I/O Systems

I/O (Input/Output) Systems manage the communication between the computer and external devices. Understanding I/O is crucial for system performance and device management.

Overview

The I/O subsystem consists of:

• I/O Hardware - Physical devices and their controllers
• I/O Software - Device drivers and OS I/O modules
• I/O Interface - Standard methods for device communication

I/O Hardware

Components

Device Controllers

• Interface between device and computer
• Contains registers: data, status, command
• Converts serial bit stream to block of bytes
• Performs error detection/correction

Device Registers

I/O Communication Methods

1. Programmed I/O (Polling)

• CPU continuously checks device status
• CPU explicitly transfers each byte
• Simple but wastes CPU time

while (status != READY) {
    // Busy wait - check status
}
read_data();

Characteristics:

• CPU is busy waiting
• Simple implementation
• Inefficient for slow devices
• Used for fast, predictable devices

2. Interrupt-Driven I/O

• Device notifies CPU when ready via interrupt
• CPU can do other work while waiting
• More efficient than polling

Interrupt Handling Steps:

1. Device raises interrupt
2. CPU finishes current instruction
3. CPU acknowledges interrupt
4. CPU saves state (PC, registers)
5. CPU jumps to interrupt handler
6. Handler processes interrupt
7. CPU restores state
8. Resume interrupted process

3. Direct Memory Access (DMA)

• Specialized hardware transfers data directly to/from memory
• CPU only involved at start and end
• Best for large data transfers

DMA Transfer Steps:

1. CPU initiates DMA transfer (start address, count, direction)
2. DMA controller takes over bus
3. DMA transfers data block
4. DMA sends interrupt when complete
5. CPU handles completion
   *DMA Modes:

   Mode	Description
   Burst Mode	DMA holds bus until transfer complete
   Cycle Stealing	DMA takes bus for one cycle at a time
   Transparent	DMA uses bus only when CPU doesn't need it

I/O Software Layers

Device Drivers

• Software that controls specific device
• Translate OS requests to device commands
• Handle device-specific details
• Part of kernel (usually)

Device-Independent Software

Functions:

• Uniform interface to device drivers
• Buffering
• Error reporting
• Allocating/releasing devices
• Device-independent block size

Buffering

Temporary storage area for data being transferred.

Why Buffering?

1. Speed Mismatch - CPU faster than I/O devices
2. Data Size Mismatch - Application vs device block sizes
3. Copy Semantics - Ensure data consistency

Buffering Strategies

Caching

Keeping copies of frequently accessed data in fast storage.

Cache vs Buffer

Disk Cache

• RAM used to cache disk blocks
• Read cache: Recently read blocks
• Write cache: Blocks waiting to be written

Spooling

Simultaneous Peripheral Operations On-Line

• Buffering for slow, non-sharable devices
• Classic example: Print spooling

Benefits:

• Allows concurrent access to non-sharable device
• Process doesn't wait for slow device
• Print order management

I/O Scheduling

Disk Scheduling

Order in which disk I/O requests are serviced.
Goals:

• Minimize seek time
• Maximize throughput
• Fair access
  See Disk-Management.md for detailed disk scheduling algorithms

Blocking vs Non-Blocking I/O

Blocking (Synchronous)

• Process waits until I/O completes
• Simple programming model
• Process is suspended

// Blocking read
int n = read(fd, buffer, size);  // Waits here until data available
process_data(buffer);

Non-Blocking (Asynchronous)

• I/O call returns immediately
• Process continues execution
• Must check for completion later

// Non-blocking read
int n = read(fd, buffer, size);  // Returns immediately
if (n == -1 && errno == EAGAIN) {
    // Data not ready, do something else
} else {
    process_data(buffer);
}

Comparison

Kernel I/O Subsystem

I/O Scheduling

• Reorder requests to improve performance
• Fairness considerations

Error Handling

• Retry transient failures
• Report errors to applications
• Log errors for diagnosis

Protection

• Prevent unauthorized access
• Validate I/O parameters
• Memory protection during DMA

Power Management

• Device standby/hibernation
• Wake-on-demand
• Power-aware scheduling

Device Types

Block Devices

• Data accessed in fixed-size blocks
• Random access possible
• Examples: HDD, SSD, USB drives
• Commands: read, write, seek

Character Devices

• Data accessed as stream of characters
• Sequential access
• Examples: Keyboard, mouse, serial ports
• Commands: get, put

Network Devices

• Neither block nor character
• Special handling via sockets
• Examples: Ethernet, WiFi adapters

Clock and Timer Devices

• Maintain current time
• Generate interrupts at intervals
• Examples: System clock, hardware timers

I/O Performance

Factors Affecting Performance

1. Device Speed - Transfer rate of hardware
2. CPU Speed - Interrupt handling, driver processing
3. Memory Speed - DMA transfer rates
4. Bus Speed - Data path bandwidth
5. Software Overhead - Context switches, copying

Performance Metrics

Improving I/O Performance

1. Reduce number of context switches
2. Reduce data copying
3. Use DMA for large transfers
4. Use polling for fast, predictable devices
5. Balance between polling and interrupts
6. Use efficient data structures