Question

It is possible for a disable statement to disable a block that is currently executing a mixed language call chain. When a DPI import task or function is disabled, the C code is required to follow a simple disable protocol. The protocol gives the C code the opportunity to perform any necessary resource cleanup, such as closing open file handles, closing open VPI handles, or freeing heap memory.

An imported task or function is said to be in the disabled state when a disable statement somewhere in the design targets either it or a parent for disabling. Note that the only way for an imported task or function to enter the disabled state is immediately after the return of a call to an exported task or function. An important aspect of the protocol is that disabled import tasks and functions must programmatically acknowledge that they have been disabled. A task or function can determine that it is in the disabled state by calling the API function svIsDisabledState().

The protocol is composed of the following items:

1. When an exported task returns due to a disable, it must return a value of 1. Otherwise it must return 0.
2. When an imported task returns due to a disable, it must return a value of 1. Otherwise it must return 0.
3. Before an imported function returns due to a disable, it must call the API function svAckDisabledState().
4. Once an imported task or function enters the disabled state, it is illegal for the current function invocation to make any further calls to exported tasks or functions.

Items 2, 3, and 4 are mandatory behavior for imported DPI tasks and functions. It is the responsibility of the DPI programmer to correctly implement the behavior.

Item 1 is guaranteed by SystemVerilog simulators. In addition, simulators must implement checks to ensure that items 2, 3, and 4 are correctly followed by imported tasks and functions. If any protocol item is not correctly followed, a fatal simulation error is issued.

Note that if an exported task itself is the target of a disable, its parent imported task is not considered to be in the disabled state when the exported task returns. In such cases the exported task shall return value 0, and calls to svIsDisabledState() shall return 0 as well.

When a DPI imported task or function returns due to a disable, the values of its output and inout parameters are undefined. Similarly, function return values are undefined when an imported function returns due to a disable. C programmers can return values from disabled functions, and C programmers can write values into the locations of output and inout parameters of imported tasks or functions. However, SystemVerilog simulators are not obligated to propagate any such values to the calling SystemVerilog code if a disable is in effect.