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Differentiate between programmed I/O and interrupt driven I/O.
The differences between programmed I/O and interrupt-driven I/O are fundamental approaches to handling data transfer between the CPU and external devices. Each technique has distinct characteristics that affect system performance and resource utilization.
Programmed I/O
Programmed I/O is the simplest technique for data exchange between external devices and processors. In this method, the CPU directly controls all I/O operations and actively monitors the status of I/O devices.
The processor issues a command to the I/O module and waits continuously for the operation to complete. During this time, the CPU keeps checking the I/O module status in a loop until it detects operation completion.
Key Characteristics
Busy waiting − CPU wastes time in a tight loop waiting for I/O completion
Single instruction − Each instruction transfers only one byte/character
Four registers − Input status, input character, output status, output character
Polling mechanism − CPU continuously checks device status
Interrupt-Driven I/O
Interrupt-driven I/O allows the CPU to perform other tasks while I/O operations execute. Instead of waiting, the CPU initiates the I/O operation and continues with other work. When the I/O operation completes, the device sends an interrupt signal to notify the CPU.
Key Characteristics
Interrupt mechanism − Device notifies CPU when operation is complete
Concurrent processing − CPU can execute other tasks during I/O operations
Context switching − Requires saving and restoring CPU state during interrupts
Hardware support − Needs interrupt controller and additional circuitry
Comparison
| Aspect | Programmed I/O | Interrupt-Driven I/O |
|---|---|---|
| CPU Utilization | Poor (busy waiting) | Efficient (multitasking) |
| Performance | Slow and inefficient | Fast and efficient |
| Programming Complexity | Simple to understand | More complex (interrupt handlers) |
| Hardware Requirements | Minimal | Additional (interrupt controllers) |
| System Throughput | Decreases with more I/O devices | Not affected by number of devices |
| Stack Initialization | Not required | Required for context switching |
Examples
Programmed I/O − Early PC ATA interfaces, simple embedded systems with single I/O operations
Interrupt-Driven I/O − Modern keyboard/mouse input, network card data reception, disk drive operations
Conclusion
Interrupt-driven I/O is superior to programmed I/O due to better CPU utilization and system performance. While programmed I/O is simpler to implement, interrupt-driven I/O enables true multitasking and scales better with multiple I/O devices, making it the preferred approach in modern computer systems.
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