Section 17.7.3, page 246 (IEEE Std 1800™-2005)

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The use of these functions is not limited to assertion features; they can be used as expressions in procedural

code as well. The clocking event, although optional as an explicit argument to the functions, is required for

their semantics. The clocking event is used to sample the value of the argument expression.

With

The use of these functions is not limited to assertion features; they can be used as expressions in procedural code as well. The clocking event, although optional as an explicit argument to the functions $past, $rose, $stable and $fell, is required for their semantics. The clocking event is used to sample the value of the argument expression.

The function $sampled does not use a clocking event, although one can be optionally provided. The optional clocking event is ignored, and its use is deprecated. The value of $sampled is updated in the preponed scheduling region.

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Function $sampled returns the sampled value of the expression with respect to the last occurrence of the clocking event. When $sampled is invoked prior to the occurrence of the first clocking event, the value of X is returned. The use of $sampled in assertions, although allowed, is redundant, as the result of the function is identical to the sampled value of the expression itself used in the assertion.

With

The $sampled function returns the value of the expression sampled in the preponed region of the simulation time step in which the function is called. No clocking event is ever inferred. The value is stable throughout the simulation step.

When $sampled is invoked prior to the occurrence of the first clocking event, the value of X is returned. The sampled value in the preponed region at time 0 of an expression is the default initial value of the expression type. For example, if $sampled(y) is called at time 0, and y is of type logic, the value returned is X.

The use of $sampled in assertions, although allowed, is redundant, as the result of the function is identical to the sampled value of the expression itself used in the assertion.

Page 247

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When these functions are called at or before the first clock tick of the clocking event, the results are computed by comparing the current sampled value of the expression to X.

With

When these functions are called at or before the first clock tick of the clocking event, the results are computed by comparing the current sampled value of the expression to X the default initial value of the type of the argument.

Page 248

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A clock tick is based on clocking_event. If the specified clock tick in the past is before the start of simulation, the returned value from the $past function is a value of X.

With

A clock tick is based on clocking_event. If the specified clock tick in the past is before the start of simulation, the returned value from the $past function is a value of X the default initial value of the type of the argument.