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