Operating systems

Question 1

What is an operating system?

Answer 1

An operating system (OS) is a type of system software that acts as a layer of software between application programs and the computer’s hardware and provides an interface through which a user can run an application program.

Question 2

How is an operating system loaded on a computer?

Answer 2

• Computers cannot run without software, but it must be running before it can load an OS. There must be a small program to start the process of loading the OS.
• A small firmware program runs to start the loading process, stored in flash memory in PCs and laptops.
• 1st program – performs ‘power-on-self-test’ (POST) to prevent the computer from starting to run in an unstable state. Checks the integrity of firmware, externals devices and size of internal memory. Identifies the most promising disk drive from which to load an operating system.
• 2nd program – ‘boot loader’ loads first part of OS into RAM. The process in which the operating system loads itself then takes control of the computer is called ‘bootstrapping’.

Question 3

List the functions of an operating system

Answer 3

• To manage the users, hardware, software and peripherals (input and output devices that are not integral or necessary to the system)
• To manage user accounts and security.
• To provide a user interface through which a user can run application programs and perform housekeeping tasks such as defragmenting or file indexing.
• To manage the allocation of internal memory and processor time, as well as interrupt signals to the processor
• Provide ‘spooling’ – temporary storage of input or output data in a queue to allow application programs to proceed with other tasks while peripherals operate relatively slowly.

Question 4

What is a batch operating system?

Answer 4

• Dedicated to managing a succession of batch ‘jobs’ or packages of work, each consisting of a program and a batch of data
• A batch of data is prepared, validates, often verified by double entry, and entered into a transaction file before any processing takes place.
• The processing runs to completion without any intervention from an operator unless an error occurs. Output received in the form of updated files or printer output
• Allows the operator to specify:
  
  • The owner of the particular job
  • Programs
  • Data files
  • Priority (position in queuing)
  • Max processor time allowance, memory and printing lines
  • Actions to be taken in case of program error

Question 5

List the advantages and disadvantages of a batch operating system

Answer 5

Advantages:

• Quick processing as no delays by waiting for user to enter data
• Especially useful when the program and data are reliable and accurate

Disadvantages:

• Less useful when data is unreliable, as it cannot be modified while the program is running

Question 6

What is a multi-tasking operating system?

Answer 6

• Typical modern operating system for any sort of general-purpose computer
• Simultaneously runs several processes and services and more than one application program
• On any computer with only a single processor, only one program can run at any instant, but there is an illusion of simultaneous running by sharing processing time between programs

Question 7

What is a multi-access operating system?

Answer 7

• Allows more than one user to use the computer at the same time (multiple log-ins)
• Each user interacts with the computer through a ‘terminal’ consisting of a keyboard and display screen. The terminals do not have their own processors and are not computers.
• The OS shares processing time between the users, who each have all the computer’s resources available to them.

Question 8

What is a real-time transaction processing operating system?

Answer 8

• A real-time OS supports application programs that can process input within a guaranteed maximum time to produce the output required to keep pace with the user’s needs for information or control.
• A real-time transaction processing OS supports application software for processing transactions e.g. bookings as they occur, with a maximum response time within the range of seconds.

Question 9

What is a real-time process control operating system?

Answer 9

• Controls a physical or chemical process by continuously monitoring conditions, set-point data and sensor feedback data.
• Data is processed to calculate and make decisions of the output required to the actuators.
• Frequent sampling and rapid processing is necessary, with a max. response time within the range of microseconds to milliseconds.

Question 10

What is a network operating system?

Answer 10

• Manages communication between computers and peripherals equipped with a ‘network interface card’ (NIC) and connected to a network.
• Most OSs for laptops and PCs for small office and home use have built-in support for ‘peer-to-peer’ networking for sharing resources, peripherals and internet access.
• Other OSs support server-based networks, which have a ‘server’ computer running a ‘server operating system’ which controls access for the ‘client’ computers which run ‘client operating systems’.

Question 11

What is a user interface?

Answer 11

A user interface consists of all the hardware and software through which a user provides input to a computer or receives information from it.

Two types of user interface:

• command line interface (CLI)
• graphical user interface (GUI)

Question 12

Describe the command line interface (CLI)

Answer 12

• User relies on keyboard for input, having to remember the available commands and their syntax, and type commands accurately.
• Output restricted to text only
• Useful for experts as it enables them to work quickly and access a wide range of commands

Question 13

Describe the graphical user interface (GUI)

Answer 13

• A GUI often features:
  
  • Windows
    
    • each task displayed in separate window
    • each window is a rectangular region of the screen through which a running program, document or dialogue box can be viewed
  • Icons
    
    • small pictorial symbol representing a command, file or shortcut
  • Menus
  • Pointing devices
• OSs with GUIs support folders and sub-folders to represent directory contents. The hierarchical structure of folders can often be viewed in a navigational file manager

Question 14

How does file management work?

Answer 14

• Folders are used as virtual containers to store files in so they are easier to organise and find.
• The OS stores an index of the contents of each storage drive, known as the drive’s root directory, which lists the files and folders within the drive.
• Each folder has its own file directory, and each sub-folder has its own sub-directory

Question 15

What user commands for file management are there?

Answer 15

• User commands for file management include listing directory contents by opening a folder, moving and copying files and folders, and printing files
• List
  
  • Opening a storage device lists the folders, files and subfolders it contains. Files and folders can be sorted, renamed, searched and created.
• Move
  
  • Moving files and folders to different places
• Copy
  
  • Useful for making backups
• Print

Question 16

What is a buffer?

Answer 16

• An area of memory to hold data temporarily
• Commonly used for storing data that the CPU is manipulating before transferring it to another device, and to compensate for the difference in processing speeds of the operating system and its peripheral devices (the OS usually works faster than the peripherals can supply or accept data)

Question 17

What is polling?

Answer 17

• When the operating system periodically interrogates each peripheral device in turn to discover its status, as peripheral devices cannot control the transmission of data to and from the computer.
• Disadvantage of significant processing time being used in polling all devices, even when inactive.

Question 18

What is an interrupt?

Answer 18

• A signal sent to the processor from a peripheral device (hardware) or program (software) to indicate that the sender needs attention; this cuts off the current thread of processing so the processor can resolve the needs of the device or program
• One way of responding to an interrupt when multiple peripherals are in use is by polling the devices, so polling only occurs when a peripheral is known to need attention.
• Another system of response is known as ‘vectored interrupt’, in which a device supplies an internal memory address called a ‘vector’, which contains the start address of the appropriate interrupt handler program.
• An application program can use a software interrupt to request a service from the OS, such as opening a file or exiting the application. This is known as a ‘system call’.
• Since the OS passed control to the application in the first place, the application has, in effect, interrupted itself to fulfil the user’s request.

Question 19

What is handshaking?

Answer 19

• The computers’ way of indicating the start and end of communication (e.g. indicating whether or not the other device has been acknowledged), as well as negotiating protocol for communication in a session. (e.g. data transfer rate, odd/even parity, flow control)
• May involve sending electronic signals as special codes down the normal data channel or sending extra signals down a short separate hardware wire cable.

Question 20

Describe the steps in the process for transmitting one data packet from a computer to a printer

Answer 20

(repeated possibly 1000s of times until the printer sends an interrupt signalling the data transfer is complete):

1. The computer’s printer driver program fills the print buffer and requests the host controller to send the data to the printer.
2. The host controller sends an OUT token packet immediately followed by a data packet to the printer. Although the OUT packet is not technically a handshake packet, it marks the beginning of a handshaking flow control process.
3. The printer’s USB hardware sends an ACK handshake packet to inform the host controller that it has successfully received the data packet in its input buffer.
4. The printer’s USB hardware generates an interrupt signal to the printer’s microcontroller.
5. The printer’s firmware handles the interrupt by reading and processing the contents of its input buffer.

Question 21

What is a checksum?

Answer 21

An arithmetic summarisation of a block of data that is sent with the data along with the calculation performed to reach the checksum – the receiving device performs the same calculation on the data and compares the result with the checksum that was sent to confirm its integrity.

If the checksums do not match, the data is rejected; else, the integrity of the data has been proven to be maintained.