CS-3310 - 04 I/O and Disks - I/O

What are the two main components of an I/O device?

A hardware interface and an internal structure.

Why do I/O devices need a hardware interface?

It allows the operating system to control and communicate with the device.

What are the three common device registers in a canonical I/O protocol?

Status register, command register, and data register.

What does the status register do?

It indicates whether the device is ready or busy.

What does the command register do?

It tells the device to perform a specific task or operation.

What does the data register do?

It transfers data between the device and the CPU.

What is programmed I/O (PIO)?

A simple I/O protocol where the CPU directly manages data transfer and polls the device for readiness.

Why is programmed I/O inefficient?

Because the CPU must wait for the device, wasting cycles on polling instead of doing other work.

How do interrupts reduce CPU overhead in I/O operations?

They let the CPU perform other tasks and notify it when the device is ready via an interrupt.

What is an interrupt service routine (ISR)?

A special function in the OS that runs when a hardware interrupt occurs to handle the device's completion event.

What is the main advantage of interrupts over polling?

They allow overlap between computation and I/O, improving overall system utilization.

When might interrupts not be ideal?

For very fast devices where the interrupt overhead outweighs the benefit.

What is a precise interrupt?

An interrupt that leaves the machine in a well-defined state: all prior instructions complete, none after have started.

What is an imprecise interrupt?

An interrupt that occurs when the CPU state is not fully consistent, making recovery difficult.

What is Direct Memory Access (DMA)?

A hardware mechanism that transfers data between memory and devices without CPU intervention.

How does DMA improve performance?

It offloads data transfer work from the CPU, reducing context switches and wait time.

What is a limitation of DMA?

It's slower than the CPU for small data transfers and requires setup overhead.

What are the two methods for device interaction with the CPU?

Port-mapped I/O (using special instructions) and memory-mapped I/O (using memory addresses for device registers).

What is memory-mapped I/O?

A method where hardware makes device registers accessible via normal memory addresses.

What is a device driver?

A kernel module that implements the specifics of a device, allowing the OS to communicate with it through a standard interface.

Why is abstraction important in I/O systems?

It hides hardware-specific details so the OS can remain device-neutral.

I/O layers

user process, device-independent software, device drivers, interrupt handlers, hardware

which I/O software layer is skipped on the way down?

interrupt handlers because they are only triggered by the hardware

lowest level software I/O layer

interrupt handler

top layer of I/O software

user process

second layer of I/O software

device-independent software

third layer of I/O software layer

device drivers