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I/O Systems

Chapter 13

I/O Systems

• I/O Hardware
• Application I/O Interface
• Kernel I/O Subsystem
• Transforming I/O Requests to Hardware Operations
• Streams
• Performance

I/O Hardware

• Incredible variety of I/O devices
• Common concepts
  • Port
  • Bus ( daisy chain or shared direct access)
  • Controller ( host adapter )
• I/O instructions control devices
• Devices have addresses, used by
  • Direct I/O instructions
  • Memory-mapped I/O

A Typical PC Bus Structure

Polling

• Determines state of device
  • command-ready
  • busy
  • Error
• Busy-wait cycle to wait for I/O from device

Interrupts

• CPU Interrupt-request line triggered by I/O device
• Interrupt handler receives interrupts
• Maskable to ignore or delay some interrupts
• Interrupt vector to dispatch interrupt to correct handler - Based on priority - Some nonmaskable
• Interrupt mechanism also used for exceptions

Intel Pentium Processor Event-Vector Table

Direct Memory Access

• Used to avoid programmed I/O for large data movement
• Requires DMA controller
• Bypasses CPU to transfer data directly between I/O device and memory

Application I/O Interface

• I/O system calls encapsulate device behaviors in generic classes
• Device-driver layer hides differences among I/O controllers from kernel
• Devices vary in many dimensions
  • Character-stream or block
  • Sequential or random-access
  • Sharable or dedicated
  • Speed of operation
  • read-write, read only, or write only

A Kernel I/O Structure

Block and Character Devices

• Block devices include disk drives
  • Commands include read, write, seek
  • Raw I/O or file-system access
  • Memory-mapped file access possible
• Character devices include keyboards, mice, serial ports - Commands include get, put - Libraries layered on top allow line editing

Network Devices

• Varying enough from block and character to have own interface
• Unix and Windows NT/9 x /2000 include socket interface - Separates network protocol from network operation - Includes select functionality
• Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes)

Blocking and Nonblocking I/O

• Blocking - process suspended until I/O completed
  • Easy to use and understand
  • Insufficient for some needs
• Nonblocking - I/O call returns as much as available
  • User interface, data copy (buffered I/O)
  • Implemented via multi-threading
  • Returns quickly with count of bytes read or written
• Asynchronous - process runs while I/O executes
  • Difficult to use
  • I/O subsystem signals process when I/O completed

Two I/O Methods

Synchronous Asynchronous