Source

Address Decoder (Behavioural)

A programmable address decoder. Works for any address range at any starting point, and the base and bound addresses can be changed at runtime. We express this behaviourally with two unsigned integer comparisons to check if the address lies between the base and bound of a range, inclusively.

This decoder scales to larger address ranges (20 bits or more) without hitting any Verilog implementation vector width limits or requiring long optimization of enormous netlists, as with the Static Address Decoder.

How this circuit synthesizes depends on your CAD tool. In the past, I saw this code synthesize to the expected arithmetic comparisons via subtraction, but a more recent version of the same CAD tool synthesizes to log2(ADDR_WIDTH) levels of LUT logic. But it is unclear if that will be faster than arithmetic circuitry. And of course, if you make the base and bound addresses constant, the logic will optimize further towards its minimal form.

The base, bound, and input addresses all use the same width, which avoids the need for selecting parts of vectors and makes the enclosing code cleaner and more general.

This is the implementation I recommend for general use. You can force a non-arithmetic implementation (but with a fixed range) by using the Static Address Decoder, or conversely, you can force an arithmetic implementation by using the Arithmetic Address Decoder.

`default_nettype none

module Address_Decoder_Behavioural
#(
    parameter       ADDR_WIDTH          = 0
)
(
    input   wire    [ADDR_WIDTH-1:0]    base_addr,
    input   wire    [ADDR_WIDTH-1:0]    bound_addr,
    input   wire    [ADDR_WIDTH-1:0]    addr,
    output  reg                         hit
);

    initial begin
        hit = 1'b0;
    end

    reg base_or_higher = 1'b0;
    reg bound_or_lower = 1'b0;

    always @(*) begin
        base_or_higher = (addr >= base_addr);
        bound_or_lower = (addr <= bound_addr);
        hit            = (base_or_higher == 1'b1) && (bound_or_lower == 1'b1);
    end

endmodule

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