Chapter 1 - Introduction

Front 1

What is an operating system?

Back 1

The basic software required to operate a computer.

Front 2

What kind of program does not belong to OS? Give an example of each.

Back 2

User programs: compilers, word processors

Most utility programs: mkdir

Command language interpreter: UNIX shell

Front 3

Which program executions are not controlled by the kernel?

Back 3

All program executions are controlled by the kernel.

Front 4

What are other names for the kernel?

Back 4

monitor

supervisor

executive

core of the OS

Front 5

How do programs interact with the kernel?

Back 5

System calls

Front 6

Are all IO operations performed through system calls?

Back 6

Yes.

Front 7

What are the four basic functions of an OS?

Back 7

1) proved a better user interface

2) manage the system resources

3) protect users' programs and data

4) let programs exchange information

Front 8

Why is improvement of the UI a basic function of an OS?

Back 8

Directly accessing hardware would be a nightmare.

Front 9

Describe time sharing. How was it advantageous?

Back 9

Several users interact with a single computer at the same time. It was an economical solution for when computers were expensive.

Front 10

What advantages do file systems offer users?

Back 10

1) lets users create and delete files without worrying about disk allocation

2) some can tolerate disk failures (RAID)

Front 11

When are the advantages file systems not seen as such?

Back 11

1) when users need to specify how their files are stored on the disk, for example, database designers usually bypass the file system.

Front 12

What are the two main techniques for optimizing IO access?

Back 12

1) managing disk access more efficiently

2) doing something else while waiting for an IO operation

Front 13

Is disk access always necessary for read operations?

Back 13

No, most small operations do not need any.

Front 14

How much does an OS typically read during a disk access? In blocks? In KB?

Back 14

Entire blocks are read (4KB +)

Front 15

Where are blocks read by OS stored?

Back 15

In the IO buffer.

Front 16

What system of optimizing reads well enough for most systems to use it?

Back 16

Buffered reads

Front 17

What is the major limitation of buffered reads?

Back 17

When trying to move too far ahead of the program data that will never be used may be brought in.

Front 18

What is caching in the context of read access optimization?

Back 18

Keeping recently accessed blocks in a buffer in the hopes that they will be accessed again

Caching is a fundamental technique of OS and database design!

Front 19

Why do small writes require disk access?

Back 19

Most of them do not!

Front 20

Does an OS write actively, or does it write in groups?

Back 20

It writes "in groups," delaying a write for a few seconds then writing an entire block

Front 21

What is the major drawback of delayed writes?

Back 21

Data loss may occur if system or program crashes.

Front 22

Why would a crash cause a system that uses delayed write to lose data?

Back 22

The program may have issued a write but the data has not necessarily been saved to the disk.

Front 23

What does multi-programming allow a computer to do?

Back 23

It is when the OS lets the CPU run another program while the program waits for an IO

Front 24

Why has multi-programming been advantageous for business applications?

Back 24

1) many applications use peripherals more than the CPU

2) the CPU was the most expensive component of a computer for a long time

3) multi-programming keeps the CPU busy

4) it made time sharing possible

5) allows a PC to run several applications simultaneously

Front 25

How many tasks can a n core(s) CPU work on at any given time?

Back 25

n tasks.

Front 26

How does the OS reassert control of the CPU?

Back 26

It uses interrupts.

Front 27

How does multiprogramming effect the relationship between the CPU and IOs?

Back 27

It ensures that the CPU doesnt waste any time waiting for completion of IOs

Front 28

In what scenarios does the OS need to regain control of the CPU?

Back 28

1) a task has exhausted its fair share of CPU time

2) something else needs to be done

Front 29

What is an interrupt?

Back 29

A request to interrupt the flow of execution of the CPU

Front 30

What detects an interrupt? When is it detected?

Back 30

It is detected by the CPU hardware.

Detection occurs after it has executed the current instruction and before it starts the next instruction.

Front 31

Write pseudo code in which interrupts are detected at the proper time.

Back 31

check_for_interrupts();

fetch_instruction();

decode_instruction();

execute_instruction();

Front 32

What happens when an interrupt occurs?

Back 32

1) the current state of the CPU is saved, typically on the top of a stack

2) a new CPU state is fetched

Front 33

Does a process "know" if has been interrupted?

Back 33

No.

Front 34

Does the new state that accompanies and interrupt have a program counter that is defined by the artifact?

Back 34

The PC is defined by the hardware. What even is an artifact in this context that doesn't even make sense

Front 35

Name the four types of interrupts presented in Chapter 1

Back 35

1) IO completion interrupts

2) timer interrupts

3) traps

4) system calls

Front 36

What does an IO completion interrupt do?

Back 36

it notifies the OS that an IO operation has completed

Front 37

What does a time interrupt do?

Back 37

it notifies the OS that a task has exceeded its quantum of CPU time (T)

Front 38

What does a trap interrupt do?

Back 38

notifies the OS of a program error (division be zero, illegal address, ...)

notifies the OS of a hardware failure

Front 39

What does a system call interrupt do?

Back 39

it notifies the OS that the running task wants to submit a request to the OS

Front 40

Do programs interrupt themselves?

Back 40

Yes.

Front 41

What is the overhead of any simiple system call?

Back 41

two context switches

Front 42

What two context switches result from an interrupt?

Back 42

1) running task is interrupted

2) task regains CPU

Front 43

Are context switches expensive?

Back 43

Yes.

Front 44

Can interrupt requests occur while a system is already processing another interrupt?

Back 44

Yes. Priority can exist in interrupts.

Front 45

What do vectorized interrupts entail?

Back 45

interrupts are prioritized (assigned a priority level according to their source)

new interrupt requests will be allowed to interrupt lower priority interrupts while having to still wait for the completion of other interrupts.

Front 46

In what situation would interrupts be disabled by the OS?

Back 46

before performing short critical tasks

works only for single threaded kernels

Front 47

In what situation would interrupts be disabled by the user?

Back 47

The user should never be able to disable interrupts!

Front 48

What is DMA? What does it allow the disk controller to do?

Back 48

Direct memory access (DMA)

allows the disk controller to read data from and write data to main memory without any CPU intervention

it "steals" memory cycles from the CPU

Front 49

What must users be prevented from doing to their data in order to protect their data?

Back 49

1) access

2) deleting

3) modifying

without authorization other people's program and data

Front 50

What are the two aspects of data protection?

Back 50

1) protecting user's files on disk

2) preventing programs from interfering with each other

Front 51

What is the key idea in trying to protect a user's files?

Back 51

prevent users' programs from directly accessing the disk

Front 52

What are privileged instructions?

Back 52

instructions only the kernel can execute

Front 53

What are the two CPU modes that privldeged instructions need to operate?

Back 53

1) privledged/executive mode: allows CPU to execute all instructions

2) User mode: CPU executes only safe unprivleged instructions

Front 54

What is the state of a CPU determined by in the context of privileged instructions?

Back 54

a special bit

Front 55

What is the default state that a CPU is in?

Back 55

User mode

Front 56

Who can run in supervisor mode?

Back 56

ONLY the kernel

Front 57

How does the switch between user mode and supervisor mode happen?

Back 57

It is done through an interrupt

Front 58

Is a dual mode CPU enough to protect a user's files?

Back 58

No.

Front 59

What is memory protection?

Back 59

It is the means by which rogue users are prevented from tampering with the kernel.

Front 60

What does memory protection hardware do?

Back 60

1) checks every reference issued by a program

2) generates an interrupt when it detects a protection violation

thus prevents programs from accessing any memory location outside their own address space

Front 61

What advantages does memory protection offer?

Back 61

1) prevents programs from corrupting address spaces of other programs

2) prevents programs from crashing the kernel

Front 62

Is memory protection really necessary for a multiprogramming system?

Back 62

yes

Front 63

Is having both dual mode CPU and memory protection enough to protect user's files? Justify your answer.

Back 63

No, rogue users have to be prevented from booting the system with a doctored kernel

Front 64

What are the two techniques for inter-process communication?

Back 64

1) message passing: general, not easy to use

2) shared memory: less general, easier to use, requires inter-process synchronization

Front 65

Name two instances of process abstraction

Back 65

virtualization the CPU: process scheduling

virtualization the memory: memory management

Front 66

What are the strict real time constraints?

Back 66

1) process control

2) guidance systems

3) most multimedia applications

Front 67

When must critical tasks be performed in a real time system?

Back 67

Always!

Front 68

What defines a hard real time system?

Back 68

1) must guarantee that all deadlines will be met

2) any failure could have catastrophic consequences

Front 69

What defines a soft real time system?

Back 69

guarantee that most deadlines will be met

Front 70

What kind of OSes do hard real time applications run on?

Back 70

typically they run on special real time OSes

Front 71

Can soft real time applications run on a regular OS?

Back 71

yes, if the OS supports them

Front 72

Are interactive/time sharing systems real time systems?

Back 72

No, while they try to provide a fast response time they do not try to meet specific deadlines

Front 73

What is the major advantage of master/slave multi-processing

Back 73

simplicity

Front 74

What defines a master/slave multi-processing?

Back 74

a single copy of OS runs on a dedicated processor

Front 75

What is the major disadvantage of master/slave multi-processings

Back 75

lack of scalability

Front 76

What must symmetric multiprocessing prevent? How does it do this?

Back 76

multiple copies of an OS running parallel must not interfere with one another

must add locks to all critical sections, disabling interrupts will not work

Front 77

What is a distributed system?

Back 77

an integrated network of computers or workstations sharing common resources

Front 78

Draw the client/server model.

Back 78

Front 79

What performance issues exist for a network?

Back 79

1) Network latency is added to disk latency, negatively affecting the response time.

2) file consistency management

3) user authentication (a problem on insecure networks

Front 80

How does a network mask associated latencies?

Back 80

Client caching

Front 81

What are the three basic kernel organizations?

Back 81

1) monolithic kernel

2) layered kernels

3) microkernels

Front 82

How are monolithic kernels organized?

Back 82

There is no particular organization, all kernel functions share the same address space, including device drivers and other kernel extensions

Front 83

What problems do monolithic kernels introduce?

Back 83

Hard to manage, extend, and debug due to the lack of internal organization.

Front 84

What does BIOS stand for?

Back 84

basic input output system (BIOS)

Front 85

What is BIOS stored on?

Back 85

a chip (EEPROM)

Front 86

What does BIOS do? When does it do it?

Back 86

It takes control of the CPU when system is turned on, identifying system components and initiating a boot of the OS. It also provides low level IO access routines.

Front 87

What does a monolithic kernel contain?

Back 87

Everything that is not device specific including the file system and networking code.

Front 88

Define layered kernel.

Back 88

A hierarchy of layers, each of which define a new data object. Lower layers hide some of their functions from the higher layers.

Front 89

Give an example of a layered operating system kernel.

Back 89

User programs

Buffering for IO devices

Operator Console device driver

memory managment

CPU scheduling

hardware

Front 90

What are the limitations of a layered design?

Back 90

There is no clear ordering of layers, for example, the memory management uses file system features and vice versa. This makes it less suitable for kernel design.

Front 91

Where does layered design do well?

Back 91

In networking code, each layer offering its own functionality is very useful.

Front 92

What is the key idea behind microkernels?

Back 92

Make the kernel smaller by delegating non-essential tasks to trusted user-level servers.

Front 93

What happens when a microkernel cannot delegate a task?

Back 93

It handles the task itself.

Front 94

Describe the process of microkernels.

Back 94

1) receives request from user program

2) decides to forward it to a user level server

3) waits for reply form server

4) forwards it to user program

Front 95

Does a trusted server run inside the kernel? What does that affect?

Back 95

No.

It cannot execute privileged instructions

Front 96

What are the advantages of a microkernel?

Back 96

1) smaller

2) easier to secure and manage

3) servers running outside of the kernel so they cannot crash it

4) it is easier to extend kernel functionality by adding new servers

Front 97

What is the major disadvantage of microkernels? What causes this?

Back 97

too slow

four context switches instead of two!

Front 98

What are modular kernels?

Back 98

Object files whose contents can be linked to and unlinked from the kernel at any time.

Front 99

Where do modular kernels run with reguard to the kernel address space?

Back 99

They run inside of the kernel address space.

Front 100

What kind of kernel is used to add kernel device drivers for new devices?

Back 100

Modular kernels

Front 101

What are the advantages of using a modular kernel?

Back 101

1) extensibility: add new features to kernel, process is transparent to user

2) no performance penalty on account of modules running in kernel address space

Front 102

What are the disadvantages of using a modular kernel?

Back 102

1) they are not as reliable, a single bad module can corrupt the entire kernel and crash the system.

2) many device drivers are poorly written and result in OS failures

Front 103

WHY?

Back 103

Unix has a monolithic kernel (which makes it fast) and does not allow extensions (which makes it both safe and non extensible)

Windows has a monolithic kernel (which makes it fast) and allows extensions (which makes it both extensible and unsafe)

Mach allows extensions in user space (which makes it extensible, safe and slow)