I/O Systems | Computer Science

------------- | -------------------------- | -------------- | | Human-readable | Keyboard, screen, printer | Low |“

7.1 I/O Hardware

Category	Examples	Data rate
Human-readable	Keyboard, screen, printer	Low
Machine-readable	Disk, tape, sensors	Medium to high
Communication	Network interfaces, modems	Variable

7.2 Device Drivers

A device driver is kernel-mode software that translates OS I/O requests into device-specific Operations. It provides a uniform interface to the OS while hiding hardware details.

Driver architecture (layered):

User application
       │
   System call interface (read, write, ioctl)
       │
   File system / block layer
       │
   Device driver (kernel module)
       │
   Device controller (hardware)

Types:

Block device drivers: Manage fixed-size block access (disks, SSDs). Use a buffer cache to reduce physical I/O. The OS can reorder and merge block requests for performance.
Character device drivers: Manage byte-stream access (terminals, serial ports). No buffering or reordering by the OS.

Driver loading. Most modern OSes support loadable kernel modules (LKMs): drivers loaded at Runtime without rebooting. On Linux: insmod``modprobe. This allows third-party hardware support Without kernel recompilation.

7.3 I/O Scheduling

I/O schedulers reorder and merge requests to improve performance:

FCFS: Simple but may cause starvation.
SSTF: Service the request closest to the current head position. Minimises seek but can starve distant requests.
SCAN (elevator): Head moves in one direction servicing requests, then reverses.
C-SCAN: Like SCAN but only services in one direction; returns without servicing.
LOOK / C-LOOK: Optimised SCAN/C-SCAN that reverses when no requests remain ahead.

Theorem 7.1. C-SCAN provides more uniform wait times than SCAN.

Proof. SCAN services requests in both directions, so requests at the extremes of the disk wait Longer. C-SCAN treats the disk as a circular queue, ensuring every request is serviced within one Full sweep. $\blacksquare$

Worked Example 7.1 — Disk Scheduling Comparison

Disk with 200 cylinders (0—199). Request queue (sorted): 98, 183, 37, 122, 14, 124, 65, 67. Current head position: 53, moving toward higher cylinders.

FCFS: 53 → 98 → 183 → 37 → 122 → 14 → 124 → 65 → 67. Total seek = $\lvert 53-98 \rvert + \lvert 98-183 \rvert + \lvert 183-37 \rvert + \lvert 37-122 \rvert + \lvert 122-14 \rvert + \lvert 14-124 \rvert + \lvert 124-65 \rvert + \lvert 65-67 \rvert = 45 + 85 + 146 + 85 + 108 + 110 + 59 + 2 = 640$ cylinders.

SSTF: 53 → 65 → 67 → 37 → 14 → 98 → 122 → 124 → 183. Total seek = $12 + 2 + 30 + 23 + 84 + 24 + 2 + 59 = 236$ cylinders.

SCAN (elevator, moving right): 53 → 65 → 67 → 98 → 122 → 124 → 183 → 199 → 37 → 14. Total seek = $12 + 2 + 31 + 24 + 2 + 59 + 16 + 162 + 23 = 331$ cylinders.

C-SCAN (moving right): 53 → 65 → 67 → 98 → 122 → 124 → 183 → 199 → 0 → 14 → 37. Total seek = $12 + 2 + 31 + 24 + 2 + 59 + 16 + 199 + 14 + 23 = 382$ cylinders.

LOOK (stops at last request): 53 → 65 → 67 → 98 → 122 → 124 → 183 → 37 → 14. Total seek = $12 + 2 + 31 + 24 + 2 + 59 + 146 + 23 = 299$ cylinders.

Algorithm	Total Seek	Max Wait	Fairness
FCFS	640	335	Fair
SSTF	236	130	Starves
SCAN	331	313	Good
C-SCAN	382	329	Best
LOOK	299	299	Good

SSTF minimises total seek but starves requests at the extremes. C-SCAN provides the most uniform Wait time at the cost of higher total seek.

7.4 Direct Memory Access (DMA)

DMA allows I/O devices to transfer data directly to/from memory without per-word CPU Intervention.

Without DMA: CPU reads each word from the device controller to memory. For a 4 KiB read, the CPU Is interrupted 4096 times.

With DMA: CPU programs the DMA controller with source, destination, and count. DMA handles the Transfer, interrupting the CPU only when complete.

DMA transfers are cycle-stealing: the DMA controller uses the system bus, pausing the CPU.
Modern systems use bus mastering: the I/O device controller performs the transfer autonomously.

7.5 Common Pitfalls

Confusing polling (busy-waiting) with interrupt-driven I/O. Polling wastes CPU cycles; interrupts are preferred for low-frequency devices.
Assuming DMA eliminates all CPU overhead. The CPU must still set up DMA descriptors and handle the completion interrupt.
Forgetting that I/O schedulers only apply to block devices. Character devices use FIFO ordering.
Mixing up virtual and physical addresses when programming DMA controllers. The DMA controller operates on physical addresses.

flowchart TD
    A[7_I O Systems] --> B[Key Concepts]
    A --> C[Core Principles]
    A --> D[Practical Applications]
    B --> E[Fundamental definitions]
    C --> F[Design patterns]
    D --> G[Real-world usage]

Intuition

I/O systems are the nervous system connecting the CPU to the outside world. Polling is like checking the mailbox every minute — simple but wasteful. Interrupts are like a doorbell — the device signals when it needs attention, and the CPU drops what it is doing to respond. DMA is like hiring a delivery service — the device transfers data directly to memory without bothering the CPU for every byte. Buffering is the waiting room — data is collected in a holding area before being processed in bulk, smoothing out the mismatch between fast producers and slow consumers.

Cross-References

Synchronisation: I/O synchronisation and interrupt handling.
File Systems: File system I/O operations.
Process Management: Process scheduling and I/O wait states.
Discrete Mathematics
Algorithm Implementation

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