300+ [UPDATED] Digital Logic and Computer Organization Interview Questions

1. What Is Digital Logic?

Digital logic is the representation of signals and sequences of a digital circuit through numbers. It is the basis for digital computing and provides a fundamental understanding on how circuits and hardware communicate within a computer.

2. What Is Combinational Logic?

The term combinational logic refers to circuitry that transforms bits, as opposed to storing bits. For example, the ALU portion of a CPU transforms data, e.g. transforming two input word-sized bit strings into an output which is the sum of the two inputs.

3. Explain Sequential Logic?

Sequential logic stores data. Registers in a CPU, RAM and so on store data.

4. Explain Computer Architecture?

A modern meaning of the term computer architecture covers three aspects of computer design:

• instruction set architecture.
• computer organization.
• computer hardware.

5. Explain Instruction Set Architecture?

Instruction Set Architecture – ISA refers to the actual programmer-visible machine interface such as instruction set, registers, memory organization and exception handling. Two main approaches: RISC and CISC architectures.‰

A computer organization and computer hardware are two components of the implementation of a machine. ‰ Computer organization includes the high-level aspects of a design, such as the memory system, the bus structure, and the design of the internal CPU (where arithmetic, logic, branching and data transfers are implemented).‰

Computer hardware refers to the specifics of a machine, included the detailed logic design and the packaging technology of the machine.‰ For many years the interaction between ISA and implementations was believed to be small, and implementation issues were not a major focus in designing instruction set architecture. ‰

In the 1980’s, it becomes clear that both the difficulty and inefficiency of pipelining could be increased by instruction set architecture complications.

6. What Are The Tasks Of Computer Architects?

Tasks of Computer Architects ‰: Computer architects must design a computer to meet functional requirements as well as price, power, and performance goals. Often, they also have to determine what the functional requirements are, which can be a major task. ‰ Once a set of functional requirements has been established, the architect must try to optimize the design. Here are three major application areas and their main requirements: 

Desktop computers: focus on optimizing costperformance as measured by a single user, with little regard for program size or power consumption.

Server computers: focus on availability, scalability,and throughput cost-performance.

Embedded computers : driven by price and often power issues, plus code size is important.

7. What Are The Developments Of Computer Design?

Developments in Computer Design
: During the first 25 years of electronic computers both forces, technology and innovations in computer design made major contributions. ‰ Then, during the 1970’s, computer designers were largely dependent upon integrated circuit technology, with roughly 35% growth per year in processor performance.

In the last 20 year, the combination of innovations in computer design and improvements in technology has led sustained growth in performance at an annual rate of over 55%. In this period, the main source of innovations in computer design has come from RISCstyle pipelined processors.

8. Explain Risc Architecture?

RISC Architecture ‰:  
After 1985, any computer announced has been of RISC architecture. RISC designers focused on two critical performance techniques in computer design:  the exploitation of instruction-level  the exploitation of instruction level parallelism, first through pipelining and later through multiple instruction issue,  the use of cache, first in simple forms and later using sophisticated organizations and optimizations.

9. What Are The Characteristics Of Risc Isa?

RISC ISA Characteristics 

• All operations on data apply to data in registers and typically change the entire register; ‰
• The only operations that affect memory are load and store operations that move data from memory to a register or to memory from a register, respectively; 
• ‰A small number of memory addressing modes;
• The instruction formats are few in number with all instructions typically being one size;
• Large number of registers; ‰

These simple properties lead to dramatic simplifications in the implementation of advanced pipelining techniques, which is why RISC architecture instruction sets were designed this way.

10. What Is The Main Example Of Cisc Architecture Processor?

Intel IA-32 processors (in over 90% computers). ‰ Intel IA-32 processors, from 80386 processor in early 80’s to Pentium IV today, and the next one to be introduced this or next year, are of CISC architecture. All Intel IA-32 processors are having as a base the Identical instruction set architecture designed in early 1980’s.

11. What Does Mips Stand For?

Microprocessor without Interlocked Pipeline Stages. MIPS processor is one of the first RISC processors. Again, all processors announced after 1985 have been of RISC architecture.

12. Explain The Combinational Logic Design Process?

Combinational Logic Design Process : 

• Create truth table from specification
• Generate K-maps & obtain logic equations
• Draw logic diagram (sharing common gates)
• Simulate circuit for design verification
• Debug & fix problems when output is incorrect
• Check truth table against K-map population
• Check K-map groups against logic equation product terms
• Check logic equations against schematic
• Circuit optimization for area and/or performance
• Analyze verified circuit for optimization metric

•G, G_IO, G_del, P_del

• Use Boolean postulates & theorems
• Re -simulate & verify optimized design

13. What Are The Design Verification Guidelines?

Design Verification Guidelines : 

• Use all audits and analysis aids possible to help find potential design bugs 
• Investigate and correct all errors/warnings
• Simulate thoroughly but use stimuli that “eat their way into the design” testing one function at a time
• more important for complex circuits
• When circuit doesn’t work, see what works and what doesn’t to narrow down the search space for the problem
• Which outputs work
• Which outputs fail and under what conditions 
• Monitor lots of internal nodes
• Additional simulations (with different vectors) can be helpful
• Remember “debugging is just solving out a puzzle”
• Also “if something doesn’t look right, stop and check it out”
• Don’t overlook potential bugs
• Always re-run audits and simulation after correcting any problem (or after any changes)
• Another bug could be lurking, or
• The fix may have messed up something else

14. What Are Sequential Logic Design Steps?

Sequential Logic Design Steps : 

• Derive circuit state diagram from design specs 
• Create state table
• Choose flip-flops (D, T, SR, JK)
• Create circuit excitation table
• use flip-flop excitation tables
• Construct K-maps for: 
• flip-flop inputs
• primary outputs
• Obtain minimized SOP equations
• Draw logic diagram
• Simulate to verify design & debug as needed
• Perform circuit analysis & logic optimization

15. Explain About Program Counter (pc)?

• The program counter is another specialized register.
• It keeps track of the execution of a program.
• It contains the memory address of the next instruction to be fetched and executed.
• During the execution of an instruction, the contents of of the PC are updated to correspond to the address of the next instruction to be executed.
• It is customary to say that PC points to the next instruction that is to be fetched from the memory. 

16. What Are Memory Address Register (mar) & Memory Data Register (mdr)?

• These two registers facilitate communication with the memory.
• The MAR holds the address of the location to be accessed.
• The MDR contains the data to be written into or read out of the addressed location.

17. Explain The Operating Steps For Program Execution?

Operating steps for Program execution :

• Programs are stored in the memory through the input unit. 
• Execution of the program starts when the PC is set to point to the first instruction of the program.
• The contents of the PC are transferred to the MAR and a Read control signal is sent to the memory.
• After the time required to access the memory elapses, the addressed word (in this case, the first instruction of the program) is read out of the memory and loaded into the MDR.  Next, the contents of the MDR are transferred to the IR .
• At this point, the instruction is ready to be decoded and executed. 
• If the instruction involves an operation to be performed by the ALU, it is necessary to obtain the required operands.
• If an operand resides in memory ( it could also be in a generalpurpose register in the processor), it has to be fetched by sending its address to the MAR and initiating a Read cycle. When the operand has been read from the memory into the MDR, it is is transferred from the MDR to the ALU.
• After one or more operands are fetched in this way, the ALU can perform the desired operation.
• If the result of the operation is to be stored in the memory, then the result is sent to the MDR.
• The address of the location where the result is to be stored is sent to the MAR, and a write cycle is initiated. At some point during the execution of the current instruction, the contents of the PC are incremented so that the PC points to the next instruction to be executed.
• Thus, as soon as the execution of the current instruction is completed, a new instruction fetch may be started. 
• In addition to transferring data between the memory and the processor, the computer accepts data from input devices and sends data to output devices. Thus, some machine instructions with the ability to handle I/O transfers are provided. 

18. What Is A Interrupt Service Routine?

• Normal execution of a programs may be preempted if some device requires urgent servicing.
• For example, a monitoring device in a computer-controlled industrial process may detect a dangerous condition.  In order to deal with the situation immediately, the normal execution of the current program must be interrupted.  To do this, the device raises an interrupt signal.
• An interrupt is a request from an I/O device for service by the processor.  The processor provides the requested service by executing an appropriate interrupt-service routine. Because such diversions may alter the internal state of the processor, its state must be saved in the memory locations before servicing the interrupt.
• Normally, the contents of the PC, the general registers, and some control information are stored in memory.
• When the interrupt service routine is completed, the state of the processor is restored so that the interrupted program may continue. 

19. Discuss Various Parameters For Improving The Performance Of A Computer.

The most important measure of the performance of a computer is how quickly it can execute a programs.The speed with which a computer executes programs is affected by the design of its hardware and its machine language instructions.

• Elapsed time:
• The total time required to execute a program .
• This elapsed time is a measure of the performance of the entire computer system.
• It is affected by the speed of the processor, the disk and the printer. 
• Processor time:
• Here we have to consider only those periods of the elapsed time, during which the processor is active.
• The sum of these periods is called processor time.
• The processor time depends on the hardware involved in the execution of individual machine instructions.
• Cache memory: 
• The processor and a relatively small cache memory can be fabricated on a single IC chip.
• The internal speed of performing the basic steps of instruction processing on such chips is very high and considerably faster than the speed at which instructions and data can be fetched from the main memory. 
• A program will be executed faster if the movement of instructions and data between the main memory and processor is minimized, which is achieved by using the cache. 

20. Explain Various Components Of System Software.

Compiler : 
A system software program which translates the highlevel language program into a suitable machine language program.

Text editor:
Another important system program that all programmers use is a text editor. It is used for entering and editing application programs. The user of this program interactively execute commands that allow statements of a source program entered at a keyboard to be accumulated in a file.

Operating system (OS) : 
It is a key system software component. This is a large program, or actually a collection of routines , that is used to control the sharing of and interaction among various computer units as they execute application programs. The OS routines perform the tasks required to assign computer resources to individual application programs. These tasks include assigning memory to program and data files, moving data between memory and disk units, and handling I/O operations. 

21. Explain The Role Of System Software In A Computer.

• System software is responsible for the coordination of all activities in a computing system.
• System software is a collection of programs that are executed as needed to perform functions such as
• Receiving and interpreting user commands.
• Entering and editing application programs and storing them as files in secondary storage devices.
• Managing the storage and retrieval of files in secondary storage devices.  
• Running standard application programs such as word processors, spread sheets, or games, with data supplied by the user.
• Controlling I/O units to receive input information and produce output results.
• Translating programs from high level language to low level language. 
• Linking and running user-written application programs with existing standard library routines, such as numerical computation packages. 

22. Explain How Timing Differences Are Smoothed Out Among Processors, Memories And I/o Devices.

• The devices connected to a bus vary widely in their speed of operation. 
• Electro mechanical devices such as key board and printers are relatively slow.
• Others like magnetic or optical disks, are considerably faster.
• Memory and processor units operate at electronic speeds.

• A common approach to smooth out the timing differences is to include buffer registers with the devices to hold the information during transfers.
• They prevent a high speed processor from being locked to a slow I/O device during a sequence of data transfers. This allows the processor to switch rapidly from one device to another, interweaving its process activity with data transfers involving several I/O devices.
• Thus, buffer registers smooth out timing differences among processors, memories and I/O devices.