Back to Basics
Let us first recapitulate some of the definitions from the DPI
- Thanks to DPI, SystemVerilog side can call a C function when an
appropriate import declaration is present at the SystemVerilog
side. Such a C function is called imported function.
DPI also provides mechanism that allows SystemVerilog task and function
to be called from the C side. There are few possible cases (and new
definitions) when the C side calls a SystemVerilog task or function.
- The C side can call a SystemVerilog function when a proper
extern declaration is present at the C side and an export
declaration is available on the SystemVerilog side. This function
will be called exported function.
- The C side can also invoke a SystemVerilog task as long as a
proper extern declaration is present at the C side and an
export declaration is made available on the SystemVerilog
side. Note that, unlike exported function, such an exported
task can consume simulation time before the program control
goes back to the C domain. Since C functions can not consume
simulation time, this is the only way to create a time delay from
the C side.
- Lastly, consider the following case: an imported C function
called from the SystemVerilog side can, in turn, call an exported
SystemVerilog task. In such a case, the SystemVerilog task is called
an imported task.
Let us see an example of an extern declaration and how an exported
SystemVerilog function is called from the C side.
export "DPI" function exported_sv_function;
module SV_side (...);
function void exported_sv_function;
$display("Hello from SV function");
extern void exported_sv_func;
printf("Hello from C function");
How About Some Arguments?
A relevant question to ask at this point is, how does one pass arguments
to a SystemVerilog exported task or function when calling it from the
C-side? This is indeed one of the core issues of DPI C layer and we will
spend a considerable part of this tutorial on this topic.
The value of an argument on the C side is specified by a C type
(i.e. data types that C allows - int, float, double, etc.) whereas the
same value is represented by a SystemVerilog type on the SystemVerilog side.
In order for the data to be interpreted correctly by either side, the C type
and SystemVerilog type should match with each other.
DPI leaves the responsibility of matching the data types on two sides
to the user. The user needs to define the data types for the SystemVerilog
and C side that match with each other.
Since SystemVerilog and C data types are different, of course, it is natural
to ask how does one find a matching pair of data types from both sides?
To start with, most of the SystemVerilog basic types already
have C compatible representations. While passing simple argument types
(int, float, and so on) from the C side, it is
easy to find their SystemVerilog counterparts. We will see this in details in
the next section on the next page.
Previous: Pure and Context functions |
Next: Representation of basic types