I/O Subsystem

I/O Subsystem

1. I/O Hardware

   • I/O devices
     • Storage
     • Transmission
     • Human Interface
   • Controller
     • Operates ports, bussess, and devices. Contains processor, microcode, private memory, bus controller, etc.
   • Typical PC bus structure
     • OBS.: Chipset is responsible for controlling the PCIe bus.
   • Device Registers
     • Devices usually have registers where device driver places commands, addresses, and data to write or read.
     • Data-in register
     • Data-out register
     • Status Register
     • Control register
   • Device address
     • Device data and command registers mapped to processor address space.

2. Mechanism to wait for I/O

   • Polling
     • Apply the concept of busy-wait.
     • Reasonable for fast devices, but inefficient for slow devices
   • Interrupt
     • CPU interrupt-request line is triggered by I/O device. Processor checks for interruption after each clock cycle. It avoids the busy-wait necessary in polling technique.
     • Interrupt Handler
       • Receives interrupts. It the interrupt is maskable, it can be delayed or ignored.
     • Interrupt Vector
       • Dispatches interrupt to correct handler.
     • Interrupt-driven I/O cycle
     • Used for exceptions
     • Trap
       • Executed by system call to trigger kernel to execute the request. Interrupt generated by software.

3. Direct Memory Access (DMA)

   • Mechanism to bypass CPU allowing direct data transfer between I/O device and memory.
   • It requires a DMA controller
   • Steps

4. Characteristics of I/O devices

   • Types
     • Block I/O
     • Character I/O (stream)
     • Memory-mapped file access
     • Network socket
   • Block and Character devices
     • Block
       • Commands
         • read
         • write
         • seek
       • DMA
       • Memory-mapped file access possible
       • Ex. Disk drives
     • Character
       • Commands
         • get
         • put
       • Ex. keyboard, mouse, serial ports
   • Network devices
     • may transfer blocks or characters, depending on the characteristics of the network device
     • Ex. Socket interface
   • Clocks and Timers
     • Provide current time, elapsed time.
     • Programmable intercval timer
       • Used for timings or periodic interrupts

5. blocking/nonblocking/asynchronous I/O operation protocol

   • Blocking
     • The process is suspended until I/O is completed.
     • Blocking
   • Nonblocking
     • Small blocking operations. Partial results are returned as long as they are available.
       • select() to check if data is ready then read and write to transfer that data.
   • Asynchronous
     • Process keep running while the I/O operation is executed. After finishing i/O operation, the hardware send interruption and the I/O result is handled.
     • Asynchronous

6. Vectored I/O

   • Ex. UNIX readve() accepts a vector of multiple buffers to read/write.
   • Allows one system call to perform multiple I/O operations
   • Better than multiple individual I/O operations
     • Avoid context switching
     • Less system call overhead

7. Kernel

   • Kernel I/O structure
   • Kernel I/O subsystem
     • Scheduling
     • Buffering
       • Store data in memory while transferring between devices
       • deal with
         • speed mismatch
         • size mismatch
       • Double buffering
         • Two copies of the data
           • Kernel and user, Varying sizes, etc.
     • Device Status Table
     • Caching
       • Improve performance
     • Spooling
       • Hold output for a device
     • Device reservation
     • Error Handling
       • OS can recover from read/write failures, device unavailable.
       • Actions
         • Retry read/write
         • Return error number or code when I/O request fails
         • Sendo error report to system error logs.
     • I/O protection
       • I/O instructions defined to be privileged
       • I/O must be performed via system calls
     • Power Management
       • I/O related
       • Very important in mobile computing
         • Ex. Android system
           • Component-level power management
             • Understands relationship between components
             • Device tree representing physiucal device topology
               • Turn off unused branches
             • Turn off unused components
           • Power collapse
             • Put device into deep sleep.
       • Advanced Configuration and Power Interface (ACPI)
         • Firmware that provides code that runs routines called by kernel for device discovery, error and power management.
   • Kernel Data Structures
     • Kernel keeps state info for I/O components, including open file tables, network connections, character device state. It also has data structures to keep track of buffers, memory allocation, "dirty" blocks, etc.
     • Unix kernel I/O data structure

8. Kernel I/O subsystem summary

   • Management of the name space for files and devices
   • Access control to files and devices
   • Operation control
   • File system space allocation
   • Device allocation
   • Buffering, caching, and spooling
   • I/O scheduling
   • Device-status monitoring, error handling, and failure recovery
   • Device-driver configuration and initialization
   • Power management of I/O devices

9. I/O request to hardware operations

   • 1. Determine the device holding file;
     • 2. Translate name to device representation;
       • 3. Physically read data from disk into buffer;
         • 4. Make data available to requesting process;
           • 5. Return controll to process;

10. Life cycle of an I/O request

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