Week 1 and 2

Question 1

What is an operating system?

Answer 1

A program that acts as an intermediary between a user and the computer hardware

Question 2

Def: Resource Allocator

Answer 2

Manage and allocate resources. Provide a uniform interface to similar hardware.

Question 3

Def: Control Program

Answer 3

Controls the execution of user programs. Controls operation of I/O devices (i.e. disk)

Question 4

Def: Kernel mode

Answer 4

The one program running at all times. IPL - initial program load
- System operations including user requests

Question 5

Single Program System vs. Multiprogram System

Answer 5

Single: Early batch mainframe system > pool of jobs each run sequentially, single process at a time. CPU is idle while waiting for I/O (waster cycles)

Multi: Later Batch Mainframe system, pool of jobs (more than one program in memory at a time) When one program is doing I/O, another gets the CPU

Question 6

What are tightly coupled Parallel systems? advantages? symmetric vs asymmetric?

Answer 6

Processes share memory and I/O

Advantages: increased throughput, reliability

Symmetric: each processor identical, process run on whatever processor is available (may one on more than one during its lifetime)

Asymmetric: Master-slave relationship

Question 7

What are distributed systems? Advantage?

Answer 7

Distribute computation among several processors (loosely coupled > each process has own memory, I/O devices, communication infrastructure)

Advantages: resource sharing/ load sharing, reliability

Question 8

Hard real-time systems vs Soft real-time systems?

Answer 8

Hard: hard limits that must be met or the system fails

Soft: A missed deadline does not mean system failure, but value of result decreases with time

Question 9

Def: Interrupts

Answer 9

Can happen at any time

• Suspends execution of the current program (saves current execution location)
• Transfers control to the interrupt routine
• Determines which controller generated the interrupt (polling, vectored Interrupts)

Question 10

Def: Traps

Answer 10

Traps are like interrupts, but are generated by the program execution (i.e. Divide by zero)

Question 11

What is the difference between a Synchronous I/O and an Asynchronous?

Answer 11

Synchronous: control returns to program only after I/O is complete, common in multi user systems

Asynchronous: return directly to program, common in single user system

Question 12

Def: Main Memory

Answer 12

memory CPU can access directly

Question 13

Def: Secondary storage

Answer 13

• large nonvolatile storage

- hard disks in the middle

Question 14

Def: Cache

Answer 14

Very high-speed memory that sits between a processor and main memory. Used to hold recent memory values.

Question 15

What are the three types of Hardware protection?

Answer 15

Memory protection (user limited to memory allocated for its process) 
I/O Protection (can only have access if in kernel mode - requires special access) 
CPU Protection (Way for all programs to use CPU without it disturbing anything aka cooperative multitasking)

Question 16

Def: User Mode

Answer 16

user program doing user computation

Question 17

Def: Process

Answer 17

A program in execution and all of the required resources to run that instance of the program

Question 18

What are some OS activities?

Answer 18

• Process creation and deletion
• Suspension and resumption
• Synchronization and communication
• allocation/ deallocating and keeping track of memory
• deciding which process to load

Question 19

Def: Memory

Answer 19

a continuous sequence of bytes each of which has own address

Question 20

Def: File

Answer 20

A collection of related information (can be structured or unstructured)

Question 21

What is I/O management?

Answer 21

Hide specifics of devices behind device drivers

Question 22

What is the command interpreter?

Answer 22

• user interface
• accepts commands and executes them
• sometimes built into the system and sometimes separate

Question 23

What are system calls?

Answer 23

The interface between the running process and the operating system (like most function calls take an argument and return a value)

Question 24

What are the three communication models?

Answer 24

1. Interprocess communication (mechanism an os provides to allow processes to manage shared data - other processes need to communicate)
2. Message Model (Os system provides message passing facility)
3. Shared memory model (Both processes access same memory)

Question 25

Def: virtualization

Answer 25

multiprocessing creates the illusion that there is more than one CPU

Question 26

What is the PCB?

Answer 26

Process Control Block: Repository for information that varies from process to process - one allocated for each process and sometimes each thread - (some operating systems have a pre-allocated number of them, that is an array)

Question 27

Def: Context Switch

Answer 27

Utilizes the PCB to switch between processes due to an interrupt or system call

Question 28

Def: Ready Queue, device Queue, suspend Queue

Answer 28

Ready Queue: All process ready to run Device Queue: All process waiting for I/O Suspend Queue: All process that have been suspended

Question 29

True or False: Processors on the Wait Queue are waiting Queue are waiting for the CPU

Answer 29

False: processes on the Wait Queue are waiting for I/O to complete

Question 30

Independent processes vs cooperating processes?

Answer 30

Independent processes cannot affect or be affected by other processes Cooperating processes can affect or be affected by other processes (ex: producer generates info that is passed to a consumer process)

Question 31

Unbound buffer vs bounded buffer?

Answer 31

Unbound: assumes no bounds on size of buffer used to share data - sender never needs to wait (resource intensive, no guaranteed delivery) Bounded: there is a limit - sender may need to wait or abandon message

Question 32

What is interprocess communication?

Answer 32

Mechanism for processes to communicate and synchronize their actions

Question 33

What is Direct communication? Advantages/ Disadvantages?

Answer 33

Processes explicitly identify each other adv: simple to implement dis: processors have to identify each other (has to be some other file you read with processor id)

Question 34

What is indirect communication? Advantages/ Disadvantages?

Answer 34

Mailboxes (each mailbox has a unique id, processes share mailboxes) Only 1 process to read mailbox, first come - first serve- multiple receivers adv: don't have to know id of other process, can preconfigure which mailbox used and everyone uses it, mailbox gets mail and don't need to know whos picking up mail dis: more complex/ harder to implement (need to build storage system where processors can communicate)

Question 35

What are threads?

Answer 35

Lightweight processes

Question 36

What are user threads? Adv? Dis?

Answer 36

Threads implemented by a library (os is unaware of) adv: fast, no system call, simple scheduling dis: one thread blocks (I/O, IPC) all threads block, no multiprocessing support

Question 37

What are kernel threads? Adv? Dis?

Answer 37

Threads provided by the operating system (only diff from user is in context switch and killing a given process kills all threads) adv: OS only blocks threads doing a system call, MP support dis: not as fast as user level threads, resource intensive

Question 38

What are the three thread models?

Answer 38

Many to one (all threads mapped to a single kernel thread) One to one (each program level thread gets a kernel thread) Many to many (limit the number of kernel threads so still more program level then kernel)

Question 39

What is the race condition?

Answer 39

to do with process synchronization - several processes handle shared resources - final value depends on who finsihes first To prevent concurrent processes must be synchronized)

Question 40

Def: Critical sections

Answer 40

several processes competing for access to some shared data (the sections of data where the shared data is accessed/ modified) - each process has its own

Question 41

What are the 3 critical section requirements?

Answer 41

1. Mutual Exclusion - only one 2. Progress - if there is no process in a critical section, and more than one process wants to enter their critical section, then the selection of the process cannot be postponed indefinitely 3. Bounded Waiting - once a process is waiting, the other process can only enter and leave a bounded number of times (no starvation)