| Simulink® Release Notes | |
Conditional Execution Behavior
This release augments the conditional input branch behavior of the previous release with a more generalized behavior called conditional execution (CE) behavior. The new behavior speeds simulation of models by eliminating unnecessary execution of blocks connected to Switch, Multiport Switch, and conditionally executed blocks.
| Note The Simulink documentation has not yet been updated to reflect the new behavior. Consequently, the remainder of this release note provides a detailed explanation of how the behavior works. |
As with the conditional input branch behavior available in the previous release, the new behavior ensures that the block methods that make up an input branch of a Switch or Multiport Switch block execute only when the model selects the corresponding switch input. In addition, the new behavior option generalizes this behavior to conditionally executed subsystems. Consider, for example, the following model.
Simulink computes the outputs of the Constant block and Gain Block only when the Enabled Subsystem executes (i.e., at time steps 0, 4, 8, and so on). This is because the output of the Constant block is required and the input of the Gain block changes only when the Enabled Subsystem executes. When CE behavior is off, Simulink computes the outputs of the Constant and Gain blocks at every time step, regardless of whether the outputs are needed or change.
In this example, Simulink regards the Enabled Subsystem as defining an execution context for the Constant and Gain blocks. Although the blocks reside in the model's root system, their block methods are executed as if the blocks reside in the Enabled Subsystem.
In general, Simulink defines an execution context as a set of blocks to be executed as a unit. At model compilation time, Simulink associates an execution context with the model's root system and with each of its nonvirtual subsystems. Initially, the execution context of the root system and each nonvirtual subsystem is simply the blocks that it contains. Simulink examines whether a block's output is required only by a conditionally executed subsystem or whether the block's input changes only as a result of the execution of a conditionally executed subsystem. If so, Simulink moves the block into the execution context of the conditionally executed system. This ensures that the block methods are executed during the simulation loop only when the corresponding conditionally executed subsystem executes.
To determine the execution context to which a block belongs, select Sorted order from the model window's Format menu. Simulink displays the sorted order index for each block in the model in the upper right corner of its icon. The index has the format s:b, where s specifies the subsystem to whose execution context the block, b, belongs.
Simulink also expands the sorted order index of conditionally executed subsystems to include the system ID of the subsystem itself in curly brackets as illustrated in the following figure.
In this example, the sorted order index of the enabled subsystem is 0:1{1}. The 0 indicates that the enable subsystem resides in the model's root system. The first 1 indicates that the enabled subsystem is the second block on the root system's sorted list (zero-based indexing). The 1 in curly brackets indicates that the system index of the enabled subsystem itself is 1. Thus any block whose system index is 1 belongs to the execution context of the enabled subsystem and hence executes when it does. For example, the constant block's index, 1:0, indicates that it is the first block on the sorted list of the enabled subsystem, even though it resides in the root system.
| | Context-Sensitive Data Typing of Tunable Parameters | Function-Call Subsystem Enhancements | |
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