the process input queues, process (and procedure or services) states, and SO on. To watch a specific element, you must select Edit > Create watch, then right-click to open a shortcut menu, where the choice Add Item prompts you to enter the name of the entity you want to put in the watch. The syntax for names you enter here is the same as that for the textual command print. For example, you can enter a variable name, such as DLCs, to watch all the occurrences of variable DLCs, as shown in Figure 4.61. Watch dicaldispatch ! dics = 0 = dicaldic(1) 1 = null dicbldispatch ! dles = 0 = null 1 = null Figure 4.61 A watch window As each DLCs is an array with two elements, the watch shows the content of each element. To get the same watch windows the next time you launch the Simulator, you can select File > Watches > Save As and enter a file name such as testl. watch. Then you can add the command source testl watch into your model startup file (example: 76.startup) to create the watch windows automatically. 4.3.7.7 Examining the SDL model: trace The trace command displays in the textual trace the content of any SDL or simulation element, after every simulation step. It also writes this content in the MSC trace, every time its value changes. The syntax for names you enter here is the same as that for the textual command print. For example, to trace the value of the variable N320cnt in block DLCa, type in the Simulator command line trace dlca!dlc (1) ! n320cnt The effect in the MSC trace is depicted in Figure 4.45. Trace is handy when searching the origin of a hard to find bug: you can trace the state of each process instance, of variables, of process queues, and SO on. 102 Validation of Communications Systems with SDL 4.3.7.8 Modifying the SDL model Two commands allow you to modify during simulation the content of an element in the SDL model: the let textual command, and the change value graphical command, available in the watch windows. The let command accepts the same arguments as the print command (except that you must s