I/O Management in OS — DMA, Device Drivers, Scheduling

I/O Hardware

I/O devices communicate with the CPU through:

Ports: Connection points for I/O devices (USB, serial, parallel)
Buses: Shared communication pathways (PCI, USB, SATA, PCIe)
Device Controllers: Hardware components that manage specific devices. They have registers (data, status, control) that the CPU reads/writes.

I/O Techniques

Technique	Description	CPU Usage	Use Case
Programmed I/O (Polling)	CPU continuously checks device status (busy-waiting). CPU is fully occupied during I/O.	100% (wasteful)	Simple embedded systems
Interrupt-Driven I/O	CPU initiates I/O and continues other work. Device interrupts CPU when done.	Low (during I/O)	Most modern systems
DMA (Direct Memory Access)	DMA controller transfers data directly between device and memory without CPU involvement. CPU only involved at start and end.	Very Low	High-speed devices (disk, network)

Device Drivers

A device driver is software that provides a standard interface between the OS and a specific hardware device. It translates generic OS I/O requests into device-specific commands.

Runs in kernel mode (privileged)
Provided by device manufacturers or OS vendors
Abstracts hardware details from the OS
Examples: printer driver, graphics driver, network adapter driver

Disk Scheduling Algorithms

Disk scheduling determines the order in which disk I/O requests are serviced to minimize seek time (time for disk head to move to the correct track).

Example: Disk head at position 53. Requests: 98, 183, 37, 122, 14, 124, 65, 67

FCFS: Service in order of arrival. Simple but not optimal. Total movement: 640 cylinders.
SSTF (Shortest Seek Time First): Service the request closest to current head position. Reduces seek time but may cause starvation. Total movement: 236 cylinders.
SCAN (Elevator): Head moves in one direction, servicing requests, then reverses. Like an elevator. Total movement: 208 cylinders.
C-SCAN (Circular SCAN): Head moves in one direction only. When it reaches the end, it jumps back to the beginning without servicing. More uniform wait times.
LOOK: Like SCAN but head only goes as far as the last request in each direction (doesn't go to disk end).

Spooling and Buffering

Buffering: Temporary storage of data in memory while it's being transferred between devices or between a device and an application. Smooths out speed differences between producer and consumer.
Spooling (Simultaneous Peripheral Operations On-Line): Using disk as a buffer for I/O operations. Classic example: print spooling - print jobs are stored on disk and sent to the printer one at a time. Allows multiple processes to "print" simultaneously.