UVM Interview Questions - 4

Q26: What is p_sequencer ? OR Difference between m_sequencer and p_sequencer?

m_sequencer is the default handle for uvm_vitual_sequencer and p_sequencer is the hook up for child sequencer.

m_sequencer is the generic uvm_sequencer pointer. It will always exist for the uvm_sequence and is initialized when the sequence is started.

p_sequencer is a typed-specific sequencer pointer, created by registering the sequence to the sequencer using macros (`uvm_declare_p_sequencer) . Being type specific, you will be able to access anything added to the sequencer (i.e. pointers to other sequencers, etc.). p_sequencer will not exist if we have not registered the sequence with the `uvm_declare_p_sequencer macros.

The drawback of p_sequencer is that once the p_sequencer is defined, one cannot run the sequence on any other sequencer type.

Q27: What is the difference between Active mode and Passive mode with respect to agent?

An agent is a collection of a sequencer, a driver and a monitor.

In active mode, the sequencer and the driver are constructed and stimulus is generated by sequences sending sequence items to the driver through the sequencer. At the same time the monitor assembles pin level activity into analysis transactions.

In passive mode, only the monitor is constructed and it performs the same function as in an active agent. Therefore, your passive agent has no need for a sequencer. You can set up the monitor using a configuration object.

Q28: What is the difference between copy and clone?

The built-in copy() method executes the __m_uvm_field_automation() method with the required copy code as defined by the field macros (if used) and then calls the built-in do_copy() virtual function. The built-in do_copy() virtual function, as defined in the uvm_object base class, is also an empty method, so if field macros are used to define the fields of the transaction, the built-in copy() method will be populated with the proper code to copy the transaction fields from the field macro definitions and then it will execute the empty do_copy() method, which will perform no additional activity.

The copy() method can be used as needed in the UVM testbench. One common place where the copy() method is used is to copy the sampled transaction and pass it into a sb_calc_exp() (scoreboard calculate expected) external function that is frequently used by the scoreboard predictor.

The clone() method calls the create() method (constructs an object of the same type) and then calls the copy() method. It is a one-step command to create and copy an existing object to a new object handle.

Q29: What is UVM factory? 

UCM Factory is used to manufacture (create) UVM objects and components. Apart from creating the UVM objects and components the factory concept essentially means that you can modify or substitute the nature of the components created by the factory without making changes to the testbench. 

For example, if you have written two driver classes, and the environment uses only one of them. By registering both the drivers with the factory, you can ask the factory to substitute the existing driver in environment with the other type. The code needed to achieve this is minimal, and can be written in the test.

Q30: What are the types of sequencer? Explain each?

There are two types of sequencers :

uvm_sequencer #(REQ, RSP) :

When the driver initiates new requests for sequences, the sequencer selects a sequence from a list of available sequences to produce and deliver the next item to execute. In order to do this, this type of sequencer is usually connected to a driver uvm_driver #(REQ, RSP).

uvm_push_sequencer #(REQ, RSP) :

The sequencer pushes new sequence items to the driver, but the driver has the ability to block the item flow when its not ready to accept any new transactions. This type of sequencer is connected to a driver of type uvm_push_driver #(REQ, RSP).

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UVM Interview Questions - 3

Q21: What is analysis port?

Analysis port (class uvm_tlm_analysis_port) — a specific type of transaction-level port that can be connected to zero, one, or many analysis exports and through which a component may call the method write implemented in another component, specifically a subscriber.

port, export, and imp classes used for transaction analysis.

uvm_analysis_port

Broadcasts a value to all subscribers implementing a uvm_analysis_imp.

uvm_analysis_imp

Receives all transactions broadcasted by a uvm_analysis_port.

uvm_analysis_export

Exports a lower-level uvm_analysis_imp to its parent.

Q22: What is TLM FIFO?

In simpler words TLM FIFO is a FIFO between two UVM components, preferably between Monitor and Scoreboard. Monitor keep on sending the DATA, which will be stored in TLM FIFO, and Scoreboard can get data from TLM FIFO whenever needed.

// Create a FIFO with depth 4

      tlm_fifo = new ("uvm_tlm_fifo", this, 4);

Q23: How sequence starts?

start_item starts the sequence

virtual task start_item ( uvm_sequence_item item,  

                                          int  set_priority =  -1,

                                        uvm_sequencer_base  sequencer =  null )

start_item and finish_item together will initiate operation of a sequence item.  If the item has not already been initialized using create_item, then it will be initialized here to use the default sequencer specified by m_sequencer. 

Q24: What is the difference between UVM RAL model backdoor write/read and front door write/read ?

Fontdoor access means using the standard access mechanism external to the DUT to read or write to a register. This usually involves sequences of time-consuming transactions on a bus interface. 

Backdoor access means accessing a register directly via hierarchical reference or outside the language via the PLI. A backdoor reference usually in 0 simulation time.

Q25: What is objection?

The objection mechanism in UVM is to allow hierarchical status communication among components which is helpful in deciding the end of test.

There is a built-in objection for each in-built phase, which provides a way for components and objects to synchronize their testing activity and indicate when it is safe to end the phase and, ultimately, the test end.

The component or sequence will raise a phase objection at the beginning of an activity that must be completed before the phase stops, so the objection will be dropped at the end of that activity. Once all of the raised objections are dropped, the phase terminates.

Raising an objection: phase.raise_objection(this);

Dropping an objection: phase.drop_objection(this);

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UVM Interview Questions - 1

Q11: Difference between module & class based TB?

Ans: A module is a static object present always during of the simulation.

A Class is a dynamic object because they can come and go during the life time of simulation.

Q12: What is uvm_config_db ? What is difference between uvm_config_db & uvm_resource_db?

Ans: uvm_config_db is a parameterized class used for configuration of different type of parameter into the uvm database, So that it can be used by any component in the lower level of hierarchy.

uvm_config_db is a convenience layer built on top of uvm_resource_db, but that convenience is very important. In particular, uvm_resource_db uses a "last write wins" approach. The uvm_config_db, on the other hand, looks at where things are in the hierarchy up through end_of_elaboration, so "parent wins." Once you start start_of_simulation, the config_db becomes "last write wins."

All of the functions in uvm_config_db#(T) are static, so they must be called using the :: operator

It is extended from the uvm_resource_db#(T), so it is child class of uvm_resource_db#(T)

Q13: What is the advantage and difference of  `uvm_component_utils() and `uvm_object_utils()?

Ans: The utils macros define the infrastructure needed to enable the object/component for correct factory operation. 

The reason there are two macros is because the factory design pattern fixes the number of arguments that a constructor can have. Classes derived from uvm_object have constructors with one argument, a string name. Classes derived from uvm_component have two arguments, a name and a uvm_component parent.  

The two `uvm_*utils macros inserts code that gives you a factory create() method that delegates calls to the constructors of uvm_object or uvm_component. You need to use the respective macro so that the correct constructor arguments get passed through. This means that you cannot add extra constructor arguments when you extend these classes in order to be able to use the UVM factory.

Q14: Difference between `uvm_do and `uvm_rand_send ?

Ans: `uvm_do perform the below steps:

1. create
2. start_item
3. randomize
4. finish_item
5. get_response (optional)

while `uvm_rand_send perform all the above steps except create. User needs to create sequence / sequence_item.

Q15: Difference between uvm_transaction and uvm_seq_item?

Ans: class uvm_sequence_item extends uvm_transaction

uvm_sequence_item extended from uvm_transaction only, uvm_sequence_item class has more functionality to support sequence & sequencer features. uvm_sequence_item provides the hooks for sequencer and sequence , So you can generate transaction by using sequence and sequencer , and uvm_transaction provide only basic methods like do_print and do_record etc .

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UVM Interview Questions - 2

Q16: Is uvm is independent of systemverilog ?

Ans: UVM is a methodology based on SystemVerilog language and is not a language on its own.  It is a standardized methodology that defines several best practices in verification to enable  efficiency in terms of reuse and is also currently part of IEEE 1800.2  working group.

Q17: What are the benefits of using UVM?

Ans: Some of the benefits of using UVM are :

• Modularity and Reusability – The methodology is designed as modular components (Driver, Sequencer, Agents , env etc) which enables reusing components across unit level to multi-unit or chip level verification as well as across projects.
• Separating Tests from Testbenches – Tests in terms of stimulus/sequencers are kept separate from the actual testbench hierarchy and hence there can be reuse of stimulus across different units or across projects
• Simulator independent – The base class library and the methodology is supported by all simulators and hence there is no dependence on any specific simulator
• Better control on Stimulus generation – Sequence methodology gives good control on stimulus generation. There are several ways in which sequences can be developed which includes randomization, layered sequences, virtual sequences etc which provides a good control and rich stimulus generation capability.
• Easy configuration – Config mechanisms simplify configuration of objects with deep hierarchy. The configuration mechanism helps in easily configuring different testbench components based on which verification environment uses it and without worrying about how deep any component is in testbench hierarchy
• Factory mechanism – Factory mechanisms simplifies modification of components easily. Creating each components using factory enables them to be overridden in different tests or environments without changing underlying code base.

Q18: Can we have user defined phase in UVM?

Ans: In addition to the predefined phases available in uvm , the user has the option to add his own phase to a component. This is typically done by extending the uvm_phase class the constructor needs to call super.new which has three arguments

• Name of the phase task or function
• Top down or bottom up phase
• Task or function

The call_task  or call_func and get_type_name need to be implemented to complete the addition of new phase.

Below is a simple example 

Example

class custom_phase extends uvm_phase;

   function new();

      super.new(“custom”,1,1);

   endfunction

   task call_task  ( uvm_component parent);

     my_comp_type comp;

     if ( $cast(comp,parent) )

             comp.custom_phase();

   endtask

   virtual function string get_type_name();

      return “custom”;

   endfunction

endclass

Q19: What is uvm RAL model ? why it is required ?

Ans: In a verification context, a register model (or register abstraction layer) is a set of classes that model the memory mapped behavior of registers and memories in the DUT in order to facilitate stimulus generation and functional checking (and optionally some aspects of functional coverage). The UVM provides a set of base classes that can be extended to implement comprehensive register modeling capabilities.

Q20: What is the difference between new() and create?

Ans: We all know about new() method that is use to allocate memory to an object instance. In UVM (and OVM), the create() method causes an object instance to be created from the factory. This allows you to use factory overrides to replace the desired object with an object of a different type without having to recode.

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UVM Interview Questions

Q1: What is UVM? What is the advantage of UVM?

Ans: UVM (Universal Verification Methodology) is a standardized methodology for verifying the both complex & simple digital design in simple way.

UVM Features:

• First methodology & second collection of class libraries for Automation
• Reusability through testbench
• Plug & Play of verification IPs
• Generic Testbench Development
• Vendor & Simulator Independent
• Smart Testbench i.e. generate legal stimulus as from pre-planned coverage plan
• Support CDV –Coverage Driven Verification
• Support CRV –Constraint Random Verification
• UVM standardized under the Accellera System Initiative
• Register modeling

Q2: UVM derived from which language?

Ans: Here is the detailed connection between SV, UVM, OVM and other methodologies. 

Q3. What is the difference between uvm_component and uvm_object?

                       OR

We already have uvm_object, why do we need uvm_component which is actually derived class of uvm_object?

Ans: 

uvm_component:

• Quasi Static Entity (after build phase it is available throughout the simulation)
• Always tied to a given hardware(DUT Interface) Or a TLM port
• Having phasing mechanism for control the behavior of simulation
• Configuration Component Topology

uvm_object:

• Dynamic Entity (create when needed, transfer from one component to other & then dereference)
• Not tied to a given hardware or any TLM port
• Not phasing mechanism 

Q4: Why phasing is used? What are the different phases in uvm?

Ans: UVM Phases is used to control the behavior of simulation in a systematic way & execute in a sequential ordered to avoid race condition. This could also be done in system verilog but manually.

1. List of UVM Phases:
2. buid_phase
3. connect_phase
4. end_of_elaboration_phase
5. start_of_simulation_phase       
6. run _phase  (task)
   Sub Phases of Reset Phase:
   pre_reset_phase
   reset_phase
   post_reset_phase
   pre_configure_phase
   configure_phase
   post_configure_phase
   pre_main_phase
   main_phase
   post_main_phase
   pre_shutdown_phase
   shutdown_phase
   post_shutdown_phase
7. extract_phase
8. check_phase
9. report_phase

Below figure makes it more clear

Q5: Which uvm phase is top - down , bottom – up & parallel?

Ans: Only build phase is a top-down & other phases are bottom-up except run phase which is parallel. The build phase works top-down since the testbench hierarchy may be configure so we need to build the branches before leafs

Q6: Why build phase is top – down & connect phase is bottom – up?

Ans: The connect phase is intended to be used for making TLM connections between components, which is why it occur after build phase. It work bottom-up so that its got the correct implementation all the way up the design hierarchy, if worked top-down this would be not possible

Q7: Which phase is function & which phase is task?

Ans: Only run phase is a task (time consuming phase) & other phases are functions (non-blocking)

Q8: Which phase takes more time and why?

Ans: As previously said the run phase is implemented as task and remaining all are function. run phase will get executed from start of simulation to till the end of simulation. run phase is time consuming, where the testcase is running.

Q9: How uvm phases initiate?

Ans: UVM phases initiate by calling run_test(“test1”) in top module. When run_test() method call, it first create the object of test top & then call all phases.

Q10: How test cases run from simulation command line?

Ans: In top module write run_test(); i.e. Don't give anything in argument.

Then in command line : +UVM_TESTNAME=testname

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Intel's Skylarke Processors for PCs, Tablets and Servers

Intel is launching a full portfolio of "Skylake" processors that company officials expect will combine with Microsoft's Windows 10 operating system to help jump start a stagnant global PC market.

Executives with the chip maker for more than a year have been talking about the 14-nanometer Skylake architecture and the advanced features that are contained within it, touching on everything from graphics and imaging to security, memory, performance and wireless connectivity. In early August, Intel rolled out two Skylake chips—the Core i7-6700K and i5-6600K desktop processors—for gaming machines, and later in the month officials gave out a few more details during the Intel Developer Forum (IDF).

While Intel Corp. is going to release its code-named “Skylake” processors a little later than expected, the company keeps the plan to introduce its new micro-architecture for virtually all segments of the market continuum this year. Intel will roll-out “Skylake” central processing units for tablets, 2-in-1s, personal computers and servers this year, the chip giant confirmed this week.

“When I look at the range of what Skylake’s able to deliver from the Core M level all up to the i7 and Xeon, it’s just going to be a fantastic product,” said Intel CEO Brian Krzanich, in an interview with the IDG News Service at Mobile World Congress in Barcelona.

Intel ran into problems with production of its code-named “Broadwell” processors using 14nm manufacturing technology last year. Due to insufficient yields, the world’s largest maker of microprocessors had to delay introduction of its latest chips by about a year. However, since “Skylake” brings a lot of innovations, Intel did not want to delay it significantly. As a result, “Broadwell” products will have a relatively short lifecycle.

Intel will introduce the first “Skylake” processors in the form of dual-core Core M chips in the third quarter of this calendar year. The CPUs will power high-performance tablets, hybrid 2-in-1 personal computers and ultra-thin notebooks. It is expected that many mobile devices powered by Intel Core M “Skylake” will support Rezence wireless charging and WiGig short-range transmission technology.

In late Q3 or early Q4 the Santa Clara, California-based chip designer will introduce its first Core i3, Core i5 and Core i7 chips featuring “Skylake” micro-architecture for mainstream personal computers, including desktops and laptops. The lineup is projected to include chips with unlocked multiplier designed for enthusiast-class desktop PCs. Systems featuring the new “Skylake” processors will have improved storage performance thanks to native support of SATA Express. In addition, many “Skylake”-powered PCs will use DDR4 memory and support a variety of other innovations.

Intel also plans to introduce Xeon processors with “Skylake” cores for uniprocessor servers later this year. While there are plans to bring “Skylake” architecture to Xeon chips for dual-processor and multi-processor servers, Intel yet has to outline exact plans concerning the move.

Intel “Skylake” processors will be made using 14nm process technology and will feature a brand new micro-architecture that is designed to improve performance and power efficiency of central processing units. Unfortunately, not all “Skylake” processors will support 512-bit AVX 3.2 instructions, according to unofficial information.

Resistive Memory - ReRam

The memory tech that will eventually replace NAND flash, finally in market

What is ReRam?

ReRam is Resistive random-access memory (RRAM or ReRAM) is a type of non-volatile (NV) random-access (RAM) computer memory that works by changing the resistance across a dielectric solid-state material often referred to as a memristor. The biggest advantage of ReRAM technology is its good compatibility with CMOS technologies.

It is under development by a number of companies, and some have already patented their own versions of the technology. The memory operates by changing the resistance of special dielectric material called a memresistor (memory resistor) whose resistance varies depending on the applied voltage.

What makes ReRam?

From the viewpoint of the material choice, the advantage of ReRAM is evident. It is possible to fabricate MOM structures easily by using the oxides widely used in the current semiconductor technologies. Low-current ReRAM operation was reported in the CuOx-based MOM structure. The CuOx layer was grown by the thermal oxidation of the 0.18-μm Cu. NiO and CoO are being intensively studied as oxide materials for ReRAM, and these transition metal elements are also used in metal silicides employed as gate materials. Recently, the good scaling feasibility of ReRAM was demonstrated in an HfOx-based memory with a cell size of 30 nm. The devices in a 1-kbit array exhibited a high device yield (~100%) and robust cycling endurance (>106) with a pulse width of 40 ns. The memory cell consisted of a TiN/Ti/HfOx/TiN structure. Here, the Ti overlayer played the role of oxygen gettering for better ReRAM operation. The gettering effect has already been investigated in HfOx as a high-k material for the gate dielectric films in CMOS devices. The academic and technological knowledge about high-k materials will be very useful in the design of the stacking structure for a ReRAM device.

How ReRam Works?

RRAM is the result of a new kind of dielectric material which is not permanently damaged and fails when dielectric breakdown occurs; for a memresistor, the dielectric breakdown is temporary and reversible. When voltage is deliberately applied to a memresistor, microscopic conductive paths called filaments are created in the material. The filaments are caused by phenomena like metal migration or even physical defects. Filaments can be broken and reversed by applying different external voltages. It is this creation and destruction of filaments in large quantities that allows for storage of digital data. Materials that have memresistor characteristics include oxides of titanium and nickel, some electrolytes, semiconductor materials, and even a few organic compounds have been tested to have these characteristics.

The principal advantage of RRAM over other non-volatile technology is high switching speed. Because of the thinness of the memresistors, it has a great potential for high storage density, greater read and write speeds, lower power usage, and cheaper cost than flash memory. Flash memory cannot continue to scale because of the limits of the materials, so RRAM will soon replace flash memory.