Question

Both DPI task and functions and VPI/PLI functions might need to understand their context. However, the meaning of the term is different for the two categories of task and functions.

DPI imported tasks and functions are essentially proxies for native SystemVerilog tasks and functions. Native SystemVerilog tasks and functions always operate in the scope of their declaration site. For example, a native SystemVerilog function f() can be declared in a module m which is instantiated as top.i1_m. The top.i1_m instance of f() can be called via hierarchical reference from code in a distant design region. Function f() is said to execute in the context (aka. instantiated scope) of top.i1_m, since it has unqualified visibility only for variables local to that specific instance of m. Function f() does not have unqualified visibility for any variables in the calling code’s scope.

DPI imported tasks and functions follow the same model as native SystemVerilog tasks and functions. They execute in the context of their surrounding declarative scope, rather than the context of their call sites. This type of context is termed DPI context.

This is in contrast to VPI and PLI functions. Such functions execute in a context associated with their call sites. The VPI/PLI programming model relies on C code’s ability to retrieve a context handle associated with the associated system task’s call site, and then work with the context handle to glean information about arguments, items in the call site’s surrounding declarative scope, etc. This type of context is termed VPI context.

Note that all DPI export tasks and functions require that the context of their call is known. This occurs since SystemVerilog task and function declarations always occur in instantiable scopes, hence giving rise to a multiplicity of associated task or function instances in the simulator’s database. Thus, there is no such thing as a non-context export tasks or function. All export task and function calls must have their execution scope specified in advance by use of a context-setting API function.