Ch. 1 - Introduction

Question 1

Software can be grouped into 2 categories. Name both and give an example of each.

Answer 1

Applications Software (eg. games, apps)
Systems Software (eg. OS, utilities)

Question 2

Why do extremely fast programs not interact with the operating system?

Answer 2

Extremely fast programs interact with the hardware directly. The drawback is that it is more challenging to create a program that directly interacts with the hardware.

Question 3

Operating System (OS)

Answer 3

A resource manager of both time (on a given resource) and space (different units of the same resource).

Question 4

What are the 3 roles an operating system takes on?

Answer 4

Referee
Illusionist
Glue

Question 5

Referee (Role of OS)

Answer 5

-Ensure resources are allocated fairly
-Isolate different users/applications
-Allow for communication between users/applications (when applicable)

Question 6

Illusionist (Role of OS)

Answer 6

-Each application appears to have the entire machine to itself
-Illusion of an infinite number of processors and memory

Question 7

Glue (Role of OS)

Answer 7

OS provides libraries, and user interface widgets which “glue” everything together

Question 8

What did the earliest computers use for input?

Answer 8

Panel Switches

Question 9

Describe the steps involved in using an early batch system

Answer 9

1. Bring punch card to card reader which goes through tape drive
2. Use the tape from tape drive a Data Processing System (7094) which does computing
3. Use result tape from 7094 in printer which prints results

Question 10

What is the goal of job interleaving?

Answer 10

To decrease total idle time

Question 11

True or False: By using job interleaving, the time to execute each individual job (NOT total time for all jobs) decreases

Answer 11

False. The total time for all jobs will improve significantly, however the time for a single job may increase since the job may have to wait for another job to finish using the CPU or finish performing I/O.

Question 12

Interrupts (Definition and Example)

Answer 12

Notification of an external event that occurs asynchronously with the current activity of the processor. Exact occurrence time is not known and not predictable.

Examples: Character typed on keyboard, timer

Question 13

Traps (Definition and Example)

Answer 13

Notification of an internal event that occurs while a program is executing and thus synchronous with the current activity of the processor. They are immediate and predictable.

Examples: System call, error, page fault

Question 14

Describe the steps required to handle an interrupt

Answer 14

1. Save the current state of CPU and branch to OS
2. Determine what kind of interrupt occurred using Interrupt Service Routine (ISR)
3. Execute ISR to handle interrupt
4. Return to interrupted program (at point saved in step 1)

Question 15

True or False: A user program cannot be affected by an interrupt

Answer 15

True. After an interrupt is serviced, the state of the user program before the interrupt is restored.

Question 16

Programmed I/O

Answer 16

The CPU transfers data to or from the device buffers. After issuing an I/O operation, the CPU continuously polls for its completion. CPU cannot resume execution of program until I/O is complete.

Question 17

Interrupt-driven I/O

Answer 17

CPU issues an I/O operation and goes on to some other work. The device interrupts the CPU as each byte or word arrives. CPU handles the data transfer.

Question 18

Direct Memory Access (DMA)

Answer 18

CPU issues an I/O operation specifying:
the device
memory location of the data
block size

This allows the CPU to do other work. The DMA device interrupts the CPU upon the completion of the requested operation.

Question 19

Nesting of Interrupts

Answer 19

A new higher priority interrupt that occurs while a lower priority interrupt is being serviced will suspend the current interrupt handler until the new interrupt is processed.

Question 20

True or False: More than 1 interrupt can be triggered at the same time

Answer 20

False. When more than 1 interrupt is triggered successively, the OS prioritizes interrupts based on their priority.

Question 21

Supervisor Mode (aka Protected/Kernel Mode)

Answer 21

All basic and privileged instructions (i.e. I/O operations) available

Question 22

User Mode

Answer 22

Subset of instructions that can be used by user (basic instructions only).

Question 23

Why does the OS have 2 modes?

Answer 23

-Protect itself from the user program
-Protect multiple programs from themselves
-To give OS authority over resource management

Question 24

System Call (Definition & Implementation)

Answer 24

Enter the operating system and perform a privileged operation (changes execution mode of the CPU). Implemented as a trap

Question 25

Describe how a system call is serviced

Answer 25

1. Verify arguments and switch to supervisor mode
2. Branch to service function
3. Return from service function and switch back to user mode
4. Return from system call

Question 26

True or False: All portions of the OS are used to service the current active process

Answer 26

False. Some portions of the OS remain in main memory to service critical operations such as dispatching and interrupt handling. This portion of the OS is called the kernel