1. What is callback ?

Callback is one of the major

confusing point for a System Verilog learner. Many people have asked the same question in many forums,

but the answer doesn't seems to satisfy fully the quest of the person who has raised the querry. I too had

the same issue, but I learned it slowly in a hard way. I am presenting here a way in which if I had an

answer, I would have learned faster.

We can pass data member to any function. Now consider a case where you are passing a function (say

func1) as a data member to another function (say func2) and you get what is called callback. The reason

why it is called callback is that the function func2 now can call anywhere in its code function func1.

From wikipedia

In computer programming, a callback is executable code that is passed as an argument to

other code. It allows a lower-level software layer to call a subroutine (or function) defined

in a higher-level layer.

Note that SV doesn't give a straight-forward way of passing a function as argument for another function.

But we can get the same result (almost we can say!) by using OOP. The idea is to describe all the

functions (both func1 type and func2 type) in a base class (don't implement the funct2 kind of function

and make them virtual for polymorphism), and then extend the class to a derived class where you

implement the func2 type of function.

Example:-

class abc_transactor;

virtual task pre_send(); endtask

virtual task post_send(); endtask

task xyz();

// Some code here

this.pre_send();

// Some more code here

this.post_send();

// And some more code here

endtask : xyz

endclass : abc_transactor

class my_abc_transactor extend abc_transactor;

virtual task pre_send();

... // This function is implemented here

endtask

virtual task post_send();

... // This function is implemented here

endtask

endclass : my_abc_transactor

Now let me explain how it is going to work. The base class abc_transactor has 3 tasks, 2 of which are

declared virtual and are not implemented. But they are being called from another task xyz() which is fully

implemented. The unimplemented virtual task are called callback class.

The child class, which extends from the base class, implements the previous unimplemented tasks. It

inherits the xyz() task from the base class and hence doesn't need to change it. By this we can inject

executable code to a function without modifying it.

Now the next question is why is done. There are many reasons for it.

1. The biggest advantage is that you can modify the behavior of task xyz() without modifying it in the

base or child class. It is a big advantage as no one wants to fiddle with known good functioning code.

2. Consider a case where you are writing a base class which is going to be used by multiple test

environment, and for each test environment a known part of the code, or a known function/task is

going to change. The natural choice is to implement those change-in-every-case functions/tasks as

callback method and let the user extend your base class with specifying only that part of the code

which need to be changed in his case.

Simple callback using the above approach does have some known limitations, which can be solved

using design patterns (from OOP land), the details of which can be found at Janik's article of

vmm_callback.

Suppose if you are doing transmitting data using mailboxes, Once you are send ing

data to design from genetarator . The same data you need to put back in score

board for latter comparison. This is called callbacks

2. What is factory pattern ?

Factory pattern as the name suggest, is aimed at solving the issue of creation of object. (Factory pattern

is not the only pattern to deal with creation of objects, there are a bunch of more patterns for handling

different kind of cases, and collectively they are known a creational patterns)

Let me give an example of case where we might need to use creational pattern and how to do so it in SV.

Suppose you want to create a "Toy Factory" class which needs to create multiple types of toys (say toy

aeroplane, toy tank, toy bus) depending upon the string input to it.

To create these different types of toys we need to have class defined for them. And there will be common

method and data interface for these classes, hence it make sense to put all the common data

member/task/functions in a class called toy class and then extend it.

class TOY;

// Common data memeber

string type;

// Common methods

virtual function string get_type();

endclass : TOY

class TOY_Tank extends TOY;

function new();

this.string = "Toy Tank";

endfunction : new

string function string get_type();

return this.string;

endfunction : get_type

endclass : TOY_Tank

class TOY_Bus extends TOY;

function new();

this.string = "Toy Bus";

endfunction : new

string function string get_type();

return this.string;

endfunction : get_type

endclass : TOY_Bus

Now we are done with the bothering about the objects to be created. The next problem that we need to solve is to write the toy factory class itself. For simplicity, let's consider the case where we will want to pass 1 to get an instance of tank class and 2 for getting an instance of bus class from the factory. Now the factory class will look like this.

class TOY_factory;

Toy my_toy

// Common methods

function toy get_toy(int type);

if(type == 1) this.my_toy = new TOY_Tank();

if(type == 2) this.my_toy = new TOY_Bus();

return this.my_toy;

endfunction : get_toy

endclass : TOY_factory

Note that we are using virtual function for bringing polymorphism in action and save us from having an

individual instance of the toy type in the factory class.

3. Explain the difference between data types logic and reg and wire

Wire:-

1. Wires are used for connecting different elements

2. They can be treated as a physical wire

3. They can be read or assigned

4. No values get stored in them

5. They need to be driven by either continuous assign statement or from a port of a module Reg:-

1. Contrary to their name, regs doesn't necessarily corresponds to physical registers

2. They represents data storage elements in Verilog/SystemVerilog

3. They retain their value till next value is assigned to them (not through assign statement)

4. They can be synthesized to FF, latch or combinational circuit (They might not be synthesizable

!!!) Wires and Regs are present from Verilog timeframe. SystemVerilog added a new data type called logic to them. So the next question is what is this logic data type and how it is different from our good old wire/reg. Logic:-

1. As we have seen, reg data type is bit mis-leading in Verilog. SystemVerilog's logic data type

addition is to remove the above confusion. The idea behind having a new data type called logic

which at least doesn't give an impression that it is hardware synthesizable

2. Logic data type doesn't permit multiple driver. It has a last assignment wins behavior in case of

multiple assignment (which implies it has no hardware equivalence). Reg/Wire data type give X if

multiple driver try to drive them with different value. Logic data type simply assign the last

assignment value.

3. The next difference between reg/wire and logic is that logic can be both driven by assign block,

output of a port and inside a procedural block like this

logic a;

assign a = b ^ c; // wire style

always (c or d) a = c + d; // reg style

MyModule module(.out(a), .in(xyz)); // wire style

4. What is the need of clocking blocks ?

In Verilog, the communication between blocks is specified using module ports. SystemVerilog adds the interface, a key construct that encapsulates the communication between blocks, thereby enabling users to easily change the level of abstraction at which the intermodule communication is to be modeled. An interface can specify the signals or nets through which a testbench communicates with a device under test (DUT). However, an interface does not explicitly specify any timing disciplines, synchronization requirements, or clocking paradigms. SystemVerilog adds the clocking block that identifies clock signals and captures the timing and synchronization requirements of the blocks being modeled. A clocking block assembles signals that are synchronous to a particular clock and makes their timing explicit. The clocking block is a key element in a cycle-based methodology, which enables users to write testbenches at a higher level of abstraction. Rather than focusing on signals and transitions in time, the test can be defined in terms of cycles and transactions. Depending on the environment, a testbench can contain one or more clocking blocks, each containing its own clock plus an arbitrary number of signals. The clocking block separates the timing and synchronization details from the structural, functional, and procedural elements of a testbench. Thus, the timing for sampling and driving clocking block signals is implicit and relative to the clocking block’s clock. This enables a set of key operations to be written very succinctly, without explicitly using clocks or specifying timing. These operations are as follows: — Synchronous events — Input sampling — Synchronous drives Clocking block in SystemVerilog are used for specifying the clock signal, timing, and synchronization

requirements of various blocks. It separates the timing related information from structural, functional and

procedural element of the TB. There are quite a few links on clocking block in the internet. These are

links to learn about SV clocking blocks.

- Specify synchronization characteristics of the design

- Offer a clean way to drive and sample signals

- Provides race-free operation if input skew > 0

- Helps in testbench driving the signals at the right time

- Features

- Clock specification

- Input skew,output skew

- Cycle delay (##)

- Can be declared inside interface,module or program

Example :

01.Module M1(ck, enin, din, enout, dout);

02.input ck,enin;

03.input [31:0] din ;

04.output enout ;

05.output [31:0] dout ;

06.

07.clocking sd @(posedge ck);

08.input #2ns ein,din ;

09.output #3ns enout, dout;

10.endclocking:sd

11.

12.reg [7:0] sab ;

13.initial begin

14.sab = sd.din[7:0];

15.end

16.endmodule:M1

5. What are the ways to avoid race condition between testbench and RTL

using SystemVerilog?

There are two major sources of nondeterminism in Verilog. The first one is that active events are processed in an arbitrary order. The second one is that statements without time control constructs in behavioral blocks do not execute as one event. However, from the testbench perspective, these effects are all unimportant details. The primary task of a testbench is to generate valid input stimulus for the design under test and to verify that the device operates correctly. Furthermore, testbenches that use cycle abstractions are only concerned with the stable or steady state of the system for both checking the current outputs and for computing stimuli for the next cycle. Formal tools also work in this fashion.

Statements within a program block that are sensitive to changes (e.g., update events) in design signals (declared in modules, not program blocks) are scheduled in the Reactive region. Consider a program block that contains the statement @(clk) S1; where clk is a design signal in some module. Every transition of signal clk will cause the statement S1

to be scheduled into the Reactive region. Likewise, initial blocks within program blocks are scheduled in the Reactive region; in contrast, initial blocks in modules are scheduled in the Active region. In addition, design signals driven from within the program must be assigned using nonblocking assignments and are updated in the NBA region. Thus, even signals driven with no delay are propagated into the design as one event. With this behavior, correct cycle semantics can be modeled without races, thereby making program-based testbenches compatible with clocked assertions and formal tools. Because the program schedules events in the Reactive region, the clocking block construct is very useful to automatically sample the steady-state values of previous time steps or clock cycles. Programs that read design values exclusively through clocking

blocks with #0 input skews are insensitive to read-write races. It is important to understand that simply sampling input signals (or setting nonzero skews on clocking

block inputs) does not eliminate the potential for races. Proper input sampling only addresses a single clocking block. With multiple clocks, the arbitrary order in which overlapping or simultaneous clocks are processed is still a potential source for races. The program construct addresses this issue by scheduling its execution in the Reactive region, after all design events have been processed, including clocks driven by nonblocking assignments.

6. Explain Event regions in SV.

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:

a) Preponed

b) Pre-active

c) Active

d) Inactive

e) Pre-NBA

f) NBA

g) Post-NBA

h) Observed

i) Post-observed

j) Reactive

k) Re-inactive

l) Pre-postponed

m) Postponed

The purpose of dividing a time slot into these ordered regions is to provide predictable

interactions between the design and testbench code.

Except for the Observed, Reactive, and Re-inactive regions and the Post-observed PLI

region, these regions essentially encompass IEEE 1364 reference model for simulation,

with exactly the same level of determinism. In other words, legacy Verilog code shall

continue to run correctly without modification within the new mechanism. The Postponed

region is where the monitoring of signals, and other similar events, takes place. No new

value changes are allowed to happen in the time slot once the Postponed region is

reached. The Observed, Reactive, and Re-inactive regions are new in this standard, and

events are only scheduled

into these new regions from new language constructs. The Observed region is for the

evaluation of the property expressions when they are triggered. A criterion

for this determinism is that the property evaluations must only occur once in any clock

triggering time slot.

During the 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. The new

#1step sampling delay provides the ability to sample data immediately before entering

the current time slot and is a preferred construct over other equivalent constructs

because it allows the 1step time delay

to be parameterized. This #1step construct is a conceptual mechanism that provides a

method for defining when sampling takes place and does not require that an event be

created in this previous time slot. Conceptually, this #1step sampling is identical to

taking the data samples in the Preponed region of the current time slot. The code

specified in the program block and the pass/fail code from property expressions are

scheduled in the Reactive region. A #0 control delay specified in a program block

schedules the process for resumption in

the Re-inactive region. The Re-inactive region is the program block dual of the Inactive

region (see below).

The Pre-active, Pre-NBA, and Post-NBA regions are new in this standard but support

existing PLI callbacks.

The Post-observed region is new in this standard and has been added for PLI support.

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 9.4).

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 9.4).

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 9.4).

The Post-observed region provides for a PLI callback control point that allows PLI

application routines to read values after properties are evaluated (in Observed or earlier

region).

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

The flow of execution of the event regions is specified in Figure 9-1.

The Active, Inactive, Pre-NBA, NBA, Post-NBA, Observed, Post-observed, Reactive,

Re-inactive, and Prepostponed regions are known as the iterative regions.

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.

The Active region holds current events being evaluated and can be processed in any

order.

The Inactive region holds the events to be evaluated after all the active events are

processed.

An explicit #0 delay control requires that the process be suspended and an event

scheduled into the Inactive region (or Re-inactive for program blocks) of the current time

slot so that the process can be resumed in the next inactive to active iteration.

A nonblocking assignment creates an event in the NBA region, scheduled for the current

or a later simulation time.

The Postponed region provides for a PLI callback control point that allows PLI

application routines to be suspended until after all the Active, Inactive, NBA, Observed,

Reactive, and Re-inactive regions have completed.

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.

9.3.1 The SystemVerilog simulation reference algorithm

execute_simulation {

T = 0;

initialize the values of all nets and variables;

schedule all initialization events into time 0 slot;

while (some time slot is nonempty) {

move to the next future nonempty time slot and set T;

preponed

pre-active

active

inactive

pre-NBA

NBA

post-NBA

observed

post-observed

reactive

postponed

time slot

from previous

time slot

to next

time slot

region

PLI region

Legend:

re-inactive

pre-postponed

7. What are the types of coverages available in SV ?

Using Cover Groups: Variables, expressions and their cross

Using Cover Properties

8. What is OOPS? Brief Introduction To Oop

Unlike procedural programming, here in the OOP programming model programs are

organized around objects and data rather than actions and logic. Objects represent some

concepts or things and like any other objects in the real Objects in programming language

have certain behavior, properties, type, and identity. In OOP based language the principal

aim is to find out the objects to manipulate and their relation between each other. OOP

offers greater flexibility and compatibility then procedural language like verilog.

Objects are key to understanding object-oriented technology. Look around right now and

you'll find many examples of real-world objects: your system, your desk, your chair.

Real-world objects share two characteristics: They all have state and behavior. System

have state (name, color) and behavior (playing music,switch off). Identifying the state and

behavior for real-world objects is a great way to begin thinking in terms of object-oriented

programming.

SystemVerilog is a object oriented programming and to understand the functionality of

OOP in SystemVerilog, we first need to understand several fundamentals related to

objects. These include class, method, inheritance, encapsulation, abstraction,

polymorphism etc.

Class

It is the central point of OOP and that contains data and codes with behavior. In

SystemVerilog OOPS , everything happens within class and it describes a set of objects

with common behavior. The class definition describes all the properties, behavior, and

identity of objects present within that class.

Object

Objects are the basic unit of object orientation with behavior, identity. As we mentioned

above, these are part of a class but are not the same. An object is expressed by the

variable and methods within the objects. Again these variables and methods are

distinguished from each other as instant variables, instant methods and class variable and

class methods.

Methods

We know that a class can define both attributes and behaviors. Again attributes are

defined by variables and behaviors are represented by methods. In other words, methods

define the abilities of an object.

Inheritance

This is the mechanism of organizing and structuring program. Though objects are

distinguished from each other by some additional features but there are objects that share

certain things common. In object oriented programming classes can inherit some common

behavior and state from others. Inheritance in OOP allows to define a general class and

later to organize some other classes simply adding some details with the old class

definition. This saves work as the special class inherits all the properties of the old general

class and as a programmer you only require the new features. This helps in a better data

analysis, accurate coding and reduces development time. In Verilog , to write a new

definition using the exesisting definitioan is done inserting `ifdef conpilation controls into

the exesisting code.

Abstraction

The process of abstraction in SystemVerilog is used to hide certain details and only show

the essential features of the object. In other words, it deals with the outside view of an

object.

Encapsulation

This is an important programming concept that assists in separating an object's state from

its behavior. This helps in hiding an object's data describing its state from any further

modification by external component. In SystemVerilog there are three different terms

used for hiding data constructs and these are public, private and protected . As we know

an object can associated with data with predefined classes and in any application an

object can know about the data it needs to know about. So any unnecessary data are not

required by an object can be hidden by this process. It can also be termed as information

hiding that prohibits outsiders in seeing the inside of an object in which abstraction is

implemented.

Polymorphism

It describes the ability of the object in belonging to different types with specific behavior

of each type. So by using this, one object can be treated like another and in this way it

can create and define multiple level of interface. Here the programmers need not have to

know the exact type of object in advance and this is being implemented at runtime.

9. What is the need of virtual interfaces ? An interface encapsulate a group of inter-related wires, along with their directions (via modports) and

synchronization details (via clocking block). The major usage of interface is to simplify the connection

between modules.

But Interface can't be instantiated inside program block, class (or similar non-module entity in

SystemVerilog). But they needed to be driven from verification environment like class. To solve this issue

virtual interface concept was introduced in SV.

Virtual interface is a data type (that implies it can be instantiated in a class) which hold reference to an

interface (that implies the class can drive the interface using the virtual interface). It provides a

mechanism for separating abstract models and test programs from the actual signals that make up the

design. Another big advantage of virtual interface is that class can dynamically connect to different

physical interfaces in run time.

10. Explain Abstract classes and virtual methods.

A set of classes can be created that can be viewed as all being derived from a common base class.

For example, a common base class of type BasePacket that sets out the structure of packets but

is incomplete would never be instantiated. From this base class, however, a number of useful

subclasses could be derived, such as Ethernet packets, token ring packets, GPSS packets, and

satellite packets. Each of these packets might look very similar, all needing the same set of

methods, but they could vary significantly in terms of their internal details.

A base class sets out the prototype for the subclasses. Because the base class is not intended to be

instantiated, it can be made abstract by specifying the class to be virtual: virtual class BasePacket;

Abstract classes can also have virtual methods. Virtual methods are a basic polymorphic

construct. A virtual method overrides a method in all the base classes, whereas a normal method

only overrides a method in that class and its descendants. One way to view this is that there is

only one implementation of a virtual method per class hierarchy, and it is always the one in the

latest derived class. Virtual methods provide prototypes for subroutines, i.e., all of the

information generally found on the first line of a method declaration: the encapsulation criteria,

the type and number of arguments, and the return type if it is needed. Later, when subclasses

override virtual methods, they must follow the prototype exactly. Thus, all versions of the virtual

method look identical in all subclasses: virtual class BasePacket;

virtual function integer send(bit[31:0] data);

endfunction

endclass

class EtherPacket extends BasePacket;

function integer send(bit[31:0] data);

// body of the function

...

endfunction

endclass

EtherPacket is now a class that can be instantiated. In general, if an abstract class has any

virtual methods, all of the methods must be overridden (and provided with a method body) for the

subclass to be instantiated. If any virtual methods have no implementation, the subclass needs to

be abstract.

An abstract class can contain methods for which there is only a prototype and no implementation

(i.e., an incomplete class). An abstract class cannot be instantiated; it can only be derived.

Methods of normal classes can also be declared virtual. In this case, the method must have a

body. If the method does have a body, then the class can be instantiated, as can its subclasses.

Polymorphism: dynamic method lookup

Polymorphism allows the use of a variable in the superclass to hold subclass objects and to

reference the methods of those subclasses directly from the superclass variable. As an example,

assume the base class for the Packet objects, BasePacket, defines, as virtual functions, all of

the public methods that are to be generally used by its subclasses. Such methods include send,

receive, and print. Even though BasePacket is abstract, it can still be used to declare a variable: BasePacket packets[100];

Now, instances of various packet objects can be created and put into the array: EtherPacket ep = new; // extends BasePacket

TokenPacket tp = new; // extends BasePacket

GPSSPacket gp = new; // extends EtherPacket

packets[0] = ep;

packets[1] = tp;

packets[2] = gp;

If the data types were, for example, integers, bits, and strings, all of these types could not be

stored into a single array, but with polymorphism, it can be done. In this example, because the

methods were declared as virtual, the appropriate subclass methods can be accessed from the

superclass variable, even though the compiler did not know—at compile time—what was going

to be loaded into it.

For example, packets[1] packets[1].send();

shall invoke the send method associated with the TokenPacket class. At run time, the system

correctly binds the method from the appropriate class. This is a typical example of polymorphism

at work, providing capabilities that are far more powerful than what is found in a nonobject-

oriented framework. 11. What is the use of the abstract class?

With inheritance we are able to force a subclass to offer the same properties like their

superclasses. Consequently, objects of a subclass behave like objects of their superclasses.

Sometimes it make sense to only describe the properties of a set of objects without

knowing the actual behaviour beforehand

Abstract classes are those which can be used for creation of handles. However their

methods and constructors can be used by the child or extended class. The need for

abstract classes is that you can generalize the super class from which child classes can

share its methods. The subclass of an abstract class which can create an object is called as

"concrete class".

EXAMPLE: useing abstract class

virtual class A ;

virtual task disp ();

$display(" This is class A ");

endtask

endclass

class EA extends A ;

task disp ();

$display(" This is Extended class A ");

endtask

endclass

program main ;

EA my_ea;

A my_a;

initial

begin

my_ea = new();

my_a = my_ea;

my_ea.disp();

my_a.disp();

end

endprogram

RESULT

This is Extended class A

This is Extended class A

EXAMPLE: creating object of virtual class

virtual class A ;

virtual task disp ();

$display(" This is class A ");

endtask

endclass

program main ;

A my_a;

initial

begin

my_a = new();

my_a.disp();

end

endprogram

RESULT

Abstract class A cannot be instantiated

Virtual keyword is used to express the fact that derived classes must redefine the

properties to fulfill the desired functionality. Thus from the abstract class point of view,

the properties are only specified but not fully defined. The full definition including the

semantics of the properties must be provided by derived classes.

Definition (Abstract Class) A class A is called abstract class if it is only used as a superclass

for other classes. Class A only specifies properties. It is not used to create objects. Derived

classes must define the properties of A.

12. What is the difference between mailbox and queue?

13. What data structure you used to build scoreboard

Queue

14. What are the advantages of linkedlist over the queue ?

15. How parallel case and full cases problems are avoided in SV

16. What is the difference between pure function and cordinary function ?

17. What is the difference between $random and $urandom?

$random system function returns a 32-bit signed random number each time it is called

$urandom system function returns a 32-bit unsigned random number each time it is called. (newly added

in SV, not present in verilog)

18. What is scope randomization

19. List the predefined randomization methods.

20. What is the dfference between always_combo and always@(*)?

21. What is the use of packages?

22. In Verilog declaration of data/task/function within modules are specific to the module only. They can't be

shared between two modules. Agreed, we can achieve the same via cross module referencing or by

including the files, both of which are known to be not a great solution.

The package construct of SystemVerilog aims in solving the above issue. It allows having global

data/task/function declaration which can be used across modules. It can contain

module/class/function/task/constraints/covergroup and many more declarations (for complete list please

refer section 18.2 of SV LRM 3.1a )

The content inside the package can be accessed using either scope resolution operator (::), or using

import (with option of referencing particular or all content of the package).

package ABC;

// Some typedef

typedef enum {RED, GREEN, YELLOW} Color;

// Some function

void function do_nothing()

endfunction : do_nothing

// You can have many different declarations here

endpackage : ABC

// How to use them

import ABC::Color; // Just import Color

import ABC::*; // Import everything inside the package

23. What is the use of $cast?

24. How to call the task which is defined in parent object into derived class ?

25. What is the difference between rand and randc?

26. What is $root? $root refers to the top level instance in SystemVerilog

1.package ABC;

2.$root.A; // top level instance A

3.$root.A.B.C; // item C within instance B within top level instance

A

27. What is $unit?

28. What are bi-directional constraints?

29. What is solve...before constraint ?

30. Without using randomize method or rand,generate an array of unique

values?

...

2.int UniqVal[10];

3.foreach(UniqVal[i]) UniqVal[i] = i;

4.UniqVal.shuffle();

5....

31. Explain about pass by ref and pass by value? Pass by value is the default method through which arguments are passed into functions and

tasks. Each subroutine retains a local copy of the argument. If the arguments are changed

within the subroutine declaration, the changes do not affect the caller.

In pass by reference functions and tasks directly access the specified variables passed as

arguments.Its like passing pointer of the variable.

example:

task pass(int i) // task pass(var int i) pass by reference

{

delay(10);

i = 1;

printf(" i is changed to %d at %d\n",i,get_time(LO) );

delay(10);

i = 2;

printf(" i is changed to %d at %d\n",i,get_time(LO) );

}

32. What is the difference between bit[7:0] sig_1; and byte sig_2; byte is signed whereas bit [7:0] is unsigned.

33. What is the difference between program block and module ?

Program block is newly added in SystemVerilog. It serves these purposes

1. It separates testbench from DUT

2. It helps in ensuring that testbench doesn't have any race condition with DUT

3. It provides an entry point for execution of testbench

4. It provides syntactic context (via program ... endprogram) that specifies scheduling in the

Reactive Region. Having said this the major difference between module and program blocks are

1. Program blocks can't have always block inside them, modules can have.

2. Program blocks can't contain UDP, modules, or other instance of program block inside them.

Modules don't have any such restrictions.

3. Inside a program block, program variable can only be assigned using blocking assignment and

non-program variables can only be assigned using non-blocking assignments. No such restrictions

on module

4. Program blocks get executed in the re-active region of scheduling queue, module blocks get

executed in the active region

5. A program can call a task or function in modules or other programs. But a module can not call a

task or function in a program.

34. What is final block ?

35. How to implement always block logic in program block ? Use of forever begin end. If it is a complex always block statement like always (@ posedge clk or negedge reset_)

always @(posedge clk or negedge reset_) begin

if(!reset_) begin

data <= '0;

end else begin

data <= data_next;

end

end

// Using forever : slightly complex but doable

forever begin

fork

begin : reset_logic

@ (negedge reset_);

data <= '0;

end : reset_logic

begin : clk_logic

@ (posedge clk);

if(!reset_) data <= '0;

else data <= data_next;

end : clk_logic

join_any

disable fork

end

36. What is the difference between fork/joins, fork/join_none fork/join_any ?

I have added histstat hit counter for my blog to have an idea about traffic/visitor/search engine

trends. One interesting I am observing is that quite a few hit to my site is coming for fork/join

interview questions in Google. Hence I thought, why not have a real useful post on fork/join{x} of

SystemVerilog and its associated disable/wait command.

Fork-join statement has come from Verilog. It is used for forking out parallel processes in test

bench. SV substantially improved fork/join construct to have much more controllability in

process creation, destruction, and waiting for end of the process.

The basic syntax of fork join block looks like this:

01.fork

02.begin : First_thread

03.// Code for 1st thread

04.end

05.begin : Second_thread

06.// Code for 2nd thread

07.end

08.begin : Third thread

09.// Code for 3rd branch

10.end

11....

12.join // Can be join_any, join_none in SV

There are 3 different kind of join keyword in SV, each specifying a different way of waiting for completion of the threads/process created by the fork.

o join : waits for completion of all of the threads

o join_any : waits for the completion of the 1st thread, then comes out of fork loop, but lets the

other process/thread execute as usual

o join_none : doesn't wait for completion of any thread, just starts then and immediately exits

fork loop. Now, suppose you have exited the fork loop by join_none or join_any and after some

steps, you want to wait till completion of all the threads spanned by the previous fork

loop. SV has "wait fork" for the same.

Now, suppose you have exited the fork loop by join_none or join_any and after some

steps, you want to killall the threads spanned by the previous fork loop. SV has "disable

fork" for the same.

Next interesting scenario: you have exited fork loop by join_none or join_any and after

some steps, you want to kill just one thread (out of many). The solution, have named

begin end block and call "disable ". (For example, in the last example if you want to kill

only the 2nd thread after exiting the loop via join_any/join_none, then add "disable

Second_thread;" at the point where you want to disable the second thread.

I have created a image to pictorially depict fork/join in SV. Hope that will help

understand this in a much better way.

37. What is the use of modports ?

Modports are part of Interface. Modports are used for specifing the direction of the signals with respect to various modules the interface connects to.

1....

2.interface my_intf;

3.wire x, y, z;

4.modport master (input x, y, output z);

5.modport slave (output x, y, input z);

6.endinterface

Please refer section 19.4 of SV LRM for more details

38. Write a clock generator without using always block.

39. What is forward referencing and how to avoid this problem?

40. What is circular dependency and how to avoid this problem ?

41. What is cross coverage ?

42. Describe the difference between Code Coverage and Functional Coverage

Which is more important and Why we need them Code Coverage indicates the how much of RTL has been exercised. The Functional Coverage

indicates which features or functions has been executed. Both of them are very

important. With only Code Coverage, it may not present the real features coverage. On the

other hand, the functional coverage may miss some unused RTL coverage.

43. How to kill a process in fork/join?

44. Difference between Associative array and Dynamic array ? 1) Difference between Associative array and Dynamic array ?

Answer:

Dynamic arrays are useful for dealing with contiguous collections of variables whose number

changes dynamically.

e.g. int array[];

When the size of the collection is unknown or the data space is sparse, an associative array

is a better option. In associative array, it uses the transaction names as the keys in associative

array.

e.g. int array[string];

45. Difference b/w Procedural and Concurrent Assertions?

46. What are the advantages of SystemVerilog DPI? SystemVerilog introduces a new foreign language interface called the Direct Programming

Interface (DPI). The DPI provides a very simple, straightforward, and efficient way to connect

SystemVerilog and foreign language code unlike PLI or VPI.

47. How to randomize dynamic arrays of objects?

class ABC;

// Dynamic array

rand bit [7:0] data [];

// Constraints

constraint cc {

// Constraining size

data.size inside {[1:10]};

// Constraining individual entry

data[0] > 5;

// All elements

foreach(data[i])

if(i > 0)

data[i] > data[i-1];

}

endclass : ABC

48. What is randsequence and what is its use?

49. What is bin? A coverage-point bin associates a name and a count with a set of values or a sequence of value

transitions. If the bin designates a set of values, the count is incremented every time the

coverage point matches one of the values in the set. If the bin designates a sequence of value

transitions, the count is incremented every time the coverage point matches the entire

sequence of value transitions.

e.g.

program main;

bit [0:2] y;

bit [0:2] values[$]= '{3,5,6};

covergroup cg;

cover_point_y : coverpoint y

{ option.auto_bin_max = 4 ; }

endgroup

cg cg_inst = new();

initial

foreach(values[i])

begin

y = values[i];

cg_inst.sample();

end

endprogram

50. Why always block is not allowed in program block?

51. Which is best to use to model transaction? Struct or class ?

52. How SV is more random stable then Verilog?

53. Difference between assert and expect statements?

54. How to add a new processs with out disturbing the random number

generator state ?

55. What is the need of alias in SV?

56. What is the need to implement explicitly a copy() method inside a

transaction , when we can simple assign one object to other ?

57. How different is the implementation of a struct and union in SV.

58. What is "this"?

59. What is tagged union ?

60. What is "scope resolution operator"?

61. What is the difference between Verilog Parameterized Macros and

SystemVerilog Parameterized Macros?

62. What is the difference between view source print?

1.logic data_1;

2.var logic data_2;

3.wire logic data_3j;

4.bit data_4;

5.var bit data_5;

63. What is the difference between bits and logic?

64. Write a Statemechine in SV styles.

65. What is the difference between $rose and posedge?

66. What is advantage of program block over clockcblock w.r.t race condition?

67. How to avoid the race condition between programblock ?

68. What is the difference between assumes and assert?

69. What is coverage driven verification?

70. What is layered architecture ?

71. What are the simulation phases in your verification environment?

72. How to pick a element which is in queue from random index?

73. What data structure is used to store data in your environment and why ?

74. What is casting? Explain about the various types of casting available in SV.

75. How to import all the items declared inside a package ?

76. Explain how the timescale unit and precision are taken when a module

does not have any timescalerdeclaration in RTL?

77. What is streaming operator and what is its use?

78. What are void functions ?

79. How to make sure that a function argument passed has ref is not changed

by the function?

80. What is the use of "extern"?

81. What is the difference between initial block and final block?

There are many difference between initial and final block. I am listing the few differences that is coming to mind now.

1. The most obvious one : Initial blocks get executed at the beginning of the simulation, final block

at the end of simulation

2. Final block has to be executed in zero time, which implies it can't have any delay, wait, or non-

blocking assignments. Initial block doesn't have any such restrictions of execution in zero time

(and can have delay, wait and non-blocking statements) Final block can be used to display statistical/genaral information regarding the status of the execution like this:-

1.final begin

2.$display("Simulation Passed");

3.$display("Final value of xyz = %h",xyz);

4.$display("Bye :: So long, and Thanks for all the fishes");

5.end

82. How to check weather a handles is holding object or not ?

83. How to disable multiple threads which are spawned by fork...join