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os

computer architecture

design and organization of a computer's core components

  • CPU (Central Processing Unit): executes instructions
    • Control Unit: directs operations of the processor
    • Arithmetic Logic Unit (ALU): Performs arithmetic and logic operations
    • registers: Small, fast storage locations inside the CPU for temporary data
  • Memory (RAM, cache, registers): stores data and programs
    • RAM (Random Access Memory): volatile memory used for storing data and instructions currently being used
    • Cache: Small, fast memory close to the CPU for frequently accessed data
    • Storage (e.g. SSD/HDD): non-volatile memory for long-term data storage
  • Input/Output systems: allow communication between computer and the outside world
    • e.g. keyboard, mouse, display, printer, network card

memory

  • primary: stores the OS, running programs and the data those programs use during computer usage

    • directly accessible by the CPU
    • fast and temporary
    • e.g. RAM, cache, registers

  • secondary: non-volatile memory used for long-term data storage

    • non-volatile: data is preserved when power is off
    • slower (compared to RAM)
    • cheaper (per GB)
    • e.g. HDD, SSD, USB flash drives

  • how they work together:

    • when running a program:
      • the program is loaded from secondary memory into RAM
      • the CPU accesses the data from RAM, but it first pulls frequently used data into cache and ultimately into registers for immediate use
      • frequently used data may even go into cache for faster access

memory hierarchy

the closer to the cpu, the faster and more expensive the memory

speed and cost: NVM (SSD/HDD) < DRAM (RAM) < SRAM (Cache) < Registers

types of memory technology

  • DRAM (Dynamic RAM)
    • used in main memory (RAM sticks in computer)
    • volatile: loses data when power is off
    • slower than SRAM, but cheaper
    • needs refresh: stores data using capacitors that leak charge, so it must be refreshed thousands of times per second
  • SRAM (Static RAM)
    • used in CPU cache (L1, L2, L3)
    • volatile
    • faster than DRAM
  • NVM (Non-Volatile Memory)
    • used in SSDs, USB drives, BIOS/firmware
    • non-volatile
    • slower than DRAM/SRAM, but persistent

speed comparison: NVM is slower than DRAM is slower than SRAM

system buses

transfer data between components

instruction cycle (fetch-decode-execute)

The CPU executes programs by repeating this cycle:

  1. fetch: get the instruction from memory
  2. decode: interpret the instruction
  3. execute: perform the operation

Instruction Set Architecture (ISA)

defines the machine language understood by a CPU

examples: x86, ARM, RISC-V

  • defines:
    • supported instructions (e.g. add, load, jump)
    • data types
    • registers
    • memory addressing modes

performance factors

  • Clock speed (GHz): Determines how many cycles per second the CPU can perform
  • CPI (Cycles Per Instruction): A measure of how efficiently instructions are executed
  • Pipelining: Overlaps instruction stages to improve throughput
  • Parallelism: Uses multiple cores or threads for simultaneous operations

von neumann architecture

computer design model that forms the foundation for most modern computers

  • key features
    • single memory for data and instructions
      • both program instructions and data are stores in the same memory space
      • this allows the CPU to fetch and execute instruction sequentially
    • Central Processing Unit (CPU)
      • Arithmetic Logic Unit (ALU)
      • Control Unit (CU)
    • input/output system
    • sequential execution of instructions
    • program stored in memory
  • innovations
    • programs can be written, stored and modified electronically
      • no need to rewire circuits for each task
      • previously, computers were hardwired to perform specific tasks
    • a computer can load and run different programs without hardware changes
    • programming languages and compilers became possible
    • software can be debugged, improved and reused

von neumann architecture

  • Memory: where data and instructions are stored, it allows for the storage and retrieval of information
  • Control Unit: manages the operations of the computer, it directs the flow of data between the CPU and other components
  • Arithmetic Logic Unit (ALU): performs arithmetic and logical operations
    • responsible for calculations and decision-making processes
  • Input: This refers to the devices or methods through which data is entered into the computer system
  • Output: This refers to the devices or methods through which data is presented to the user or other systems
  • Central Processing Unit (CPU): central component that carries out the instructions of a computer program
    • contains the ALU and Control Unit
  • Accumulator: This is a register in the CPU that stores intermediate results of arithmetic and logic operations
  • firmware: software that is stored in hardware and tells it how to work
    • e.g. BIOS or UEFI (starts boot process in computers)