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SystemVerilog Tutorial PART19: by Abhiram Rao

Assertions in SystemVerilog - Part2


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Boolean expressions

The expressions used in sequences are evaluated over sampled values of the variables that appear in the expressions. The outcome of the evaluation of an expression is Boolean and is interpreted the same way as an expression is interpreted in the condition of a procedural if statement. That is, if the expression evaluates to X, Z, or 0, then it is interpreted as being false. Otherwise, it is true. There are certain restrictions on the expressions that can appear in concurrent assertions. The restrictions on operand types, variables, and operators are specified in the following sections.

Expressions are allowed to include function calls, but certain semantic restrictions are imposed.

  • Functions that appear in expressions cannot contain output or ref arguments (const ref are allowed).

  • Functions should be automatic (or preserve no state information) and have no side effects.

 Operand types

The following types are not allowed:

  • non-integer types (shortreal, real and realtime)

  • string

  • event

  • chandle

  • class

  • associative arrays

  • dynamic arrays

Fixed size arrays, packed or unpacked, can be used as a whole or as part selects or as indexed bit or part selects. The indices can be constants, parameters, or variables. The following example shows some possible forms of comparison of members of structures and unions:

                            typedef int [4] array;

                            typedef struct { int a, b, c, d } record;

                            union { record r; array a; } p, q;

The following comparisons are legal in expressions:

                            p.a == q.a


                            p.r == q.r

The following example provides further illustration of the use of arrays in expressions.

logic [7:0] arrayA [0:15], arrayB[0:15];

The following comparisons are legal:

arrayA == arrayB;

arrayA != arrayB;

arrayA[i] >= arrayB[j];

arrayB[i][j+:2] == arrayA[k][m-:2];

(arrayA[i] & (~arrayB[j])) == 0;



The variables that can appear in expressions must be static design variables or function calls returning values of types described in in the above section. Static variables declared in programs, interfaces or clocking blocks can also be accessed. If a reference is to a static variable declared in a task, that variable is sampled as any other variable, independent of calls to the task.


All operators that are valid for the types described in the above section are allowed with the exception of assignment operators and increment and decrement operators. SystemVerilog includes the C assignment operators, such as +=, and the C increment and decrement operators, ++ and --. These operators cannot be used in expressions that appear in assertions. This restriction prevents side effects.

Sequential Expressions

Having defined the proper sampling semantics for signals in assertions, SystemVerilog also includes the ability to specify sequential expressions or sequences of Boolean expressions with clear temporal relationships between them. To determine a match of the sequence, the Boolean expressions are evaluated at each successive sample point, defined by a clock that gets associated with the sequence.

If all expressions are true, then a match of the sequence occurs. The most basic sequential expression is something like “a followed by b on the next clock” which is represented in SystemVerilog as

a ##1 b

In this example, the “##1” indicates a one-clock delay between successive Boolean expressions in the sequence.

Notice the similarity between the “##” cycle delay operator and the “#” delay operator, which specifies a number of time units to delay. This natural extension of an existing Verilog construct preserves the notion of delay, as Verilog users are used to, and is used elsewhere in SystemVerilog to indicate an explicit clockcycle delay. This is but one instance where including the definition of assertions and sequences as part of SystemVerilog allows for common concepts to use the same syntax throughout different areas of the language.

It is important to understand that in SystemVerilog, each element of a sequence may be either a Boolean expression or another sequence. In terms of sequences, a Boolean expression is simply the degenerate case of a sequence of length 1. Thus, the expression

s1 ##1 s2

means that sequence s2 begins on the clock after sequence s1 ends.

##1 s2                               
                         Figure: Sequence concatenation: Sequence s2 starts on the cycle after s1 sequence

                                               s1 ##0 s2 
                          Figure: Sequence Overlap: Sequence s2 starts on the same cycle at which s1 finishes

While sequential expressions are useful for specifying temporal relationships between expressions, it is important to be able to capture sequential expressions as elements in the language so that they can be reused and referenced. SystemVerilog thus introduces the notion of a sequence which can be declared and reused elsewhere in the language, either to build other sequential expressions or, as we shall see, as part of properties to be asserted.

Sequences are declared in SystemVerilog using the following syntax:

sequence_declaration ::=

sequence name [ ( formal_item {, formal_item } ) ] ;

{ assertion_variable_declaration }

sequence_spec ;


The optional list of formal arguments allows for specification of sequences as a generic temporal relationship that is applied to the actual arguments passed in when the sequence is instantiated. For example, the sequence

sequence seq1 (a, b);

        a ##2 b;


represents a sequence of two expressions. When this sequence is instantiated as


the actual arguments e and f are substituted for the formal arguments (a and b, respectively) in the sequence definition, so that the temporal relationship between e and f is

e ##2 f



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