Keywords and Identifiers in Verilog

In Verilog, the distinction between keywords and identifiers forms the bedrock of syntactical integrity and code clarity. Understanding their roles, rules, and interactions is fundamental to crafting robust and readable Verilog code.

Keywords in Verilog

Keywords, as the name suggests, are reserved words within the Verilog language that possess predefined meanings and functionalities. They play pivotal roles in defining the language’s syntax and facilitating specific actions within the code. Examples of Verilog keywords include module, input, output, always, if, else, endmodule, and others.

Verilog keywords are off-limits for use as identifiers or user-defined names within the code. Attempting to repurpose keywords as identifiers can lead to syntax errors and confusion in code interpretation. Thus, developers must adhere strictly to Verilog’s predefined set of keywords and avoid using them as identifiers.

Identifiers in Verilog

Identifiers represent user-defined names used to label various elements within Verilog code. These elements could include modules, variables, ports, instances, and other user-created entities. Identifiers follow specific rules and conventions:

  • Naming Rules:
    • Must start with a letter or an underscore (_), followed by letters, numbers, or underscores.
    • Cannot begin with a number or any Verilog keyword.
  • Case Sensitivity:
    • Verilog is usually case-sensitive, distinguishing between uppercase and lowercase letters in identifiers. For instance, myModule, MyModule, and mymodule would be treated as distinct identifiers.

Example of Keywords and Identifiers

module ExampleModule (
  input A,
  output reg B
  reg internal_reg;  // 'internal_reg' is an identifier

  always @* begin
    if (A == 1)      // 'if' is a keyword; 'A' is an identifier
      B <= 1;        // '<=' is an operator; 'B' is an identifier
      B <= 0;


In the provided Verilog snippet:

  • module, input, output, reg, always, if, else, endmodule are keywords.
  • A, B, internal_reg are identifiers, representing input/output ports and an internal register, respectively.