How I/O Systems Work โ 10 Minutes of OS, Day 12
๐
Before getting started ๐
Computers cannot function properly without input devices such as mice and keyboards and output devices such as monitors and printers.
The
I/O system
exchanges information between these devices and the operating system.
This article explains I/O hardware and how the OS controls and manages it, using practical examples.What is I/O hardware? ๐
I/O hardware includes every device that communicates with the outside of the computer.
Typical examples include:
- Input devices: Keyboards, mice, and scanners.
- Output devices: Monitors, printers, and speakers.
- Storage devices: Hard drives, SSDs, and USB drives.
- Communication devices: Network cards and Bluetooth modules.
These devices connect to the OS through
controllers
.
A controller manages a device's actual operation and acts as an intermediary transferring data to and from the CPU and memory.The kernel I/O subsystem ๐
Rather than controlling every device directly, the kernel manages them through
drivers
and the I/O subsystem
.Components ๐
- Device driver: A program controlling specific hardware, such as a printer driver.
- Buffering and caching: Store and organize data in memory for faster processing.
- Interrupt handling: Notify the CPU that a device has completed its work.
- DMA (Direct Memory Access): Transfer data directly between a device and memory without CPU involvement.
Example flow ๐
When a user prints a document:
- The program makes a system call.
- The kernel forwards the request to the printer driver.
- The driver sends commands to the printer controller.
- An interrupt signals completion to the CPU.
Applications and I/O interfaces ๐
Applications request I/O from the operating system rather than handling it directly.
These requests take the form of
system calls
.Examples include read(), write(), open(), and close().
The OS passes the request to the appropriate driver and returns the result to the application.
Developers can therefore perform I/O without knowing the device's internal details.
What is STREAMS? ๐
UNIX-like systems use a structure called
STREAMS
to organize I/O flows.- Connect several I/O modules into a stream.
- Modules exchange data, with each able to process it independently.
- Used for networking, terminal I/O, and related tasks.
Like connecting filters along a water pipe, this makes I/O processing flexible.
I/O and performance ๐
I/O strongly affects overall system speed.
Techniques for improving it include:
- Asynchronous I/O: Continue with the next task without waiting after submitting a request.
- Buffering and caching: Compensate for differences between I/O and CPU speeds.
- Interrupt-driven processing: Let devices signal completion instead of polling.
- Block I/O: Process multiple data items together.
Example I/O scheduling layers ๐
Operating systems use queues at several layers to handle requests efficiently.
- Request queue: Requests waiting in front of a device driver.
- Scheduling algorithm: Disk algorithms such as FIFO, SSTF, LOOK, and C-LOOK.
- Merging: Combine adjacent requests to improve processing speed.
These techniques use system resources efficiently.
Summary ๐
We examined the basic structure of I/O hardware and how operating systems control it.
It is important to understand how system calls, drivers, DMA, and STREAMS connect in a concrete flow.
The next post covers
file-system interfaces
.References ๐
- Operating System Concepts (10th Edition)
- https://en.wikipedia.org/wiki/I/O_schedulingโ
- https://www.geeksforgeeks.org/i-o-scheduling-in-operating-systems/โ
