverse object has no value changes the handle shall point to the minimum time (of the trace), so that calls to vpi_get_time() can return a valid time. It is up to the user to call an initial vpi_goto() to move to the desired initial pointing location.</p></div>
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<a href="00434.html" target="topic">30.8 Reading data</a></p></div>
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<a href="00441.html" target="topic">30.8.4.1 Traversing value changes of objects</a> </div></td><td class="Element206" valign="top" width="50%">
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After getting a traverse vpiHandle, the application can do a forward or backward walk or jump traversal by using vpi_goto() on a vpiTrvsObj object type with the new traverse properties.<br><br>Here is a sample code segment for the complete process from handle creation to traversal. </div></td></tr><tr>
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<a href="00442.html" target="topic">30.8.4.2 Jump Behavior</a> </div></td><td class="Element206" valign="top" width="50%">
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Jump behavior refers to the behavior of vpi_goto() with a vpiTime control constant, vpiTrvsObj type, and a jump time argument. The user specifies a time to which he or she would like the traverse handle to jump, but the specified time might or might not have value changes. In that case, the traverse handle shall point to the latest VC equal to or less than the time requested.<br><br>In the example below, the whole simulation run is from time 10 to time 65, and a variable has value changes at time 10, 15 and 50. If a value change traverse... <a href="00442.html" target="topic">more</a> </div></td></tr><tr>
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When accessing a database, it is likely that there are gaps along the value time-line where possibly the data recording (e.g. dumping from simulator) was turned off. In this case the starting point of that interval shall be marked as a VC if the object had a stored value before that time. vpi_goto(), whether used to jump to that time or using next VC or previous VC traversal from a point before or after respectively, shall stop at that VC. Calling vpi_get_value() on the traverse object pointing to that VC shall have no effect on the value argument passed; the... <a href="00443.html" target="topic">more</a> </div></td></tr></table></div></div>
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