Input/Output Management

• manage various input/output (I/O) devices, including

  • mouse

  • keyboards

  • touch pad

  • disk drives

  • display adapters

  • USB devices

  • Bit-mapped screen

  • LED

  • Analog-to-digital converter

  • On/off switch

  • network connections

  • audio I/O

  • printers

• An I/O system is required to take an application I/O request and send it to the physical device, then take whatever response comes back from the device and send it to the application

• I/O devices can be divided into two categories:

  • Block devices:

    • A block device is one with which the driver communicates by sending entire blocks of data.

    • For example, hard disks, USB cameras, Disk-On-Key, and so on.

  • Character Devices:

    • A character device is one with which the driver communicates by sending and receiving single characters (bytes, octets).

    • For example, serial ports, parallel ports, sounds cards, and so on.

• The CPU must have a way to pass information to and from an I/O device.

  • Special Instruction I/O

    • This uses CPU instructions that are specifically made for controlling I/O devices.

    • These instructions typically allow data to be sent to an I/O device or be read from an I/O device.

  • Memory-mapped I/O

    • When using memory-mapped I/O, the same address space is shared by memory and I/O devices.

    • The device is connected directly to certain main memory locations so that the I/O device can transfer block of data to/from the memory without going through the CPU.

    • While using memory mapped I/O, the OS allocates buffer in the memory and informs the I/O device to use that buffer to send data to the CPU.

    • The I/O device operates asynchronously with the CPU, and interrupts the CPU when finished.

    • The advantage to this method is that every instruction which can access memory can be used to manipulate an I/O device. Memory-mapped I/O is used for most high-speed I/O devices like disks and communication interfaces.

  • Direct memory access (DMA)

    • Slow devices like keyboards will generate an interruption to the main CPU after each byte is transferred. If a fast device, such as a disk, generated an interruption for each byte, the operating system would spend most of its time handling these interruptions. So a typical computer uses direct memory access (DMA) hardware to reduce this overhead.

    • Direct Memory Access (DMA) means the CPU grants the I/O module authority to read from or write to memory without involvement.

    • The DMA module itself controls the exchange of data between the main memory and the I/O device. The CPU is only involved at the beginning and end of the transfer and interrupted only after the entire block has been transferred.

    • Direct Memory Access needs special hardware called a DMA controller (DMAC) that manages the data transfers and arbitrates access to the system bus.

      • The controllers are programmed with source and destination pointers (where to read/write the data), counters to track the number of transferred bytes, and various settings. These include the I/O and memory types and the interruptions and states for the CPU cycles.