Appendix¶

yosys> help $alu¶

A building block supporting both binary addition/subtraction operations, and indirectly, comparison operations. Typically created by the alumacc pass, which transforms: $add, $sub, $lt, $le, $ge, $gt, $eq, $eqx, $ne, $nex cells into this $alu cell.

Simulation model (verilog)¶

Listing 113 simlib.v¶

module \$alu (A, B, CI, BI, X, Y, CO);

    parameter A_SIGNED = 0;
    parameter B_SIGNED = 0;
    parameter A_WIDTH = 1;
    parameter B_WIDTH = 1;
    parameter Y_WIDTH = 1;

    input [A_WIDTH-1:0] A;      // Input operand
    input [B_WIDTH-1:0] B;      // Input operand
    output [Y_WIDTH-1:0] X;     // A xor B (sign-extended, optional B inversion,
                                //          used in combination with
                                //          reduction-AND for $eq/$ne ops)
    output [Y_WIDTH-1:0] Y;     // Sum

    input CI;                   // Carry-in (set for $sub)
    input BI;                   // Invert-B (set for $sub)
    output [Y_WIDTH-1:0] CO;    // Carry-out

    wire [Y_WIDTH-1:0] AA, BB;

    generate
        if (A_SIGNED && B_SIGNED) begin:BLOCK1
            assign AA = $signed(A), BB = BI ? ~$signed(B) : $signed(B);
        end else begin:BLOCK2
            assign AA = $unsigned(A), BB = BI ? ~$unsigned(B) : $unsigned(B);
        end
    endgenerate

    // this is 'x' if Y and CO should be all 'x', and '0' otherwise
    wire y_co_undef = ^{A, A, B, B, CI, CI, BI, BI};

    assign X = AA ^ BB;
    // Full adder
    assign Y = (AA + BB + CI) ^ {Y_WIDTH{y_co_undef}};

    function get_carry;
        input a, b, c;
        get_carry = (a&b) | (a&c) | (b&c);
    endfunction

    genvar i;
    generate
        assign CO[0] = get_carry(AA[0], BB[0], CI) ^ y_co_undef;
        for (i = 1; i < Y_WIDTH; i = i+1) begin:BLOCK3
            assign CO[i] = get_carry(AA[i], BB[i], CO[i-1]) ^ y_co_undef;
        end
    endgenerate

endmodule

yosys> help $lcu¶: Lookahead carry unit A building block dedicated to fast computation of carry-bits used in binary arithmetic operations. By replacing the ripple carry structure used in full-adder blocks, the more significant bits of the sum can be expected to be computed more quickly. Typically created during techmap of $alu cells (see the “_90_alu” rule in +/techmap.v).