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36 - IO devices.md

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  1. Introduction:
    • Importance of I/O in computer systems
    • Challenge: Efficient integration of I/O into systems
  2. System Architecture:
    • Components: CPU, memory, I/O bus, peripheral bus
    • Hierarchical structure due to physics and cost
    • Modern systems: specialized chipsets and point-to-point interconnects
  3. Canonical Device:
    • Hardware interface (registers) and internal structure
    • Interface: status, command, and data registers
    • Protocol: poll status, write data, write command, poll status for completion
  4. Inefficiencies in Canonical Protocol:
    • Problem: Polling wastes CPU time
    • Solution: Interrupts
      • Allow CPU to switch tasks while waiting for I/O
      • Enable overlap of computation and I/O
    • Note: Interrupts not always better (fast devices, interrupt floods)
  5. Programmed I/O (PIO) Inefficiency:
    • Problem: CPU wastes time on data transfer
    • Solution: Direct Memory Access (DMA)
      • Offloads data transfer from CPU
      • DMA engine handles transfers, interrupts CPU when done
  6. Methods of Device Interaction:
    • Explicit I/O instructions (e.g., in/out on x86)
    • Memory-mapped I/O: device registers as memory locations
    • Both methods in use today
  7. Device Drivers:
    • Problem: Keeping OS device-neutral
    • Solution: Device drivers encapsulate device-specific details
    • Example: Linux file system stack
    • Downside: Generic interfaces may not use all device capabilities
    • Note: Device drivers are a large portion of OS code (70%+ in Linux)
  8. Case Study: IDE Disk Driver
    • IDE disk interface: control, command block, status, error registers
    • Basic protocol: wait for readiness, write parameters, start I/O, handle data/interrupts
    • xv6 IDE driver functions:
      • ide_rw(): queue or issue request
      • ide_start_request(): send request to disk
      • ide_wait_ready(): ensure drive readiness
      • ide_intr(): handle interrupts
  9. Historical Notes:
    • Interrupts and DMA introduced in early 1950s
    • Exact origins debated (UNIVAC, DYSEAC, IBM SAGE)
    • Ideas arose from need to handle slow I/O devices with fast CPUs
  10. Summary:
    • Interrupts and DMA improve device efficiency
    • Device access: I/O instructions or memory-mapped I/O
    • Device drivers make OS device-neutral