Operating Systems

Question 1

What is an operating system?

Answer 1

The program that manages a computer’s resources, especially the allocation of those resources among other programs (CPU)

Question 2

Purpose of operating system

Answer 2

Running several applications at the same time. The Operating System needs to manage the use of hardware resources amongst several applications

Question 3

Definition of a process

Answer 3

A program in memory that is in execution.
Several processes can be in memory at the same time.
Processes can be run through several threads.
Consists of code (machine code instructions) and data in memory

Question 4

Layered Architecture of an OS

Answer 4

Kernel + libraries + tools

Question 5

Advantages of UNIX

Answer 5

Highly modular– includes only features you need
Highly scalable and portable– from mobile phones, desktops to supercomputer
Highly reliable– update mechanism, desktop, servers and cloud
Highly cost-effective – Linux and Android are free

Question 6

Describe the five different states a process can obtain during its lifespan

Answer 6

running- a process currently has CPU
ready- process is ready to get CPU
blocked- process is waiting for some event
new- created but not admitted
exit- process destruction

Question 7

What is a message identifier?

Answer 7

Named constant that identifies the purpose of the message

Question 8

What are message parameters?

Answer 8

Specify data or the location of data used by a window procedure when processing a message

Question 9

How are threads and processes scheduled?

Answer 9

CPU priority. They wait in a queue for their turn on the CPU. A process/thread can be in different states, i.e. waiting or running. The OS scheduler determines how long each process/thread can run before it is taken off the CPU.

Question 10

How is the memory managed?

Answer 10

All memory areas, allocated or available, are preceded and followed by structures known as memory links. These links contain information about the area that they enclose. Using this information, the operating system can determine if the area is available or in use. If the area is currently in use, the operating system can also decide whether the area can be moved to virtual memory or moved to another location in the main memory.

Question 11

How is the address translation performed?

Answer 11

When the system allocates a frame to any page, it translates this logical address into a physical address and creates an entry into the page table to be used throughout the execution of the program. When a process is to be executed, its corresponding pages are loaded into any available memory frames

Question 12

How is the disk space for files organised?

Answer 12

divide files into blocks of fixed size

file is stored as blocks that are not necessarily adjacent

Question 13

Explain the FAT file system

Answer 13

The FAT file system divides the disk space into
blocks. Pointers to each block are used in the File Allocation Table. Files are broken up into many blocks which are linked together. The entries in the File Allocation Table are the block number of the next linked block. The directory entry contains a pointer to the first file block.

Question 14

Characteristics of process

Answer 14

Process environment- lives in a protected (memory) environment. No interference between processes. But communication between processes is possible (IPC - inter-process communication)
Resource Ownership- allocated virtual address space
(protected memory). Assigned from main memory. Control of I/O channels. Files
Code Execution- Single thread (classic process). Multi-threaded (modern process)

Question 15

What are threads?

Answer 15

A sequence of instructions which exist 
within the same process. 
Every process has at least one thread.
Threads of the same process share the same process resources, including memory and file access. 
Threads can run in parallel.

Question 16

What is multiprocessing?

Answer 16

Method to run several processes and threads

Question 17

What is a scheduler?

Answer 17

Part of the OS kernel process management that decides which processes to run next.

Question 18

What is the purpose of a memory manager?

Answer 18

Ensures that the available memory is used effectively

Question 19

What are the roles of the memory manager?

Answer 19

allocate memory (RAM) to processes during execution
de-allocates memory when no longer needed
re-locates memory
prevent processes from running into each other’s
memory space (security/protection)
virtualise memory
swapping process memory between RAM and disk
when memory cannot hold all process(es)

Question 20

What is monoprogramming?

Answer 20

Only one process runs at a time
At any given moment, the operating system and one user program reside in RAM
MS-DOS

Question 21

Explain the virtual memory process

Answer 21

Each process gets its own (virtual) linear address space but only the used elements need to be available in physical memory
combined with paging which reduces memory fragmentation

Question 22

What is paging?

Answer 22

Process of breaking up the memory into much smaller

blocks (pages)

Question 23

What is a page frame?

Answer 23

A page in physical memory

Question 24

What is a page table?

Answer 24

Contains the mappings between the virtual address and physical address for each process
Every process has their own page table

Question 25

Describe the address translation steps

Answer 25

Take the 4 higher-order bits (HOB) of the virtual address
Convert HOB into a decimal number. This is the page number
The page number is used as an index into the page table (as with arrays)
Obtain the corresponding page frame from the page table in physical memory
At position 4 of the page table, there are 3 Higher-Order Bits of the new physical address
New Physical address = 3 HOB + 12-bit offset

Question 26

What is a page fault?

Answer 26

Occurs if a program attempts to access an unmapped page (virtual page) that is not in memory

Question 27

What are the file management tasks?

Answer 27

To organise the file data in an efficient way
Hard disk partitioning
Block allocation
Free space allocation
File system layout organisation
Security (file access control)

Question 28

What are file operations?

Answer 28

Allow data to be stored and retrieved

Question 29

Describe linked list allocation

Answer 29

each file is kept as a link list of disk blocks
the first word of each disk block contains a pointer to next
disk blocks are no longer consecutive

Question 30

Advantages of linked list allocation

Answer 30

only internal fragmentation

directory entry stores the first block address

Question 31

Disadvantages of linked list allocation

Answer 31

random access is slow

some disk space is used for pointer and not for data

Question 32

Advantages of linked list with index

Answer 32

the entire block is available for data

single address entry for the file

Question 33

Disadvantages of linked list with index

Answer 33

The entire table must be brought into memory

potentially high overhead (disk space for tables)