Bitwise Operators

Part of the Fundamentals section of Coddy's Verilog journey. Lesson 23 of 90.

Bitwise operators perform operations on each individual bit of a number. Unlike arithmetic operators (+, -, *, /) that treat numbers as whole values, bitwise operators work bit-by-bit.

Available Bitwise Operators

Operator	Operation	What It Does
`&`	AND	Output bit is 1 only if both input bits are 1
`\|`	OR	Output bit is 1 if at least one input bit is 1
`^`	XOR	Output bit is 1 if input bits are different
`~`	NOT	Flips all bits (0 becomes 1, 1 becomes 0)

When you use bitwise operators, Verilog treats the numbers as binary behind the scenes.

reg [3:0] a, b;

a = 4'd10;      // Decimal 10 = binary 1010
b = 4'd12;      // Decimal 12 = binary 1100
c = a & b;      // Works the same as using binary directly

You can write numbers in decimal, hex, or binary: Verilog converts them to binary for bitwise operations.

How Bitwise Works

Each operator works on pairs of bits (or single bits for NOT):

Example with 4-bit numbers:

a = 1010  (binary)
b = 1100  (binary)

AND:  a & b = 1000
      (1&1=1, 0&1=0, 1&0=0, 0&0=0)

OR:   a | b = 1110
      (1|1=1, 0|1=1, 1|0=1, 0|0=0)

XOR:  a ^ b = 0110
      (1^1=0, 0^1=1, 1^0=1, 0^0=0)

NOT:  ~a = 0101
      (~1=0, ~0=1, ~1=0, ~0=1)

Code Example

module bitwise_demo;
  reg [3:0] a, b;
  reg [3:0] and_res, or_res, xor_res, not_res;
  
  initial begin
    a = 4'b1010;
    b = 4'b1100;
    
    and_res = a & b;   // 1010 & 1100 = 1000
    or_res  = a | b;   // 1010 | 1100 = 1110
    xor_res = a ^ b;   // 1010 ^ 1100 = 0110
    not_res = ~a;      // ~1010 = 0101
    
    $display("a = %b", a);
    $display("b = %b", b);
    $display("a & b = %b", and_res);
    $display("a | b = %b", or_res);
    $display("a ^ b = %b", xor_res);
    $display("~a    = %b", not_res);
    $finish;
  end
endmodule

Output:

a = 1010
b = 1100
a & b = 1000
a | b = 1110
a ^ b = 0110
~a    = 0101

Challenge

Calculate the bitwise results for the given values.

What to do:

Calculate a & b and store in and_res
Calculate a | b and store in or_res
Calculate a ^ b and store in xor_res
Calculate ~a and store in not_res

Try it yourself

module bitwise_challenge;
  reg [3:0] a, b;
  reg [3:0] and_res, or_res, xor_res, not_res;
  
  initial begin
    a = 4'b1101;
    b = 4'b1011;
    
    and_res = ______;   // a & b
    or_res  = ______;   // a | b
    xor_res = ______;   // a ^ b
    not_res = ______;   // ~a
    
    $display("a = %b", a);
    $display("b = %b", b);
    $display("a & b = %b", and_res);
    $display("a | b = %b", or_res);
    $display("a ^ b = %b", xor_res);
    $display("~a    = %b", not_res);
    $finish;
  end
endmodule