Concepts

Question 1

What is an Operating System?

Answer 1

An operating system is the one program running at all times on a computer (called the kernel) that manages a computer’s hardware. Everything else is either a system or application program.

Question 2

First 3 OS phases?

Answer 2

Serial Processing → Simple Batch Systems → Multi-programmed Batch Systems

Question 3

Why is an Operating System likened to a government?

Answer 3

Because, like a government, it can’t execute a useful function on its own, but can create an environment within which other programs can execute them.

Question 4

How does an OS act as an intermediary between a user and computer hardware?

Answer 4

An OS acts as an intermediary by acting as a manager of these resources and controlling and allocating resources. A computer’s resources refer to its hardware such as CPU time, memory space, I/O devices (for e.g. printer, keyboard, mouse). The OS must decide how to allocate these to specific programs and users so it can operate the computer system efficiently.

Question 5

What do System Calls do?

Answer 5

Provide an interface to the services made available by an operating system.

Question 6

Why would an application programmer prefer programming according to
an API rather than invoking actual system calls?

Answer 6

Because an API can specify a set of functions and parameters for the programmer, and actual systems. Direct systems calls require more integration effort because all integration logic has to be manually coded.

Question 7

What is the microkernel approach, and why was it developed?

Answer 7

As an OS expanded, it became difficult to manage. Microkernel removes all nonessential components from a kernel and implementing them as system and user-level programs. This results in smaller kernels, requiring less processes and memory management.

Question 8

One pro and con of the microkernel approach.

Answer 8

Pro: The resulting OS is easier to port from one hardware design to another. The microkernel also provides more security and reliability, since most services are running as user—rather than kernel— processes. If a service fails, the rest of the operating system remains untouched.

Con: Performance of microkernels can suffer due to increased system-function overhead (slower processing speed, less memory, etc).

Question 9

What is the Modules approach?

Answer 9

Involves using loadable kernel modules for an OS design - so the kernel has it’s core functions, and then other additional functions are added dynamically.

Question 10

Why is the Modules Approach similar but better than the Microkernel Approach?

Answer 10

Like the microkernel approach, the primary module has only core functions, but it also has nonessential modules added as layers. This means modules do not need to invoke message passing up and down those layers to communicate.

Question 11

What are hybrid systems? Provide examples with reasoning.

Answer 11

In reality, most OS combine different structures to address performance, security and usability issues.

For example, Linux and Solaris are monolithic - where all core components are in one place - which provides efficient performance. However, they are also modular, so that new functionality can be dynamically added to the kernel.

Question 12

What is the definition of a process?

Answer 12

A program in execution / a program that’s been activated.

Question 13

How does an OS execute a program (to turn it into a process)?

Answer 13

By loading the program into memory

Question 14

What is a process made up of (in memory)?

Answer 14

text (the code), data (global and static variables), heap (memory dynamically allocated during run time), stack (keeps track of active function calls)

Question 15

What 2 parts of a process grow toward each other?

Answer 15

The stack grows downward in memory, whilst the heap grows up in memory.

Question 16

What happens when the heap and stack overlap?

Answer 16

The program could crash, but the OS monitors this to avoid a collision.

Question 17

What different states can a process be in during execution?

Answer 17

New (process is being created)
Running (executing)
Waiting (for an event)
Ready (to be assigned)
Terminated (finished)

Question 18

What is a Process Control Block (PCB)?

Answer 18

A structure that contains all the information about a process:
Process state
Program counter
CPU registers
Scheduling Info
Memory-management info
I/O status

Question 19

In brief what does a PCB serve as?

Answer 19

A repository for any information that may
vary from process to process.

Question 20

What is a thread when compared to processes?

Answer 20

A process is a container for threads - one single process can have one or multiple threads of execution, allowing a process to perform more than one task at a time.

Question 21

Benefits of multithreading?

Answer 21

1) Responsiveness - let’s a program keep running even if one thread is busy or blocked.
2) Threads share the memory and resources of the process by default - no need for message passing.
3) More economical if resources are being shared

Question 22

What is a context switch?

Answer 22

When CPU switches to another process, the system must SAVE the
state of the old process (in the PCB) and LOAD the saved state of the new process - aka, a context switch

Question 23

What is process creation?

Answer 23

A process may create several new processes, like how a parent would create children.

Question 24

What 2 options are there for parent/child process execution?

Answer 24

1. The parent and children process execute concurrently.
2. The parent waits until children have terminated.

Question 25

Can a child process share resources with parent resource?

Answer 25

Yes and no - they may be able to, or may be constrained to a subset of resources.

Question 26

Can processes communicate with one another?

Answer 26

Yes. this is called Inter-Process Communication (IPC) which allows processes to work together

Question 27

How can they communicate?

Answer 27

Using a) message passing or b) shared memory.

Question 28

Message passing may either be... a or b

Answer 28

a) blocking (synchronous) or b) non-blocking (asynchronous)

Question 29

What is an independent vs cooperating process?

Answer 29

An independent process executes without affecting or being affected by other processes. Cooperating process - the opposite.

Question 30

What is concurrency?

Answer 30

Where multiple programs can be executed simultaneously.

Question 31

What is a race condition?

Answer 31

When multiple processes access and manipulate the same data concurrently and the outcome of the execution depends on an order of the order of the execution.

Question 32

What aims to guard against race conditions occurring?

Answer 32

Process Synchronisation - they have to be in some way, to ensure only one process can manipulate data at a time.

Question 33

What is the critical-section problem?

Answer 33

When 2 or more processes are executing in their critical-sections simultaneously.

Question 34

What 3 requirements must be satisfied for a solution to the critical-section problem?

Answer 34

1) Must be mutually exclusive 2) Progress - can't prevent other processes from entering their critical-section when no process is executing theirs. 3) Bounded Waiting - limit on number of times other processes are allowed to enter their critical-sections AFTER a process has made a request to enter its critical section, and BEFORE that request is granted.

Question 35

What is Peterson's solution to the critical-section problem?

Answer 35

Processes A and B have their own lock + a mask for their locks. A arrives at critical region first, sets its lock, pushes mask to the other lock and enters. B arrives and sets its lock and pushes its mask, but must wait until A's lock is reset. A exits CR and resets its lock - B can now enter.

Question 36

Does Peterson's solution to the critical-section problem work?

Answer 36

Yes, in pure software - but not guaranteed to work on modern computer architectures - too much overhead.

Question 37

What's the Test-Set-Lock (TSL) solution?

Answer 37

Helps us build a shared lock mechanism. A reaches CR, finds a key and takes it. B arrives, sees no key, waits. A leaves CR, key returned to start. B can now take the key and enter.

Question 38

Test-set-lock is similar to the speaker's baton...

Answer 38

Only one person can hold it at a time

Question 39

Pro and con of test-set-lock solution?

Answer 39

Pro: Mutual exclusion is achieved. Con: Bounded waiting and progress are NOT GUARANTEED, also requires special hardware.

Question 40

What is an atomic instruction?

Answer 40

An operation that is indivisible - either completes fully, or doesn't happen. like a light switch - either ON or OFF, never inbetween.

Question 41

How does atomic instruction help in the critical-section problem - use a real world analogy.

Answer 41

Imagine signing a form at a reception desk. You can only do it if no one else is already writing. An atomic instruction is like locking the pen and clipboard to your hand until you’re done — nobody else can mess with it

Question 42

What's a mutex lock (mutual exclusion lock)?

Answer 42

Where a process must acquire the lock before entering the critical-section. Releases the lock when it leaves the critical-section.

Question 43

Main disadvantage of mutex lock?

Answer 43

Requires busy waiting - other processes not in the critical-section must loop continuously, wasting CPU cycles.

Question 44

TF is a Semaphore?

Answer 44

A more sophisticated approach to process synchronisation. It's a special integer value, only accessible through 2 atomic operations: wait() and signal().

Question 45

What do the atomic operations wait and signal do to a semaphore?

Answer 45

Wait() decrements the value - if semaphore value becomes negative, process becomes blocked. Signal() increments - if semaphore becomes positive, it can unblock a previously blocked process.

Question 46

Pro and con of semaphore

Answer 46

Pro: ensures mutual exclusion, can cause synchronisation in multiple processes, effective for signalling between processes (like saying 'I'm done, your turn') Con: incorrect use of signal() can cause deadlocks, resource leaks. Can cause spinlocks (busy waiting), wasting CPU cycles.

Question 47

Detail one classic problem of synchronisation.

Answer 47

SINGLE LANE BRIDGE. A road is only wide enough for one way traffic at a time. Cars can move concurrently if moving in same direction. Deadlock: Without proper synchronisation, cars from both directions block each other. Starvation: favouring one side can starve the other.

Question 48

What is CPU Scheduling?

Answer 48

Allowing one process to use the CPU whilst another is waiting for I/O, making use of lost CPU cycles.