Explore Types of Subsystems

Subsystems can be virtual or nonvirtual.

• Virtual subsystems help visually organize a block diagram. When you group blocks in a virtual subsystem or expand a virtual subsystem, you can affect the execution order of blocks because of the changes to the block paths.

• Nonvirtual subsystems, also known as atomic subsystems, help functionally organize a block diagram. When you group blocks in a nonvirtual subsystem or expand a nonvirtual subsystem, you change the model behavior. Each nonvirtual subsystem executes as a single block, or atomic unit, when the parent model executes. The blocks in a nonvirtual subsystem execute consecutively.

Tip

For controllers and other standalone components, define a hard boundary around the related blocks by using a nonvirtual subsystem or referenced model. Defining a hard boundary upfront avoids costly refactoring when you want to generate code for the component.

To determine whether a subsystem is virtual or nonvirtual, use either of these strategies:

• Check the border of the block. A virtual subsystem has a thin border. A nonvirtual subsystem has a thick border.

  

• Use the get_param function to query the Boolean block parameter IsSubsystemVirtual. For an example, see Determine Whether Subsystem Is Virtual.

This table classifies some of the types of nonvirtual subsystems.

Basis of Subsystem Execution	Subsystem Description	Subsystem Block Name	More Information
Control signal value	Subsystem whose execution is enabled by external input.	Enabled Subsystem	Using Enabled Subsystems
	Subsystem whose execution is triggered by external input.	Triggered Subsystem	Using Triggered Subsystems
	Subsystem whose execution is enabled and triggered by external inputs.	Enabled and Triggered Subsystem	Using Enabled and Triggered Subsystems
	Subsystem whose block states reset with an external trigger.	Resettable Subsystem	Using Resettable Subsystems
If-else logic	Subsystem whose execution is enabled by an If block.	If Action Subsystem	Select Subsystem Execution
Switch logic	Subsystem whose execution is controlled by a Switch Case block.	Switch Case Action Subsystem	Select Subsystem Execution
Function-call event	Subsystem whose execution is controlled by an external function-call input.	Function-Call Subsystem	Using Function-Call Subsystems
Number of iterations	Subsystem that repeats execution during a simulation time step for a specified number of iterations.	For Iterator Subsystem	Iterate Subsystem Execution with While Iterator and For Iterator Subsystems
	Subsystem that repeats execution during a simulation time step while a logical condition is true.	While Iterator Subsystem	Iterate Subsystem Execution with While Iterator and For Iterator Subsystems
Number of elements	Subsystem that repeats execution on each element or subarray of input signal and concatenates results.	For Each Subsystem	Repeat an Algorithm Using a For-Each Subsystem
Message	Subsystem that executes conditionally at each time step based on whether a message is available at the control port.	Message Polling Subsystem	Using Message Polling and Message Triggered Subsystems
	Subsystem that executes whenever a message is available at the control port, independent of sample time.	Message Triggered Subsystem	Using Message Polling and Message Triggered Subsystems
Execution order only	Subsystem that executes as a single block, or atomic unit, when the parent model executes.	Atomic Subsystem	Specify Whether Subsystem Is Atomic