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Verilog-XL Reference

Product Version 3.1August 2000

1990-2000 Cadence Design Systems, Inc. All rights reserved.Printed in the United States of America.

Cadence Design Systems, Inc., 555 River Oaks Parkway, San Jose, CA 95134, USA

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Contents

1Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18The Verilog Hardware Description Language . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18The Verilog-XL Logic Simulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Major Features of Verilog-XL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Verilog-XL Licenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2Lexical Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22White Space and Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

String Variable Declaration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25String Manipulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Special Characters in Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Identifiers, Keywords, and System Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Escaped Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Text Substitutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3Data Types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Value Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Registers and Nets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Nets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

August 2000 2 Product Version 3.1


Signed Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Net and Register Declaration Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Declaration Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Specifying Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Vector Net Accessibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Strengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Charge Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Drive Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Implicit Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Net Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Net Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

wire and tri Nets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39Wired Nets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40trireg Net . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40tri0 and tri1 Nets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44Supply Nets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Memories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45Integers and Times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46Real Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Real Number Declaration Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47Specifying Real Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47Operators and Real Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Binary Operator Precedence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52Numeric Conventions in Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Arithmetic Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Arithmetic Expressions with Registers and Integers . . . . . . . . . . . . . . . . . . . . . . . . . 54Relational Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55



Equality Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55Logical Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Bit-Wise Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57Reduction Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58Syntax Restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59Shift Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59Arithmetic Shift Operators for Signed Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60Conditional Operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60Concatenations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Operands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62Net and Register Bit Addressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62Memory Addressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64String Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64String Value Padding and Potential Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64Null String Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Minimum, Typical, Maximum Delay Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65Expression Bit Lengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

An Example of an Expression Bit Length Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . 67Verilog Rules for Expression Bit Lengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

5Assignments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70Continuous Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

The Continuous Assignment Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72The Net Declaration Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76Calling Functions in a Continuous Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Procedural Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78Accelerated Continuous Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Restrictions on Accelerated Continuous Assignments . . . . . . . . . . . . . . . . . . . . . . . . 79Controlling the Acceleration of Continuous Assignments . . . . . . . . . . . . . . . . . . . . . . 86The Effects of Accelerated Continuous Assignments . . . . . . . . . . . . . . . . . . . . . . . . . 87



Procedural Continuous Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93The assign and deassign Procedural Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . 93The force and release Procedural Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

6Gate and Switch Level Modeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97Gate and Switch Declaration Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

The Gate Type Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99The Drive Strength Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99The Delay Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100The Primitive Instance Identifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100The Range Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100Primitive Instance Connection List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101Rules for Using an Array of Instances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

and, nand, nor, or, xor, and xnor Gates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104buf and not Gates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105bufif1, bufif0, notif1, and notif0 Gates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106MOS Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107Bidirectional Pass Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109cmos Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110pullup and pulldown Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111Implicit Net Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112Logic Strength Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113Strengths and Values of Combined Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

Combined Signals of Unambiguous Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115Ambiguous Strengths: Sources and Combinations . . . . . . . . . . . . . . . . . . . . . . . . . 116Ambiguous Strength Signals and Unambiguous Signals . . . . . . . . . . . . . . . . . . . . . 123Wired Logic Net Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

Mnemonic Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130Strength Reduction by Non-Resistive Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130Strength Reduction by Resistive Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130Strengths of Net Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

tri0 and tri1 Net Strengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131trireg Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131



supply0 and supply1 Net Strengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131Gate and Net Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

min/typ/max Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135trireg Net Charge Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

Gate and Net Name Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

7User-Defined Primitives (UDPs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141UDP Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142UDP Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

UDP Terminals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144UDP Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144Sequential UDP initial Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144UDP State Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

Summary of UDP Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146Combinational UDPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146Level-Sensitive Sequential UDPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148Edge-Sensitive UDPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148Sequential UDP Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149Mixing Level-Sensitive and Edge-Sensitive Descriptions . . . . . . . . . . . . . . . . . . . . . . . 152Level-Sensitive Dominance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153UDP Instances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154Compilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155Reducing Pessimism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156Processing of Simultaneous Input Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157Memory Usage and Performance Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158UDP Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

8Behavioral Modeling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161Structured Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

always Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163initial Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163



Procedural Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164Blocking Procedural Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165Non-Blocking Procedural Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165Processing Blocking and Non-Blocking Procedural Assignments . . . . . . . . . . . . . . 171

Conditional Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172Multi-Way Decision Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

if-else-if Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173case Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174Using case Statements with Inconsequential Conditions . . . . . . . . . . . . . . . . . . . . . 176

Looping Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177forever Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178repeat Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178while Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179for Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

Procedural Timing Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180Delay Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181Zero-Delay Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181Event Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182Named Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182Event OR Construct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183Level-Sensitive Event Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184Intra-Assignment Timing Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

Block Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187Sequential Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187Parallel Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189Block Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190Start and Finish Times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

Behavior Model Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

9Tasks and Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194Distinctions Between Tasks and Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194Tasks and Task Enabling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

Defining a Task . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195



Task Enabling and Argument Passing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196Task Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197Effect of Enabling an Already Active Task . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

Functions and Function Calling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198Defining a Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198Returning a Value from a Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199Calling a Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199Function Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200Function Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

10Disabling of Named Blocks and Tasks . . . . . . . . . . . . . . . . . . . . . . . . 202Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202disable Statement Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

11Hierarchical Structures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

Top-Level Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208Module Instantiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208Module Definition and Instance Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208

Overriding Module Parameter Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210Using the defparam Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211Using Module Instance Parameter Value Assignment . . . . . . . . . . . . . . . . . . . . . . . 212Parameter Dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213

Macro Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213Constructs Allowed in Macro Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213Specifying Macro Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214Instances of Macro Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214Using Parameters with Macro Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214Effect on Decompilation and Tracing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

Ports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216Port Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216



Port Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217Connecting Module Ports by Ordered List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217Connecting Module Ports by Name . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218Real Numbers in Port Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220Port Collapsing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220Port Connection Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221Port Connections in Macro Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224

Hierarchical Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225Data Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228Macro Modules and Hierarchical Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228Upwards Name Referencing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

Automatic Naming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230Scope Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231

12Using Specify Blocks and Path Delays. . . . . . . . . . . . . . . . . . . . . . . . 234Understanding Specify Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234

Specparam Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235Understanding Path Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236

Driving Wired Logic Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239Simulating Distributed Delays as Inertial and Transport Delays . . . . . . . . . . . . . . . . 240Simulating Path Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

Describing Module Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243Establishing Parallel or Full Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245Specifying Transition Delays on Module Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248Calculating Delay Values for X Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250Specifying Module Path Polarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251Using Path Delays in Behavioral Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252Simulating Path Outputs that Drive Other Path Outputs . . . . . . . . . . . . . . . . . . . . . . 253Understanding Strength Changes on Path Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . 254Specifying Global Pulse Control on Module Paths . . . . . . . . . . . . . . . . . . . . . . . . . . 254Specifying Local Pulse Control for Module Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . 256Pulse Filtering for Module Path Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257Pulse Filtering and Cancelled Schedules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259Pulse Filtering and Cancelled Schedule Dilemmas . . . . . . . . . . . . . . . . . . . . . . . . . 263



Using State-Dependent Path Delays (SDPDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266Evaluating SDPD Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267Using Edge Keywords in SDPDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270Making SDPDs Function as Unconditional Delays . . . . . . . . . . . . . . . . . . . . . . . . . . 271Working with Distributed Delays and SDPDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

Working with Multiple Path Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272Effects of Unknowns on SDPDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273Effects of Unknowns on Edge-Sensitive Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274Possible Effects of Internal Logic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274

Enhancing Path Delay Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276Invoking the accu_path Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276Comparing the Default and accu_path Delay Selection Algorithms . . . . . . . . . . . . . 278Limits of the accu_path Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282

13Timing Checks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285Using Timing Checks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285

Understanding Timing Violation Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286Using Edge-Control Specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287Using Notifiers for Timing Violations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288Enabling Timing Checks with Conditioned Events . . . . . . . . . . . . . . . . . . . . . . . . . . 289

Using the Timing Check System Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291$hold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291$nochange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293$period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294$recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295$recrem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297$removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301$setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302$setuphold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303$skew . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307$width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308

Using Negative Timing Check Limits in $setuphold and $recrem . . . . . . . . . . . . . . . . . 309Effects of Delayed Signals on Timing Checks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310



Calculation of Delayed Signals and Limit Modification . . . . . . . . . . . . . . . . . . . . . . . 312Explicitly Defining Delayed Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314Effects of Delayed Signals on Path Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316Restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317Exception Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318

14System Tasks and Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319Filename Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320Display and Write Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

Escape Sequences for Special Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322Format Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322Size of Displayed Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324Unknown and High-Impedance Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325Strength Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 326Hierarchical Name Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328String Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328

Strobed Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328Continuous Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329Monitoring Interconnect Delay Signal Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330File Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331Default Base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333Signed Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334Simulation Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334Stop and Finish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335Random Number Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335Tracing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336Saving and Restarting Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340

Incremental Save and Restart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342Command-Line Restart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343Limitations for Saving and Restarting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343



Command History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343Command Input Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344Log File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344Key File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345Setting the Interactive Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345Showing the Hierarchy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346Showing Variable Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346Showing Net Expansion Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347Showing Module Port Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347Showing Number of Drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348Displaying the Delay Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350Storing Interactive Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350Interactive Source ListingDecompilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351

$list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351$listcounts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351$list_forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352

Disabling and Enabling Warnings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353$disable_warnings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353$enable_warnings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355

Loading Memories from Text Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355Setting a Net to a Logic Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357Fast Processing of Stimulus Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358Incremental Pattern File Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359

$incpattern_write . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359$incpattern_read . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360$compare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363$strobe_compare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364Examples of Response Checking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 366

Functions and Tasks for Reals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367Functions and Tasks for Timescales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368Protecting Data in Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369Value Change Dump File Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369Running the Behavior Profiler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370

$startprofile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371$reportprofile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371$listcounts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372



$stopprofile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372Resetting Verilog-XLStarting Simulation Over Again . . . . . . . . . . . . . . . . . . . . . . . . . 372

$reset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373$reset_count . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378$reset_value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379

SDF Annotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380$sdf_annotate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381Controlling $sdf_annotate Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383$sdf_annotate Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384Annotating Path Delay or Timing Check Vector Bits in Specify Blocks . . . . . . . . . . . 388

Using the $dlc System Task . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391Using the $system System Task . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392

15Programmable Logic Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394Array Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395Array Logic Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395Logic Array Personality Declaration and Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396Logic Array Personality Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396PLA Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398

Synchronous Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398And-Or Array Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399PAL16R8 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 400PAL16R4 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405

16Interconnect Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411Module Import Port Delays (MIPDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413

How MIPDs Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413Specifying MIPDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418Restrictions on Ports for MIPDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419Monitoring Nets Internal to MIPDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420



Displaying Status Information for Nets Internal to MIPDs . . . . . . . . . . . . . . . . . . . . 420An Application of MIPDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

Single-Source/MultiSource Interconnect Transport Delays (S/MITDs) . . . . . . . . . . . . . 422Controlling MIPD and S/MITD Creation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423S/MITDs and Pulse Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426Resolving Ambiguous S/MITD Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427PLI Tasks for S/MITDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428

17Timescales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429The timescale Compiler Directive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430

Effects of Timescales on Simulation Performance . . . . . . . . . . . . . . . . . . . . . . . . . . 432Usage Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432

Timescale System Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432$time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433$realtime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434$scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434

The Timescale System Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435$printtimescale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436$timeformat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436Timescales Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439

18Delay Mode Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443Delay Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444

Unit Delay Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444Zero Delay Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444Distributed Delay Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445Path Delay Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445Default Delay Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446

Reasons to Select a Delay Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446Setting a Delay Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446

Compiler Directives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446



Command-Line Plus Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447Precedence in Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447Timescales and Simulation Time Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 448Overriding Delay Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449

PLI 1.0 or VPI Access Routines and Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449Parameter Attribute Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450

Delay Mode Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451Decompiling with Delay Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452$showmodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452acc_fetch_delay_mode Access Routine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452Macro Module Expansion and Delay Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452Summary of Delay Mode Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453

19The Behavior Profiler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454How the Behavior Profiler Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 454Behavior Profiler System Tasks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456

$startprofile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456$reportprofile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457$stopprofile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458$listcounts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 458

Behavior Profiler Data Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460Profile Ranking by Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460Profile Ranking by Module Instance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463Profile Ranking by Statement Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464Profile Ranking by Statement Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465

Recommended Modeling Practices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472Invoke the Behavior Profiler After You Initialize Your Design . . . . . . . . . . . . . . . . . . 472Put Statements on Separate Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472

How Verilog-XL Affects Profiler Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472Using a Variable to Drive Mulitple Module Instances . . . . . . . . . . . . . . . . . . . . . . . . 472Expanded Vector Nets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473Accelerated Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473



Behavior Profiler Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473

20The Value Change Dump File. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481Creating the Value Change Dump File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481

Specifying the Dump File Name ($dumpfile) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482Specifying Variables for Dumping ($dumpvars) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483Stopping and Resuming the Dump ($dumpoff/$dumpon) . . . . . . . . . . . . . . . . . . . . 484Generating a Checkpoint ($dumpall) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484Limiting the Size of the Dump File ($dumplimit) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485Reading the Dump File During Simulation ($dumpflush) . . . . . . . . . . . . . . . . . . . . . 485Sample Source Description Containing VCD Tasks . . . . . . . . . . . . . . . . . . . . . . . . . 486

Format of the Value Change Dump File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486Contents of the Dump File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486Structure of the Dump File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487Formats of Dumped Variable Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487Using Keyword Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489Description of Keyword Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489Syntax of the VCD File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494Value Change Dump File Format Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495

Using the $dumpports System Task . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498$dumpports Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 498$dumpports Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499$dumpports Restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503$dumpports_close . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503



AFormal Syntax Definition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505Summary of Syntax Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505Source Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509Primitive Instances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511Module Instantiations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511Behavioral Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512Specify Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517General Syntax Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518Switch-Level Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519

BVerilog-XL Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 521Keywords from Compiler Directives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 522Keywords from Specify Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524Keywords from Neither Compiler Directives nor Specify Blocks . . . . . . . . . . . . . . . . . . 524

CVerilog-XL and Standards Compliance. . . . . . . . . . . . . . . . . . . . . . . 527Supported Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 527Known Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 527

VPI Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 527Wire with same name as a Port . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 528

Index.............................................................................................................................. 529



1Introduction

This chapter describes the following information:

Overview on page 18

The Verilog Hardware Description Language on page 18

The Verilog-XL Logic Simulator on page 20

Overview

This reference manual describes the features of the Verilog-XL digital logic simulator and theVerilog Hardware Description Language you use to model a design for simulation byVerilog-XL.

The Verilog Hardware Description Language

The Verilog Hardware Description Language (HDL) describes a hardware design or part of adesign. Verilog models are descriptions of designs in the Verilog HDL. The Verilog HDL isboth a behavioral and a structural language. Models in the Verilog HDL can describe both thefunction of a design and the components and the connections to the components in a design.

Verilog models can be developed for different levels of abstraction. These levels of abstractionand their corresponding model types are described in Table 1-1 on page 18.

Table 1-1 Verilog models and their level of abstraction

algorithmic a model that implements a design algorithm in high-level languageconstructs

RTL a model that describes the flow of data between registers and how adesign processes that data


Verilog-XL ReferenceIntroduction

The basic building block of the Verilog-XL HDL is the module. The module format facilitatestop-down and bottom-up design. A module contains a model of a design or part of a design.Modules can incorporate other modules to establish a model hierarchy that describes howparts of a design are incorporated in an entire design. The constructs of the Verilog HDL,such as its declarations and statements, are enclosed in modules.

The Verilog HDL behavioral language is structured and procedural like the C programminglanguage. The behavioral language constructs are for algorithmic and RTL models. Thebehavioral language provides the following capabilities:

structured procedures for sequential or concurrent execution

explicit control of the time of procedure activation specified by both delay expressionsand by value changes called event expressions

explicitly named events to trigger the enabling and disabling of actions in otherprocedures

procedural constructs for conditional, if-else, case, and looping operations

procedures called tasks that can have parameters and non-zero time duration

procedures called functions that allow the definition of new operators

arithmetic, logical, bit-wise, and reduction operators for expressions

The Verilog HDL structural language constructs are for gate-level and switch-level models.The structural language provides the following capabilities:

a complete set of combinational primitives

primitives for bidirectional pass and resistive devices

the ability to model dynamic MOS models with charge sharing and charge decay

Verilog structural language models can accurately model signal contention. In the VerilogHDL, structural modeling accuracy is enhanced by primitive delay and output strengthspecification. Signal values can have different strengths and a full range of ambiguous valuesto reduce the pessimism of unknown conditions.

gate-level a model that describes the logic gates and the connections betweenlogic gates in a design

switch-level a model that describes the transistors and storage nodes in a deviceand the connections between them



The Verilog-XL Logic Simulator

The Verilog-XL digital logic simulator is a software tool that allows you to perform the followingtasks in the design process without building a hardware prototype:

determine the feasibility of new design ideas

try more than one approach to a design problem

verify functionality

identify design errors

To use Verilog-XL, you develop models that describe your design and its environment in theVerilog HDL and then supply Verilog-XL with the file names that contain these models. Youalso need a Verilog-XL license. This section describes the major features of Verilog-XL andthe Verilog-XL license.

Major Features of Verilog-XL

Verilog-XL provides you with the following simulation capabilities:

setting break points during simulation that stops the simulation and allows you to enteran interactive mode to examine and debug your design

displaying information about the current state of the design and to specifying the formatof that information

applying stimulus during simulation

patching circuits during simulation

tracing the execution flow of the statements in your model

traversing the model hierarchy to various regions of your design to examine the state ofthe simulation in that region

stepping through the statements of a design and executing them one at a time

displaying the active statements in a design

displaying and disabling the operations you entered in interactive mode

reading data from a file and writing data to that file

saving the current state of a simulation in a file and restoring that simulation at anothertime



investigating the performance ramifications of architectural decisionstochasticmodeling

simulating with SimVision, the Verilog-XL graphical user interface

Verilog-XL Licenses

To use the Verilog-XL logic simulator you need a license. The SoftShare application handlesall licenses for Verilog-XL and provides a variety of license management tools and options.

When you invoke Verilog-XL, SoftShare searches for a license file and checks out a licensefor you if one is available. You can queue a request for a license if one is not currentlyavailable. When you queue license requests, you can automatically run a number ofsimulations as licenses become available. Requests in the queue are first-in-first-out with allrequests at the same priority level. There is no time-out on queues, meaning that you cannotwait for a license for a fixed time. To remove a request from the queue, you must provide aninterrupt signal.

The following license features can be queued:

VERILOG-XL (Verilog-XL)

VXL-LMC-HW-IF (Verilog-XL LMC Hardware Interface). This feature is checked outduring compilation whenever there is a LMSI (LMC Hardware Interface) system task,$lm_*(), present in the design.

To enable the queuing, use the following command-line plus options:

+licq_vxl (queue only the VERILOG-XL license)

+licq_lmchwif (queue only the VXL-LMC-HW-IF license)

+licq_all (queue all of the above licenses)



2Lexical Conventions

This chapter describes the following:

Overview on page 22

Operators on page 22

White Space and Comments on page 23

Numbers on page 23

Strings on page 25

Identifiers, Keywords, and System Names on page 27

Text Substitutions on page 28

Overview

Verilog language source text files are a stream of lexical tokens. A token consists of one ormore characters, and each single character is in exactly one token. The layout of tokens in asource file is free format that is, spaces and newlines are not syntactically significant.However, spaces and newlines are very important for giving a visible structure and format tosource descriptions. A good style of format, and consistency in that style, are an essentialpart of program readability.

This manual uses a syntax formalism based on the Backus-Naur Form (BNF) to define theVerilog language syntax. Appendix A, Formal Syntax Definition contains the complete setof syntax definitions in this format, plus a description of the BNF conventions used in thesyntax definitions.

Operators

Operators are single, double, or triple character sequences and are used in expressions.Chapter 4, Expressions, discusses the use of operators in expressions.


Verilog-XL ReferenceLexical Conventions

Unary operators appear to the left of their operand. Binary operators appear between theiroperands. A ternary operator has two operator characters that separate three operands. TheVerilog language has one ternary operator theconditional operator. See ConditionalOperator on page 60 for an explanation of the conditional operator.

White Space and Comments

White space can contain the characters for blanks, tabs, newlines, and formfeeds. The Veriloglanguage ignores these characters except when they serve to separate other tokens.However, blanks and tabs are significant in strings.

The Verilog language has two forms to introduce comments. A one-line comment starts withthe two characters // and ends with a newline. A block comment starts with /* and ends with*/. Block comments cannot be nested, but a one-line comment can be nested within a blockcomment.

Numbers

You can specify constant numbers in decimal, hexadecimal, octal, or binary format. TheVerilog language defines two forms to express numbers. The first form is a simple decimalnumber specified as a sequence of the digits 0 to 9 which can optionally start with a plus orminus. The second takes the following form:

The element contains decimal digits that specify the size of the constant in terms ofits exact number of bits. For example, the specification for two hexadecimal digits is8, because one hexadecimal digit requires four bits. The specification is optional.The contains a letter specifying the numbers base, preceded by thesingle quote character (). Legal base specifications are one of d, h, o, or b, for the basesdecimal, hexadecimal, octal, and binary respectively. (Note that these base identifiers can beupper- or lowercase.)

The element contains digits that are legal for the specified .The element must physically follow the , but can be separatedfrom it by spaces. No spaces can separate the single quote and the base specifier character.

Alphabetic letters used to express the or the hexadecimal digits a to f canbe in upper- or lowercase.

The following example shows unsized constant numbers.659 // is a decimal numberh 837FF // is a hexadecimal number



o7460 // is an octal number4af // is illegal (hexadecimal format requires h)

The following example shows sized constant numbers4b1001 // is a 4-bit binary number5 D 3 // is a 5-bit decimal number3b01x // is a 3-bit number with the least significant bit unknown12hx // is a 12-bit unknown number16hz // is a 16-bit high-impedance number

In the Verilog language a plus or minus preceding the size constant is a sign for the constantnumberthe size constant does not take a sign. A plus or minus between the and the is illegal syntax. In the following example, the firstexpression is a syntax error. The second expression legally defines an 8-bit number with avalue of minus 6.

8 d -6 // this is illegal syntax-8 d 6 // this defines the twos complement of 6,

// held in 8 bitsequivalent to -(8d 6)

The number of bits that make up an un-sized number (which is a simple decimal number ora number without the specification) is the host machine word sizefor mostmachines this is 32 bits.

In the Verilog language, an x expresses the unknown value in hexadecimal, octal, and binaryconstants. A z expresses the high-impedance value. See Value Set on page 30 for adiscussion of the Verilog value set. An x sets four bits to unknown in the hexadecimal base,three bits in the octal base, and one bit in the binary base.

Similarly, a z sets four, three, and one bit, respectively, to the high-impedance value. If themost significant specified digit of a constant number is an x or a z, then Verilog-XLautomatically extends the x or z to fill the higher order bits of the constant. This makes it easyto specify complete vectors of the unknown and the high-impedance values. The followingexample illustrates this value extension:

reg [11:0] a;initialbegin a = h x; // yields xxx a = h 3x; // yields 03x a = h 0x; // yields 00xend

The question mark (?) character is a Verilog HDL alternative for the z character. It sets fourbits to the high-impedance value in hexadecimal numbers, three in octal, and one in binary.Use the question mark to enhance readability in cases where the high-impedance value is adont-care condition. See the discussion of casez and casex in case Statements onpage 174 and the discussion on personality files in Logic Array Personality Formats onpage 396 .



The underline character is legal anywhere in a number except as the first character. Use thisfeature to break up long numbers for readability purposes. The following example illustratesthis.

27_195_00016b0011_0101_0001_111132 h 12ab_f001

Underline characters are also legal in numbers in text files read by the $readmemb and$readmemh system tasks.

Note: A sized negative number is not sign-extended when assigned to a register data type.

Strings

A string is a sequence of characters enclosed by double quotes and all contained on a singleline. Verilog treats strings used as operands in expressions and assignments as a sequenceof eight-bit ASCII values, with one eight-bit ASCII value representing one character. Thefollowing shows examples of strings:

"this is a string""print out a message\n""bell!\007"

String Variable Declaration

To declare a variable to store a string, declare a register large enough to hold the maximumnumber of characters the variable will hold. Note that no extra bits are required to hold atermination character; Verilog does not store a string termination character.

For example, to store the string Hello world! requires a register 8*12, or 96 bits wide,as follows:

reg [8*12:1] stringvar;initialbegin

stringvar = Hello world!;end

String Manipulation

Verilog permits strings to be manipulated using the standard Verilog HDL operators. Keep inmind that the value being manipulated by an operator is a sequence of 8-bit ASCII values,with no special termination character.



The code in the following example declares a string variable large enough to hold 14characters and assigns a value to it. The code then manipulates this string value using theconcatenation operator.

module string_test;reg [8*14:1] stringvar;

initialbeginstringvar = Hello world;

$display(%s is stored as %h,stringvar,stringvar);stringvar = {stringvar,!!!};$display(%s is stored as %h,stringvar,stringvar);

endendmodule

Note: When a variable is larger than required to hold a value being assigned, Verilog padsthe contents on the left with zeros after the assignment. This is consistent with the paddingthat occurs during assignment of non-string values.

The following strings display as the result of executing Verilog-XL in the previous example:

Hello world is stored as 00000048656c6c6f20776f726c64Hello world!!! is stored as 48656c6c6f20776f726c64212121

Special Characters in Strings

Certain characters can only be used in strings when preceded by an introductory charactercalled an escape character. The following table lists these characters in the right-handcolumn with the escape sequence that represents the character in the left-hand column.

Specifying special characters in strings

Escape String Character Produced by Escape String

\n new line character

\t tab character

\\ slash (\) character

\ double quote () character

\ddd a character specified in 1-3 octal digits (0


Identifiers, Keywords, and System Names

An identifier is used to give an object, such as a register or a module, a name so that it canbe referenced from other places in a description. An identifier is any sequence of letters,digits, dollar signs ($), and the underscore (_) symbol.

Important

The first character must not be a digit or $; it can be a letter or an underscore.

Upper- and lowercase letters are considered to be different (unless the uppercase option isused when compiling). Identifiers can be up to 1024 characters long. Examples of identifiersfollow:

shiftreg_abusa_indexerror_conditionmerge_ab_bus3n$657

Escaped Identifiers

Escaped identifiers start with the backslash character (\) and provide a means of includingany of the printable ASCII characters in an identifier (the decimal values 33 through 126, or21 through 7E in hexadecimal). An escaped identifier ends with white space (blank, tab,newline). Note that this also applies when using bit- or part-selects on the escaped identifier,in which case the bit- or part- select operator must be preceded by a space.

Neither the leading backslash character nor the terminating white space is considered to bepart of the identifier.

The primary application of escaped identifiers is for translators from other hardwaredescription languages and CAE systems, where special characters may be allowed inidentifiers; do not use escaped identifiers under normal circumstances.Examples of escaped identifiers follow:

\busa+index\-clock\***error-condition***\net1/\net2\{a,b}\a*(b+c)\p1$i14/data [2]

Note: Remember to terminate escaped identifiers with white space, otherwise charactersthat should follow the identifier are considered as part of it.



Keywords

Keywords are predefined non-escaped identifiers that are used to define the languageconstructs. A Verilog HDL keyword preceded by an escape character is not interpreted as akeyword.

All keywords are defined in lowercase only and therefore must be typed in lowercase insource files (unless the -u uppercase option is used when compiling).

See Appendix B, Verilog-XL Keywords, for a complete list of Verilog-XL keywords.

Text Substitutions

You can define a text macro name, assign a value to it, and use the name repetitivelythroughout your design. Verilog-XL substitutes the assigned value whenever it encounters thetext macro name. To change the value of the text macro throughout the design, you need onlymodify the definition statement. Text macros are espcially useful for constant values.

You can also define and use text macros in the interactive mode. For example, you can assignthe value of often-used interactive commands to a text macro.

The syntax for text macro definitions is as follows:

::= define

::=

The syntax for using a text macro is as follows:

::=

The accent grave (), also called tick, must precede the text macro name.

You can reuse names that are used as identifiers elsewhere. For example, signal_name andsignal_name are different.

Do not use compiler directive keywords as text macro names. For example, definedefine and define accelerate are illegal because define and accelerate arecompiler directives.

The value for is any text specified on the same line as the. A one-line comment (specified with the characters //) does notbecome part of the text substituted. The text for can be blank, in which casethe text macro is defined to be empty and no text is substituted when the macro is used.



Once you define a text macro name, you can use it anywhere in a source description or in aninteractive command; there are no scope restrictions.

The folllowing example shows how to define and use two text macros called wordsize andtyp_nand. The macro wordsize has a value of 8. The macro typ_nand has a value of nand#5.

define wordsize 8 // assign a value of 8 to the wordsize macroreg [1:wordsize] data;// translates to reg [1:8] data;

define typ_nand nand #5 // define a nand gate with typical delaytyp_nand g121 (q21, n10, n11);// translates to nand #5 g121 (q21, n10, n11);

Do not split the text specified for across the following lexical tokens:

comments

numbers

strings

identifiers

keywords

double or triple character operators

For example, the following is illegal syntax in the Verilog language because it is split acrossa string:

define first_half "start of string$display(first_half end of string"); // illegal syntax

You can redefine text macros; the latest definition of a particular text macro read by thecompiler prevails when the macro name is encountered in the source text.



3Data Types

This chapter describes the following information:

Overview on page 30

Value Set on page 30

Registers and Nets on page 31

Vectors on page 36

Strengths on page 37

Implicit Declarations on page 38

Net Initialization on page 39

Net Types on page 39

Memories on page 45

Integers and Times on page 46

Real Numbers on page 47

Parameters on page 49

Overview

The set of Verilog HDL data types is designed to represent the data storage and transmissionelements found in digital hardware.

Value Set

The Verilog HDL value set consists of four basic values:

0 represents a logic zero, or false condition


Verilog-XL ReferenceData Types

1 represents a logic one, or true condition

x represents an unknown logic value

z represents a high-impedance state

The values 0 and 1 are logical complements of one another.

When the z value is present at the input of a gate, or when it is encountered in an expression,the effect is usually the same as an x value. Notable exceptions are the MOS primitives,which can pass the z value.

Almost all of the data types in the Verilog language store all four basic values. The exceptionsare the event data type, (which has no storage), and the trireg net data type, (whichretains its first state when all of its drivers go to the high-impedance value), and z. All bits ofvectors can be independently set to one of the four basic values.

The language includes strength information in addition to the basic value information forscalar net variables. This is described in detail in Chapter 6, Gate and Switch LevelModeling,.

Registers and Nets

There are two main groups of data types: the register data types and the net data types.These two groups differ in the way that they are assigned and hold values. They alsorepresent different hardware structures.

Nets

The net data types represent physical connections between structural entities, such asgates. A net does not store a value (except for the trireg net, discussed in trireg Net onpage 40 ). Instead, it must be driven by a driver, such as a gate or a continuous assignment.See Chapter 6, Gate and Switch Level Modeling, and Chapter 5, Assignments, fordefinitions of these constructs. If no driver is connected to a net, its value will be high-impedance (z)unless the net is a trireg.

Registers

A register is an abstraction of a data storage element. The keyword for the register data typeis reg. A register stores a value from one assignment to the next. An assignment statementin a procedure acts as a trigger that changes the value in the register. The Verilog languagehas powerful constructs that allow you to control when and if these assignment statements



are executed. Use these control constructs to describe hardware trigger conditions, such asthe rising edge of a clock, and decision-making logic, such as a multiplexer. Chapter 8,Behavioral Modeling describes these control constructs.

The default initialization value for a reg data type is the unknown value, x.

Caution

Registers can be assigned negative values, but, when a register is anoperand in an expression, its value is treated as an unsigned (positive)value. For example, a minus one in a four-bit register functions as thenumber 15 if the register is an expression operand. See NumericConventions in Expressions on page 53 for more information onnumeric conventions in expressions.

Signed Objects

You can type any object as signed (except for user system functions) using the signedkeyword in a type declaration (see Net and Register Declaration Syntax on page 34 ). Thevalue of signed quantities are represented with twos complement notation. A signed valuewill not cross hierarchical boundaries. If you want a signed value in other modules in ahierarchy, you must declare them in each of the modules where signed arithmetic isnecessary. The following example shows some sample declarations.

wire signed [3:0] signed_wire; // range -8 +7reg signed [3:0] signed_reg; // range -8 +7reg signed [3:0] signed_mem [99:0] // 100 words range -8 +7function signed [3:0] signed_func; // range -8 +7

You can type a based constant by prepending the letter s to the base type as shown in thefollowing example.

module test;

reg signed [3:0] sig_reg;reg [3:0] unsig_reg;

initialbegin

$monitor($time,,"sig_reg=%d unsig_reg=%d (-4d1)=%d (-4sd1)=%d",sig_reg, unsig_reg, -4d1, -4sd1);

#0 sig_reg = -4d1;unsig_reg = -4d1;

#10 sig_reg = -4sd1;unsig_reg = -4sd1;

end

endmodule

The output would be as follows:



0 sig_reg= -1 unsig_reg=15 (-4d1)=15 (-4sd1)= -1

The following rules determine the resulting type of an expression:

The expression type depends only on the operands. It does not depend on the left-handside (LHS) (if any).

Decimal numbers are signed.

If any operand is real, the result is real.

If all operands are signed, the result is signed, regardless of operator.

The following list shows objects that are unsigned regardless of the operands:

The result of any expression where any operand is unsigned

Based numbers

Comparison results (1, 0)

Bit select results

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