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The following function is a function of a library (The code is written in Solidity). It has an assembly instruction that I do not understand. Specifically at line call(sub(gas, 2000), 6, 0, input, 0xc0, r, 0x60).

From the docs:

call(g, a, v, in, insize, out, outsize)

call contract at address a with input mem[in..(in+insize)) providing g gas and v wei and output area mem[out..(out+outsize)) returning 0 on error (eg. out of gas) and 1 on success

function addition(G1Point p1, G1Point p2) internal returns (G1Point r) {
        uint[4] memory input;
        input[0] = p1.X;
        input[1] = p1.Y;
        input[2] = p2.X;
        input[3] = p2.Y;
        bool success;
        assembly {
            success := call(sub(gas, 2000), 6, 0, input, 0xc0, r, 0x60)
            // Use "invalid" to make gas estimation work
            switch success case 0 { invalid() }
        }
        require(success);
    }

Why the address field a is a simple integer ? Is it referring to an internal library function ? If yes how is the numbering defined ?

Tell me ff you want me to provide the full code of the library.

EDIT

I think I found it. Is the address of the a precompiled contract ?

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It refers to pre-compiled contracts. 6 refers to a the bn256Add pre-compiled contract in the Byzantium release. You should look for PrecompiledContractsByzantium in the geth source code to find a list of pre-compiled contracts, but this link could help you: https://ethereum.stackexchange.com/a/15484/47307

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