1.1 KiB
+++ title = "Calling foreign functions" weight = 1 +++
A function can be declared to be foreign which means that its implementation is not in BH.
topDefn ::= foreign varId :: type [= string ] [ , ( {string }) ]
The optional string gives the name of the external "function" to use. If no string is given the same name as the BH name is used. The optional strings in parentheses are the port names of the Verilog module that implements the function. Without port names positional arguments will be used.
Example:
foreign countOnes :: Bit n -> Bit 32 = "pop_count"
A call to countOnes will instantiate the Verilog pop`` ``count
module. It should have the same number of arguments (with the same type)
as the BH function, and an additional trailing argument which is the
result. If the function is (size) polymorphic the instantiated types
will be used as Verilog parameters.
Example: using the declaration above an action, with the type of x
being Bit 5,
y := countOnes x
will translate to something like
pop_count #(5) ires1(R_x, I_y);