Unit IV: Instruction Set Architecture PDF

# Unit IV: Instruction Set Architecture **12/9/24, 7:39 PM** **Homeb.tech** Unit IV: Instruction Set Architecture | CSE211: COMPUTER ORGANIZATION AND DESIGN | B.Tech CSE | classpdfindia.com ## Unit IV: Instruction Set Architecture | CSE211: COMPUTER ORGANIZATION AND DESIGN | B.Tech CSE **by One...

# Unit IV: Instruction Set Architecture **12/9/24, 7:39 PM** **Homeb.tech** Unit IV: Instruction Set Architecture | CSE211: COMPUTER ORGANIZATION AND DESIGN | B.Tech CSE | classpdfindia.com ## Unit IV: Instruction Set Architecture | CSE211: COMPUTER ORGANIZATION AND DESIGN | B.Tech CSE **by One - October 18, 2024. 12 min read** ## Contents - Introduction to Instruction Set Architecture and Microcode: Architecture & Microcode, Machine Models, ISA characteristics, Pipeline - Architecture & Microcode - Instruction Set Architecture (ISA) - Defines the set of instructions a computer can execute. - Acts as the interface between hardware (processor) and software (programs). - Determines fundamental aspects like the instruction formats, addressing modes, registers, and data types. - Microcode - A lower-level set of instructions or signals used by complex instruction set computing (CISC) CPUs to implement higher-level instructions. - Translates complex machine instructions into simpler, sequenced microinstructions. - Microcode is stored in special, faster storage within the CPU (like ROM or RAM). - Machine Models - Von Neumann Model - A traditional architecture where instructions and data share the same memory and data bus. - Known for the Von Neumann Bottleneck, as the single data path for instructions and data can cause delays. - Characteristics include simplicity and ease of use for general-purpose computing. - Harvard Model - A design with separate memory and buses for instructions and data. - Allows simultaneous access to instructions and data, increasing performance and reducing potential bottlenecks. - Often used in digital signal processing and microcontrollers. - Modified Harvard Model - Combines features of both Von Neumann and Harvard models, sometimes allowing transfer between instruction and data memories. - Adds flexibility and is common in modern CPUs. - ISA Characteristics - Complexity and Design (RISC vs. CISC) - RISC (Reduced Instruction Set Computer): Simple, minimal instructions, focusing on a small set of efficient operations. - CISC (Complex Instruction Set Computer): Larger set of complex instructions, often with microcoded implementation for complex instructions. - Instruction Types and Formats - Instruction Types: Includes arithmetic, logical, control, and data movement instructions. - Formats: Varying instruction sizes (fixed or variable) and formats based on operation types. - Registers - Defines the types and numbers of registers (like general-purpose and floating-point). - Registers are typically limited in number, which affects the efficiency of the ISA. - Memory Addressing Modes - Different methods to access memory, such as immediate, direct, indirect, indexed, and base-plus-offset. - Determines the flexibility and ease with which programs can access data. - Data Types - Defines supported data types (integers, floating-point numbers, characters, etc.). - Affects the range of applications the ISA can handle and the precision of operations. - Pipeline - Basic Concept - A method of instruction execution that divides operations into multiple stages (e.g., fetch, decode, execute, memory access, and write-back). - Each instruction is processed in different stages simultaneously, allowing overlap and improving throughput. - Pipeline Stages - Fetch: Retrieves the instruction from memory. - Decode: Decodes the instruction and prepares required control signals. - Execute: Performs the operation (e.g., arithmetic or logic operation). - Memory Access: Reads or writes data from/to memory if needed. - Write-back: Stores the result in a register for later use. - Pipeline Hazards - Data Hazards: Occur when instructions depend on the results of previous ones that are still in the pipeline. - Control Hazards: Happen when the pipeline encounters branch instructions, potentially causing it to fetch the wrong instruction. - Structural Hazards: Arise when hardware resources are insufficient to execute instructions in parallel. - Advantages of Pipelining - Increased Throughput: More instructions are completed in a shorter time frame. - Efficient Utilization of CPU Resources: Reduces idle time by overlapping instruction execution stages. - Improved Performance: Enables higher instruction-per-cycle (IPC) execution, which improves overall CPU efficiency. - Review: Microcoded Microarchitecture, Pipeline Basics, Structural and Data Hazards - Microcoded Microarchitecture - Microarchitecture Overview - Refers to the internal architecture of the CPU, implementing the instructions defined by the ISA. - Consists of functional blocks like the ALU (Arithmetic Logic Unit), control unit, registers, and cache. - Role of Microcode - Microcode is a layer of low-level instructions that translates complex machine instructions into sequences of simpler micro-operations. - Microinstructions control each CPU function in sequence, enabling execution of complex instructions in CPUs, especially in CISC (Complex Instruction Set Computing) architectures. - Microprogram Control Unit - The part of the CPU that stores and manages microinstructions. - Microcode is typically stored in a small, high-speed memory (e.g., control store) within the CPU. - When an instruction is decoded, the control unit uses microcode to generate signals for various CPU components, guiding them through the steps required for that instruction. - Advantages - Allows easier implementation of complex instructions by breaking them into manageable steps. - Makes updating and extending instructions easier without changing the hardware, as changes can be made in microcode. - Disadvantages - Adds latency due to the additional layer between machine instructions and execution. - Increases complexity and may slow down execution speed, especially compared to RISC architectures that rely on fewer, simpler instructions. - Pipeline Basics - Concept of Pipelining - Pipelining divides the CPU's instruction cycle into multiple stages, enabling multiple instructions to be processed simultaneously. - Common pipeline stages include Fetch (IF), Decode (ID), Execute (EX), Memory Access (MEM), and Write-back (WB). - Stages of a Pipeline - Instruction Fetch (IF): Retrieves the next instruction from memory. - Instruction Decode (ID): Decodes the instruction to identify the operation and operands. - Execute (EX): Performs the operation, like an arithmetic or logic function - Memory Access (MEM): Accesses memory for read/write operations, if needed. - Write-back (WB): Writes results back to registers or memory. - Pipeline Depth - Refers to the number of stages in the pipeline. - More stages (deeper pipeline) can improve instruction throughput but may introduce more latency, especially if hazards occur. - Benefits of Pipelining - Higher Throughput: Enables more instructions to be completed in less time. - Efficient Resource Usage: Minimizes idle by utilizing each pipeline stage simultaneously. - Reduced Latency per Instruction: Each instruction is broken down into smaller, more manageable operations, allowing faster instruction cycles. - Challenges with Pipelining - Requires careful management to avoid stalling or inefficient use of resources. - Susceptible to hazards, which can stall the pipeline and reduce efficiency. - Pipeline Hazards - Structural Hazards - Definition: Occur when multiple instructions require the same hardware resources at the same time. - Examples - When two instructions simultaneously need to access the same memory or the same arithmetic unit (ALU). - A common problem in processors with limited resources or no duplicated functional units. - Solution: Resource duplication (e.g., multiple ALUs) or effective resource scheduling to reduce conflicts. - Data hazards - Definition: Arise when an instruction depends on the results of a previous instruction that has not yet completed its execution in the pipeline. - Types of Data Hazards - Read After Write (RAW): The current instruction depends on the result of a previous instruction still in the pipeline (most common). - Write After Read (WAR): Occurs if an instruction writes to a register or memory location before a previous instruction has read from it. - Write Aftera Write (WAW): Happens if two instructions write to the same register or memory location in the wrong order. - Solutions - Data Forwarding/Bypassing: Redirects the result of a previous instruction directly to the next one without waiting for it to complete. - Stalling: Pauses the pipeline until the required data becomes available. - Pipeline Interlocks: Control logic that automatically inserts stalls to prevent data hazards. - Cache Review: Control Hazards - Jump, Branch, Others - Cache Review - Purpose of Cache - A cache is a small, high-speed memory located close to the CPU that stores frequently accessed data and instructions. - Reduces the average time to access data from main memory, improving CPU efficiency and performance. - Cache Types - Instruction Cache (I-Cache): Stores instructions to reduce latency in instruction fetching. - Data Cache (D-Cache): Stores data to reduce latency in data access. - Unified Cache: A single cache that stores both instructions and data. - Cache Levels - L1 Cache: Closest to the CPU, extremely fast but small in size. - L2 Cache: Larger and slightly slower, provides data if not found in L1. - L3 Cache: Found in some processors, shared among cores, larger and slower than L1 and L2. - Cache Mapping Techniques - Direct-Mapped Cache: Each memory block maps to exactly one location in the cache. - Fully Associative Cache: Any memory block can be stored in any cache location. - Set-Associative Cache: A combination where each memory block maps to a set, and can be placed in any location within that set. - Cache Performance - Performance Metrics - Cache Hit: Occurs when the requested data is found in the cache, resulting in faster data retrieval. - Cache Miss: Occurs when the requested data is not found, requiring data retrieval from main memory. - Hit Rate: The percentage of memory accesses that result in cache hits. - Miss Rate: The percentage of memory accesses that result in cache misses, directly affecting performance. - Miss Penalty: The additional time required to fetch data from main memory on a miss. - Average Memory Access Time (AMAT): Calculated as AMAT = Hit Time + (Miss Rate × Miss Penalty) - Factors Affecting Cache Performance - Cache Size: Larger caches generally reduce miss rates but may increase access time. - Block Size: Determines the amount of data transferred on each cache miss. - Associativity: Higher associativity typically reduces conflict misses but increases search complexity. - Techniques to Improve Cache Performance - Prefetching: Anticipating which data will be needed soon and loading it into the cache. - Replacement Policies: Determines which cache line to replace on a cache miss (e.g., Least Recently Used (LRU), First-In-First-Out (FIFO)). - Two-Way In-Order Superscalar - Superscalar Architecture - A processor design where multiple instructions are issued and executed per clock cycle, increasing instruction-level parallelism. - Requires multiple execution units to allow simultaneous instruction processing. - In-Order Execution: - Instructions are issued and executed in the order they appear in the program, without reordering. - Simpler design but can result in pipeline stalls if dependencies between instructions are not managed well. - Two-Way Superscalar - Allows two instructions to be issued and executed simultaneously per clock cycle. - Benefits: Improves throughput by allowing multiple operations without complex out-of-order execution mechanisms. - Challenges: Requires careful handling of dependencies and may encounter stalls if both instructions need the same resources. - Fetch Logic and Alignment - Fetch Logic - The mechanism that retrieves instructions from memory and loads them into the pipeline. - Determines how quickly and efficiently instructions are fed to the processor, impacting overall performance. - In a superscalar architecture, fetch logic must retrieve multiple instructions per cycle to keep the pipeline filled. - Instruction Alignment - Ensures that instructions are correctly positioned in memory for fast and efficient retrieval. - Critical in superscalar processors, as instructions must align properly with execution units. - Challenges in Superscalar Fetching and Alignment: - Branch Instructions: Control hazards from branches can disrupt fetch logic, requiring branch prediction to maintain efficiency. - Alignment Hardware: Special hardware may be needed to handle misaligned instructions to maintain the dual-instruction fetch rate in a two-way superscalar processor. A graphic of a computer processor is shown on the page where the CPU is illustrated with microcode. This includes: * Micro-code * HAR (Hardware Register) * Jump tables * TMP * PC (Program counter) * IAR (Instruction Address Register) * MAR (Memory Address Register) * MDR (Memory Data Register) * IR (Instruction Register) * Memory A diagram of the pipeline, including the five stages is also illustrated. * IF (Instruction Fetch) * ID (Instruction Decode) * EX (Execute) * MEM (Memory Access) * WB (Write Back) A diagram depicting the five stages of instruction fetching in the cache is presented, showing how a cache memory interacts with a CPU. It's comprised of the following functions: * Instruction Fetch * Instruction Decode * Register Fetch * Execute * Address Calc. * Memory Access * Write Back It is a graphic depiction of data flow, where information is transferred between registers, memory, and the ALU via a series of buses.