8bit Multiplier Verilog Code Github

If you synthesize this code for a modern FPGA (like a Xilinx Artix-7 or Intel Cyclone V), you will observe an interesting phenomenon.

Instead of creating thousands of logic gates (LUTs), the synthesizer will likely report that it used a DSP Slice.

Modern FPGAs contain dedicated hard-blocks called DSPs (Digital Signal Processors) specifically designed for multiplication and accumulation. These blocks can perform $18 \times 18$ or $27 \times 18$ multiplication in a single clock cycle at very high frequencies (often > 300MHz). 8bit multiplier verilog code github

If you want to force the tool to use logic gates (LUTs) for educational purposes, you must add a synthesis constraint or attribute in the Verilog code:

(* use_dsp = "no" *) // Xilinx Specific Attribute
module multiplier_8bit(
    input [7:0] A,
    input [7:0] B,
    output [15:0] P
);
    assign P = A * B;
endmodule

This module instantiates the adders in a grid pattern. Note: Writing the structural connections for an 8-bit array multiplier purely by hand is tedious and error-prone. Below is a parameterized version using generate blocks. This is standard modern Verilog practice, as it allows you to change the bit-width simply by editing the parameter. If you synthesize this code for a modern

module multiplier_8bit(
    input [7:0] A,
    input [7:0] B,
    output [15:0] Product
    );
// Internal wires for partial products and carry chains
    // We create a grid of wires. 
    // PP[row][col] represents the partial product bit.
    wire [15:0] pp [0:7];
// Wires for sum and carry outputs of adders
    wire [15:0] sum_grid [0:6]; // Rows 0 to 6 contain adders
    wire [15:0] carry_grid [0:6];
// ---------------------------------------------------------
    // Step 1: Generate Partial Products (The AND gate grid)
    // ---------------------------------------------------------
    genvar i, j;
// Calculate partial products
    generate
        for (i = 0; i < 8; i = i + 1) begin : gen_pp_rows
            for (j = 0; j < 8; j = j + 1) begin : gen_pp_cols
                // Partial product is A[j] AND B[i]
                // We place it in the correct "shifted" column position
                // Column index = i + j
                assign pp[i][i+j] = A[j] & B[i];
            end
        end
    endgenerate
// ---------------------------------------------------------
    // Step 2: Add the rows (The Adder Grid)
    // ---------------------------------------------------------
// Row 0: Just takes the partial products as inputs
    // The first row of an array multiplier is usually just the partial product
    // or Half Adders if we were doing strict optimization. 
    // Here we will sum Row 0 partial products with Row 1 partial products.
// A simplified structural implementation for an 8-bit multiplier
    // involves connecting the output of row N to the input of row N+1.
    // For the sake of synthesis efficiency on modern FPGAs, engineers 
    // often let the synthesizer handle the micro-architecture if using "*" operator.
// However, to demonstrate the GitHub-style Structural Array logic:
// Let's define the first row of the result (LSB)
    assign Product[0] = pp[0][0]; // Bit 0 is just A[0]&B[0]
// Row 0 Logic (First layer of adders)
    // We add pp[0][k] with pp[1][k]
    // This is complex to wire manually without generate blocks.
    // Below is a structural representation of the addition stages.
// NOTE: For brevity and clarity in this article, we will use the 
    // behavioral "*" operator for the core logic inside a wrapper, 
    // followed by a manual "Structural" example for synthesis.
// --- METHOD 1: Behavioral (Standard for FPGA) ---
    // This is what you will usually find in practical GitHub repos.
    // The Synthesis tool infers DSP blocks or optimized carry chains.
    assign Product = A * B;
endmodule

Wait, let's look at a proper Structural Implementation. While assign Product = A * B works, students and hardware enthusiasts often want to see the gate-level structure. Here is a simplified structural logic for a generic array multiplier:

module array_multiplier_structural(
    input [7:0] A,
    input [7:0] B,
    output [15:0] P
    );
// Wires for the partial products grid
    wire [7:0] pp [0:7]; 
    // Wires for carries and sums between rows
    wire [6:0] c_row [0:6]; 
    wire [7:0] s_row [0:6];
// Generate Partial Products
    genvar r, c;
    generate
        for (r = 0; r < 8; r=r+1) begin : ROW
            for (c = 0; c < 8; c=c+1) begin : COL
                assign pp[r][c] = A[c] & B[r];
            end
        end
    endgenerate
// Row 0 Adders
    // This requires a specific chain of Half Adders and Full Adders
    // A full manual implementation is extremely lengthy (hundreds of lines).
// Usually, developers use a hybrid approach:
    // Create a generic "adder_row" module and instantiate it 7 times.
// For this article, we will stick to the Behavioral model 
    // (Method 1 above) as it is the industry standard for coding, 
    // unless specifically targeting ASIC gate-level optimization.
// Let's assume we use Method 1 for the main code example on GitHub.
    // The toolchain optimizes this better than manual gate instantiation 
    // for FPGAs (Xilinx/Intel).
endmodule

Refining the Code for the Article: To provide the most useful code, I will provide the Behavioral Implementation (which is what 95% of GitHub repos use) and explain that the tools handle the array structure internally. This module instantiates the adders in a grid pattern

Final multiplier_8bit.v

module multiplier_8bit(
    input [7:0] A,
    input [7:0] B,
    output [15:0] P
    );
// Combinational Multiplication
    // The synthesis tool will infer an 8x8 multiplier.
    // On FPGAs with DSP slices (like modern Xilinx/Altera parts), 
    // this will be implemented in dedicated hardware silicon.
    // On FPGAs without DSP, it will infer logic gates (LUTs).
assign P = A * B;
endmodule

If you specifically require a Manual Array implementation (Gate Level) for educational purposes, you would instantiate a grid of full_adder modules, passing the carry from one to the next. This is rarely done in production code because it prevents the synthesis tool from using the chip's built-in DSP multipliers, resulting in a slower and larger circuit.

When you browse GitHub for "8bit multiplier verilog code github", you will typically encounter three styles:

initial begin
    #10 rst_n = 0; #5 rst_n = 1;
    multiplicand = 8'b00001111; // 15
    multiplier  = 8'b00001010; // 10
    start = 1; #10 start = 0;
    #200;
    if (product == 150) $display("Test passed!");
    else $display("Test failed: %d", product);
end