Stratified event scheduler

A compliant SystemVerilog simulator shall maintain some form of data structure that allows events to be dynamically scheduled, executed, and removed as the simulator advances through time. The data structure is normally implemented as a time-ordered set of linked lists, which are divided and subdivided in a well-defined manner.

The first division is by time. Every event has one and only one simulation execution time, which at any given point during simulation can be the current time or some future time. All scheduled events at a specific time define a time slot. Simulation proceeds by executing and removing all events in the current simulation time slot before moving on to the next nonempty time slot, in time order. This procedure guarantees that the simulator never goes backwards in time.

A time slot is divided into a set of ordered regions, as follows:

a) Preponed b) Pre-Active c) Active d) Inactive

e) Pre-NBA

f) NBA

g) Post-NBA

h) Pre-Observed i) Observed j) Post-Observed k) Reactive l) Re-Inactive m) Pre-Re-NBA

n) Re-NBA

o) Post-Re-NBA p) Pre-Postponed q) Postponed

The purpose of dividing a time slot into these ordered regions is to provide predictable interactions between the design and testbench code.

NOTE—These regions essentially encompass the IEEE 1364-2005 reference model for simulation, with exactly the same level of determinism. In other words, legacy Verilog code should continue to run correctly without modification within the SystemVerilog mechanism.

4.4.1 Active region sets and reactive region sets

There are two important groupings of event regions that are used to help define the scheduling of SystemVerilog activity, the active region set and the reactive region set. Events scheduled in the Active, Inactive, Pre-NBA, NBA, and Post-NBA regions are active region set events. Events scheduled in the Reactive, Re-Inactive, Pre- Re-NBA, Re-NBA, and Post-Re-NBA regions are reactive region set events.

The Active, Inactive, Pre-NBA, NBA, Post-NBA, Pre-Observed, Observed, Post- Observed, Reactive, Re-Inactive, Pre-Re-NBA, Re-NBA, Post-Re-NBA, and Pre-Postponed regions are known as the iterative regions.

In addition to the active region set and reactive region set, all of the event regions of each time slot can be categorized as simulation regions (see 4.4.2) or PLI regions (see 4.4.3).

4.4.2 Simulation regions

The simulation regions of a time slot are the Preponed, Active, Inactive, NBA, Observed, Reactive, Re-Inactive, Re-NBA and Postponed regions. The flow of execution of the event regions is specified in Figure 4-1.

4.4.2.1 Preponed events region

The #1step sampling delay provides the ability to sample data immediately before entering the current time slot. #1step sampling is identical to taking the data samples in the Preponed region of the current time slot.

Sampling in the Preponed region is equivalent to sampling in the previous Postponed region.

Preponed region PLI events are also scheduled in this region (see 4.4.3.1).

4.4.2.2 Active events region

The Active region holds the current active region set events being evaluated and can be processed in any order.

4.4.2.3 Inactive events region

The Inactive region holds the events to be evaluated after all the Active events are processed.

If events are being executed in the active region set, an explicit #0 delay control requires the process to be suspended and an event to be scheduled into the Inactive region of the current time slot so that the process can be resumed in the next Inactive to Active iteration.

4.4.2.4 NBA events region

The NBA (nonblocking assignment update) region holds the events to be evaluated after all the Inactive events are processed.

If events are being executed in the active region set, a nonblocking assignment creates an event in the NBA region scheduled for the current or a later simulation time.

VERIFICATION LANGUAGE

4.4.2.5 Observed events region

The Observed region is for evaluation of property expressions when they are triggered. During property evaluation, pass/fail code shall be scheduled in the Reactive region of the current time slot. PLI callbacks are not allowed in the Observed region.

4.4.2.6 Reactive events region

The Reactive region holds the current reactive region set events being evaluated and can be processed in any order.

The code specified by blocking assignments in program blocks and the code in action blocks of concurrent assertions are scheduled in the Reactive region. The Reactive region is the reactive region set dual of the Active region (see 4.4.2.2).

4.4.2.7 Re-Inactive events region

The Re-Inactive region holds the events to be evaluated after all the Reactive events are processed.

If events are being executed in the reactive region set, an explicit #0 delay control requires the process to be suspended and an event to be scheduled into the Re-Inactive region of the current time slot so that the process can be resumed in the next Re-Inactive to Reactive iteration. The Re-Inactive region is the reactive region set dual of the Inactive region (see 4.4.2.3).

4.4.2.8 Re-NBA events region

The Re-NBA region holds the events to be evaluated after all the Re-Inactive events are processed.

If events are being executed in the reactive region set, a nonblocking assignment creates an event in the Re-NBA update region scheduled for the current or a later simulation time. The Re-Re-NBA region is the reactive region set dual of the NBA region (see 4.4.2.4).

4.4.2.9 Postponed events region

$monitor, $strobe, and other similar events are scheduled in the Postponed region.

No new value changes are allowed to happen in the current time slot once the Postponed region is reached.

Within this region, it is illegal to write values to any net or variable or to schedule an event in any previous region within the current time slot.

Postponed region PLI events are also scheduled in this region (see 4.4.3.10).

4.4.3 PLI regions

In addition to the simulation regions, where PLI callbacks can be scheduled, there are additional PLI-specific regions. The PLI regions of a time slot are the Preponed, Pre-Active, Pre-NBA, Post-NBA, Pre-Observed, Post-Observed, Pre-Re-NBA, Post-Re-NBA and Pre-Postponed regions. The flow of execution of the PLI regions is specified in Figure 4-1.

4.4.3.1 Preponed PLI region

The Preponed region provides for a PLI callback control point that allows PLI application routines to access data at the current time slot before any net or variable has changed state. Within this region, it is illegal to write values to any net or variable or to schedule an event in any other region within the current time slot.

NOTE—The PLI currently does not schedule callbacks in the Preponed region.

4.4.3.2 Pre-Active PLI region

The Pre-Active region provides for a PLI callback control point that allows PLI application routines to read and write values and create events before events in the Active region are evaluated (see 4.5).

4.4.3.3 Pre-NBA PLI region

The Pre-NBA region provides for a PLI callback control point that allows PLI application routines to read and write values and create events before the events in the NBA region are evaluated (see 4.5).

4.4.3.4 Post-NBA PLI region

The Post-NBA region provides for a PLI callback control point that allows PLI application routines to read and write values and create events after the events in the NBA region are evaluated (see 4.5).

4.4.3.5 Pre-Observed PLI region

The Pre-Observed region provides for a PLI callback control point that allows PLI application routines to read values after the active region set has stabilized. Within this region, it is illegal to write values to any net or variable or to schedule an event within the current time slot.

4.4.3.6 Post-Observed PLI region

The Post-Observed region provides for a PLI callback control point that allows PLI application routines to read values after properties are evaluated (in the Observed or an earlier region).

NOTE—The PLI currently does not schedule callbacks in the Post-Observed region.

4.4.3.7 Pre-Re-NBA PLI region

The Pre-Re-NBA region provides for a PLI callback control point that allows PLI application routines to read and write values and create events before the events in the Re-NBA region are evaluated (see 4.5).

4.4.3.8 Post-Re-NBA PLI region

The Post-Re-NBA region provides for a PLI callback control point that allows PLI application routines to read and write values and create events after the events in the Re- NBA region are evaluated (see 4.5).

4.4.3.9 Pre-Postponed PLI region

The Pre-Postponed region provides a PLI callback control point that allows PLI application routines to read and write values and create events after processing all other regions except the Postponed region.

4.4.3.10 Postponed PLI region

The Postponed region provides a PLI callback control point that allows PLI application routines to create read-only events after processing all other regions. PLI cbReadOnlySynch and other similar events are scheduled in the Postponed region.

The SystemVerilog flow of time slots and event regions is shown in Figure 4-1.

VERIFICATION LANGUAGE

Figure 4-1—Event scheduling regions Preponed

Pre-Active

Active

Inactive

Pre-NBA NBA Post-NBA

Postponed time slot from previous

time slot

to next time slot region

PLI region Legend:

Pre-Postponed Reactive

Re-Inactive

Pre-Re-NBA Re-NBA Post-Re-NBA Pre-Observed Observed Post-Observed