chapter 36 - I/O devices

Question 1

What are the main components of classical system architecture and how are they connected?

Answer 1

• CPU and RAM: Connected via a high-speed memory bus
• I/O Bus: Connects medium-speed devices
• Peripheral Bus: Connects slow-speed devices via interfaces like USB
• High-speed devices → closer to CPU
• Low-speed devices → further from CPU

Question 2

Why is hardware organized hierarchically in a computer system?

Answer 2

Due to physical and cost constraints:

• fast buses must be short
• slower buses allow many low-cost devices to connect

Question 3

What is a canonical device in OS design?

Answer 3

device used to explain how hardware interacts with software - not a real device

Question 4

Describe the canonical protocol between OS and device using polling.

Answer 4

status(read only) - tells the OS wether device is busy/ready/done

command(write only) - tells the device what to do

data(read/write) - allows OS to send or receive dataw

Question 5

what are the 2 components in any device - explain

Answer 5

hardware interface - registers and protocols used by the OS to control it - how the OS talks to the hardware

internal structure - hidden by the OS - includes: small CPU, RAM

Question 6

what is the problem with polling

Answer 6

CPU keeps checking device status in a loop - This wastes CPU cycles + no other process can run

Question 7

what is polling

Answer 7

OS keeps checking the status register in a loop

Question 7

what is programmed I/O (PIO)

Answer 7

the CPU itself is doing all the reading/writing

Question 8

how can interrupts help the polling problem

Answer 8

OS sends the command → puts process to sleep → runs other tasks

When the device finishes, it raises an interrupt

invoking an ISR to resume the waiting process - Interrupt Service Routine (ISR)

Question 9

What is Direct Memory Access (DMA) and how does it improve performance?

Answer 9

DMA transfers data between memory and device without CPU involvement

• CPU kjører prosess 1(før I/O starter)
• DMA starter kopi av data fra minne til disk (c c c)
• Disk starter I/O parallelt
• CPU er fri og kjører prosess 2 mens DMA+disk jobber
• Når I/O er ferdig, får prosess 1 CPU igjen

se figur i notater

Question 10

Compare Programmed I/O (PIO) with DMA

Answer 10

PIO: CPU does all the copying, wasting cycles

DMA: a dedicated engine handles the copy, allowing CPU to multitask

Question 11

What are the two main methods the OS uses to interact with devices?

Answer 11

Explicit I/O instructions — Special CPU instructions used to interact directly with device registers

Memory-mapped I/O — uses regular memory operations (load/store) to access device registers

Question 12

What is a device driver and why is it important?

Answer 12

software that knows how to control a specific device – OS talks to the driver, not the hardware directly

allows the OS to be device-neutral

Question 13

ID disk interface - uses 4 types of registers

Answer 13

• Control
• Command Block
• Status
• Error