troyed. Figure 2.18 shows an example of stop. process DLC ready L_EstabReq SABME Figure 2.18 Example of stop 2.3.8 Task Figure 2.19 shows two task examples. The first one simply performs n :=n- 1 and the second one contains informal text (sometimes called informal task). 18 Validation of Communications Systems with SDL process DLC n:= n + 1 'Check the CRC' Figure 2.19 Example of tasks 2.3.9 Create SDL allows dynamic creation of process instances. Figure 2.20 shows a process DLCmaster creating instances of process DLC. The dashed line between the two processes shows who creates who, but is optional. The process instance creation is actually performed by the create request symbol seen inside DLCmaster on the right. process DLCmaster DLC3SU block DLC3 (dlc2su) L_EstabReq (su2dlc) DLCmaster DLC(0, 5) Figure 2.20 Example of process creation Every process instance contains an implicit variable called offspring. After a create, offspring contains the Pid (Process identification) of the created instance, or Null if the create failed. 2.3.10 Output Output is used to transmit a signal to another process instance. In Figure 2.21, signal s1 is transmitted, with parameter values True and 15. If more than one process instance can be reached by the signal, as in Figures 2.22 and 2.23, it is safer to use VIA or TO to specify which instance must receive the signal. Figure 2.22 shows how to use VIA to send signal red through srl, where process screen cannot have more than one instance. Figure 2.23 shows the use of TO to send signal red to a certain instance of process screen, which has a maximum of three instances. Before the output of red, variable p must be filled with the Pid of the target instance of screen: this is generally done using an array. Quick Tutorial on SDL 19 system sigOutput process bitmap ready SIGNAL ready s1(Boolean, Integer); s1 (True, 15) Figure 2.21 Signal parameters in output block colors2 process bitmap ready bitmap screen (1, 1) red VIA sr1 printer (1, 1) Figure 2.22 Using VIA in output process bitmap blo