PTU BTECH-CSE-3-SEM-COMPUTER-ARCHITECTURE-56591-MAY-2023 FULLY SOLVED

Computer Architecture B.Tech Exam Questions and Answers – Key Topics Explained

Get detailed answers to B.Tech Computer Architecture exam questions covering accumulator logic, RISC/CISC, pipelining, cache memory, and more. Perfect for semester 3 CSE students.

SECTION-A: Short Answer Questions

1a) Define Accumulator Logic

Answer: Accumulator logic refers to a special register in the CPU that stores intermediate arithmetic and logic operation results. It acts as a temporary storage location where data is processed before being transferred to other registers or memory. This central register simplifies instruction set design by serving as the implicit operand for many operations.

1b) Discuss Register Transfer Language

Answer: Register Transfer Language (RTL) is a symbolic notation used to describe microoperations and data transfers between registers in digital systems. It represents the flow of data at register level with statements like “R1 ← R2 + R3” showing data movement and processing. RTL serves as an intermediate representation between hardware design and machine instructions.

1c) Define Control Unit

Answer: The control unit is the component of a CPU that directs operations by generating timing signals and control signals to coordinate all processor activities. It fetches instructions from memory, decodes them, and manages execution by communicating with ALU, registers, and I/O devices. Control units can be hardwired or microprogrammed.

1d) What are Memory Reference Instructions?

Answer: Memory reference instructions are CPU commands that involve direct interaction with main memory. These include:

• LOAD (transfer data from memory to register)
• STORE (transfer data from register to memory)
• Branch/jump instructions
  They typically use memory addresses as operands and require memory access during execution.

1e) What is meant by Instruction Cycle?

Answer: The instruction cycle is the basic operational process of a CPU that repeats continuously to execute programs. It consists of four phases:

1. Fetch – Retrieve instruction from memory
2. Decode – Interpret the instruction
3. Execute – Perform the operation
4. Store – Write back results (if needed)
   This cycle forms the fundamental timing framework for processor operation.

1f) Write Use of Interrupts

Answer: Interrupts serve several critical functions:

• Handle asynchronous external events (I/O device ready signals)
• Implement time-sharing through timer interrupts
• Manage hardware errors and exceptions
• Support debugging with breakpoints
• Enable efficient I/O operations without polling
  They improve CPU utilization by allowing concurrent processing while waiting for slower devices.

1g) What are CPU Registers?

Answer: CPU registers are small, ultra-fast storage locations built into the processor that hold:

• Data being processed (accumulator)
• Memory addresses (address registers)
• Instruction pointers (program counter)
• Status flags (condition codes)
• Stack pointers
  Common types include general-purpose, special-purpose, and floating-point registers. Their proximity to the ALU enables single-clock-cycle access.

1h) Discuss Virtual Memory

Answer: Virtual memory creates an illusion of larger memory space by combining RAM and disk storage. Key aspects:

• Uses paging/segmentation to map virtual to physical addresses
• Enables efficient multitasking via memory isolation
• Implements demand paging to load pages only when needed
• Managed by MMU (Memory Management Unit)
  Benefits include larger address spaces, memory protection, and simplified programming.

1i) Briefly Explain Array Processors

Answer: Array processors are parallel computing systems with multiple ALUs that perform simultaneous operations on data arrays. Characteristics:

• Single Instruction Multiple Data (SIMD) architecture
• Synchronous parallel processing
• Specialized for vector/matrix operations
• Used in graphics processing, scientific computing
  Examples include GPUs and historical systems like ILLIAC IV.

1j) List Advantages of Pipelining

Answer: Pipelining improves CPU performance by:

1. Increasing instruction throughput
2. Enabling concurrent execution of multiple instructions
3. Better hardware utilization (no idle units)
4. Higher clock rates achievable
5. Maintaining linear speedup for ideal conditions
   Though limited by hazards (structural, data, control), it’s fundamental to modern processors.

SECTION-B: Detailed Questions

2. Explain Different Arithmetic Operations in Computer Architecture

Answer: Computer architecture implements various arithmetic operations:

1. Fixed-Point Operations:

• Addition/Subtraction: Using 2’s complement adder-subtractors
• Multiplication: Sequential (shift-add) or parallel (array multipliers)
• Division: Restoring/non-restoring algorithms

2. Floating-Point Operations:

• Specialized FPUs handle IEEE 754 operations
• Normalization and denormalization steps
• Exception handling (overflow, underflow)

3. Logical Operations:

• Bitwise AND, OR, NOT, XOR
• Shifts (logical, arithmetic) and rotates

4. Decimal Arithmetic:

• BCD (Binary Coded Decimal) operations
• Used in financial applications

Modern processors use ALUs with dedicated circuits for these operations, often pipelined for performance.

3. Advantages and Disadvantages of Microprogrammed Design

Answer:

Advantages:

• Simplified control unit design (replaces complex circuitry)
• Flexible – instruction set can be modified
• Easier to implement complex instructions
• Supports emulation of other architectures
• Better error detection capabilities

Disadvantages:

• Slower than hardwired control (additional memory access)
• Higher latency due to interpretation overhead
• Requires more chip area for control store
• Limited by microinstruction cycle time
• Power consumption typically higher

Microprogramming dominated CISC designs while RISC processors favor hardwired approaches for speed.

4. What is DMA? Give an Example Where DMA is Useful

Answer:

Direct Memory Access (DMA) is a data transfer method where peripherals access memory directly without CPU intervention. A DMA controller handles the transfers, only interrupting the CPU when complete.

Example Use Case: High-speed disk I/O

• When reading a large file from SSD to RAM:

1. CPU sets up DMA controller with source/destination addresses
2. DMA controller manages data transfer disk → memory
3. CPU continues executing other tasks
4. DMA interrupts CPU upon completion

This prevents CPU stalling during lengthy transfers, crucial for real-time systems and high-performance storage.

5. Role of Cache Memory in Computer Architecture

Answer: Cache memory bridges the speed gap between CPU and main memory through:

Key Functions:

• Stores frequently accessed data/instructions (temporal locality)
• Holds adjacent memory locations (spatial locality)
• Reduces average memory access time
• Decreases bus contention

Architectural Impact:

• Enables higher CPU clock speeds
• Multi-level hierarchies (L1, L2, L3)
• Uses mapping techniques (direct, associative, set-associative)
• Implements replacement policies (LRU, FIFO)
• Requires coherency protocols in multiprocessors

Modern systems achieve 90%+ hit rates, making caches indispensable for performance.

6. Inter-Processor Communication and Synchronization

Answer: In multiprocessor systems, coordination occurs through:

Communication Methods:

• Shared memory (most common)
• Message passing (distributed systems)
• Hardware interrupts
• Network-on-Chip (NoC) in manycore

Synchronization Mechanisms:

• Locks/mutexes (test-and-set instructions)
• Semaphores
• Memory barriers
• Atomic operations
• Cache coherency protocols (MESI, MOESI)

Challenges include deadlock avoidance, latency minimization, and maintaining consistency across cache hierarchies.

SECTION-C: Comprehensive Questions

7. RISC vs CISC Architecture Comparison

Answer:

RISC (Reduced Instruction Set Computer):

• Fixed-length instructions (32-bit typical)
• Load/store architecture (memory only via dedicated instructions)
• Large register sets
• Single-cycle execution for most instructions
• Hardwired control
• Examples: ARM, MIPS, RISC-V

CISC (Complex Instruction Set Computer):

• Variable-length instructions
• Memory operands allowed in most instructions
• Complex, multi-cycle instructions
• Microprogrammed control often used
• Compact code density
• Examples: x86, VAX

Modern Convergence:

• RISC designs add some CISC features (ARM conditional execution)
• CISC implementations use RISC-like micro-ops (Intel since P6)
• Performance gap has narrowed significantly

8. Need for Peripheral Devices and Data Transfer Modes

Answer:

Peripheral Necessity:

• Interface between digital systems and analog world (sensors, displays)
• Persistent storage needs (disks, SSDs)
• User interaction (keyboards, touchscreens)
• Network connectivity

Data Transfer Modes:

1. Programmed I/O: CPU actively polls devices (simple but inefficient)
2. Interrupt-Driven: Devices signal when ready (better CPU utilization)
3. DMA: Direct memory access for bulk transfers
4. I/O Processors: Offload I/O management completely
5. Memory-Mapped I/O: Treat peripherals as memory locations
6. Isolated I/O: Separate address space for I/O (IN/OUT instructions)

Modern systems combine these approaches based on performance requirements.

9. Pipelining in Computer Organization

Answer:

Pipeline Fundamentals:

• Divides instruction processing into stages (fetch, decode, execute, etc.)
• Enables concurrent execution like assembly line
• Ideal speedup = number of stages (limited by hazards)

Speed Enhancement Mechanisms:

• Superpipelining: More stages at higher clock rates
• Superscalar: Multiple pipelines parallel
• Out-of-order execution: Dynamic scheduling
• Speculation: Predict branches to keep pipeline full

Practical Considerations:

• Pipeline stalls from hazards reduce theoretical speedup
• Requires forwarding/bypassing for data dependencies
• Branch prediction essential for control flow
• Deep pipelines increase branch misprediction penalties

Modern CPUs employ 10-20 stage pipelines with sophisticated mitigation techniques for 3-5x speedups over non-pipelined designs.

Keywords for SEO: computer architecture questions, accumulator logic, RISC vs CISC, pipelining benefits, cache memory function, DMA explained, register transfer language, virtual memory, array processors, CPU registers, interrupt handling, microprogrammed control, inter-processor communication, peripheral devices, instruction cycle, memory reference instructions