[OS] Basics of OS ( I/O Structure)

• Storage is only one of many types of I/O devices within a computer

• A large portion of operating system code is dedicated to managing I/O, both becasuse of its importance to the reliability and performance of a system and because of the varying nature of the devices

  → Relibility and Performance of System greatly depends upon "How its input and output are managed in varying nature of the devices"

• A gerneral-purpose computer system consists of CPUs and multiple device controllers that are connected through a common bus

• Each device controller is in charge of a specific type of device

 

Device Controller

maintains

• Local Buffer Storage
• Set of Special Purpose Registers

 

Device Driver

• Typically, operating systems have a devcie driver for each device controller

• This device driver understands the device controller and presents a uniform interface to the device to the rest of the operating system

  → devices are very different that have a varying nature

• To start an I/O operation, the device driver loads the appropriate registers within the device controller

  → Device controller maintains Set of registers, so these registers are requred for performing that particular input/output operation

• The device controller, in turn, examines the contents of these registers to determine what action to take

• The controller starts the transfer of data from the devcie to its local buffer

• Once the transfer of data is complete, the device controller informs the device driver via an interrupt that it has finished its operation

• The device driver then returns control to the operating system

❗This form of interrupt-driven I/O is fine for moving small amounts of data but can produce high overhead when used for bulk data movement


To solve this problem, Direct Memory Access (DMA) is used

 

DMA

• After setting up buffers, pointers, and counters for the I/O device, the deivce controller transfers an entire block of data directly to or from its own buffer storage to memory, with no intervention by the CPU
• Only one interrupt is generated per block, to tell the device driver that the operation has completed ( ↔ for being transfered via CPU is interrupt per byte of data so many times )
• While the device controller is performing these operations, the CPU is available to accomplish other works

 

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