Unit 5: Processor Instructions & Processor Enhancements

1. Instruction Basics

Elements of Machine Instruction

A machine instruction is the most basic command in a computer system that the CPU can execute. Each instruction is composed of the following elements:

• Opcode: Specifies the operation to be performed (e.g., addition, subtraction, data transfer).
• Operands: The data on which the operation is performed.
• Addressing Mode: Specifies how to interpret the operands (e.g., direct, indirect).

Instruction Representation

Machine instructions are typically represented using opcodes and mnemonics.

• Opcode: The numerical code that represents a specific instruction in machine language (e.g., 1101 for addition).
• Mnemonics: Human-readable abbreviations used in assembly language (e.g., ADD for addition, MOV for data movement).
• Assembly Language Elements: Assembly language includes mnemonics, registers, memory locations, and labels, providing a user-friendly way to write low-level code.

Instruction Format

Instruction formats define how bits are allocated to the different parts of an instruction (opcode, operands, addressing mode). Formats vary by the number of addresses used:

• 0-address format: No operands are explicitly specified. Operations rely on an implicit stack.
• 1-address format: A single operand, typically using an accumulator.
• 2-address format: Two operands are specified.
• 3-address format: Three operands are explicitly defined, commonly seen in modern processors.

Types of Operands

Operands can represent different data types or locations, such as:

• Immediate: The operand is a constant value.
• Register: The operand is stored in a register.
• Memory: The operand is located in a memory address.

2. Addressing Modes

Addressing modes define how to interpret the operands in an instruction, which influences how the CPU fetches data. Common addressing modes include:

• Immediate Addressing: The operand is a constant value.
• Direct Addressing: The operand is located in memory, and the address is specified directly.
• Indirect Addressing: The operand’s memory address is stored in a register or another memory location.
• Register Addressing: The operand is stored in a CPU register.
• Indexed Addressing: The operand’s address is determined by adding a base address and an offset.
• Relative Addressing: The operand is accessed relative to the current program counter.

3. Instruction Types Based on Operations

Instructions can be classified based on the type of operations they perform. Common types include:

• Data Transfer Instructions: Move data between memory and registers (e.g., MOV, LOAD, STORE).
• Arithmetic Instructions: Perform mathematical operations (e.g., ADD, SUB, MUL, DIV).
• Logical Instructions: Carry out logical operations (e.g., AND, OR, NOT, XOR).
• Control Instructions: Manage the sequence of execution (e.g., JMP, CALL, RET).
• Input/Output Instructions: Handle data exchange with peripherals (e.g., IN, OUT).

4. RISC vs CISC

RISC (Reduced Instruction Set Computer)

• Key Characteristics:
  • Small, simple instruction set.
  • Each instruction performs a simple task.
  • Instruction execution in a single clock cycle.
  • Focus on optimizing pipelining.
• Examples: ARM architecture.

CISC (Complex Instruction Set Computer)

• Key Characteristics:
  • Large, complex instruction set.
  • Single instructions can perform complex tasks.
  • Multiple clock cycles for some instructions.
  • Focus on minimizing the number of instructions per program.
• Examples: x86 architecture.

5. Interrupts

Purpose and Types of Interrupts

An interrupt temporarily halts the CPU’s current operations and redirects it to execute a specific piece of code (Interrupt Service Routine or ISR) to handle an event. Types of interrupts include:

• Hardware Interrupts: Triggered by external devices (e.g., keyboards, timers).
• Software Interrupts: Generated by software (e.g., system calls).
• Exceptions: Interrupts caused by errors or unexpected conditions (e.g., divide by zero).

Interrupt Handling (ISR, Multiple Interrupts)

• ISR (Interrupt Service Routine): A specialized function that runs when an interrupt occurs. It handles the interrupt and returns control to the main program.
• Multiple Interrupts: Multiple interrupts can occur simultaneously. These are handled using priority levels or interrupt vectors to determine the order of servicing.

Exceptions

Exceptions are specific types of interrupts that occur during the execution of a program due to abnormal conditions like divide-by-zero or invalid instructions. The processor halts and transfers control to an exception handler.

6. Instruction Pipelining

Operation of Pipelining

Instruction pipelining divides instruction execution into discrete stages, allowing multiple instructions to be processed concurrently. This improves performance by overlapping the execution of multiple instructions.

• Stages: Typical pipeline stages include instruction fetch (IF), instruction decode (ID), execute (EX), memory access (MEM), and write-back (WB).

Speed-Up

Pipelining increases CPU performance by allowing multiple instructions to be executed in parallel, reducing idle CPU time. The speed-up depends on factors like the number of pipeline stages and how well they are balanced.

7. Multiprocessor Systems

Taxonomy of Parallel Processor Architectures

Parallel processor architectures can be classified based on how they handle memory and processes. The Flynn’s Taxonomy includes:

• SISD (Single Instruction, Single Data): Traditional sequential architecture.
• SIMD (Single Instruction, Multiple Data): Executes the same instruction on multiple data streams.
• MISD (Multiple Instruction, Single Data): Rarely used, multiple instructions operate on a single data stream.
• MIMD (Multiple Instruction, Multiple Data): Different instructions executed on multiple data streams, commonly used in modern processors.

MIMD Clusters and SMP

• MIMD Clusters: Multiple processors working on different instructions and data sets. Two main types:
  • Distributed Memory Clusters: Each processor has its own memory.
  • Shared Memory Clusters: All processors share a common memory.
• SMP (Symmetric Multiprocessing): A type of MIMD architecture where all processors share the same memory and are treated equally. Benefits include easier management and better performance for multi-threaded tasks.

Multicore Processor

A multicore processor integrates multiple processing units (cores) onto a single chip, allowing for parallel execution of programs.

• Alternatives: Different designs include homogeneous multicore (all cores are identical) and heterogeneous multicore (different types of cores optimized for different tasks).
• Advantages: Increased parallelism, improved performance, and power efficiency.

Intel Core i7 Features

The Intel Core i7 is a popular multicore processor that includes several advanced features:

• Hyper-Threading: Allows a single core to execute multiple threads simultaneously.
• Turbo Boost: Dynamically increases the clock speed when more performance is needed.
• Integrated GPU: Some models come with an integrated graphics processing unit.
• Cache Hierarchy: Includes multiple levels of cache (L1, L2, L3) to optimize data access speeds.

Conclusion

Understanding processor instructions and processor enhancements is key to optimizing the performance and efficiency of modern computer systems. From instruction formats and addressing modes to multiprocessor architectures and multicore systems, these concepts allow for deeper insight into how data is processed and managed at the hardware level.