Unit 2 Processes

Question 1

What is a process?

Answer 1

A process is a program under execution.

Question 2

What is meant by platform?

Answer 2

A particular combination of hardware and operating system.

Question 3

What is the difference between source code and a process?

Answer 3

Source code is the static code, an abstraction of the task that needs to be carried out. A process is the dynamic execution of that code, interacting with the underlying platform.

Question 4

What are the key requirements of an operating system?

Answer 4

To maximise the use of resources.
To mediate the sharing of resources.
To keep the system responsive to all users.

Question 5

What is the difference between cooperative and preemptive multitasking?

Answer 5

Cooperative - writing a programme in such a way that it allows other programs to run. Not so common.
Preemptive - when the operating system requires one process to give up access to resources to allow another process to run ( as long as it is safe to do so.

Question 6

What is the role of the Runtime Environment

Answer 6

Load and execute the program, organise its storage and how it interacts with software libraries and hardware, as well as removal from memory when it has done

Question 7

What are registers

Answer 7

Fastest parts of memory. Have a data width of 32 or 64 bit. Used by the CPU to hold an instruction plus the data it works on during a fetch-execute cycle.

Question 8

What is held in the RAM?

Answer 8

This is the main storage for processes and their data.

Question 9

What is Disk Storage?

Answer 9

This is the virtual memory . It is persistent.

Question 10

What are the functions of the CPU?

Answer 10

1. Read and write binary from and to memory.
2. Perform arithmetical calculations on those numbers.
3. To determine what instructions come next, this may be based on a condition.

Question 11

Describe the Fetch-Excecute Cycle.

Answer 11

The program counter tracks which instruction is needed next from a list of instructions held in memory.
Fetch - get the next instruction and place it in an ‘instruction register’.
Execute - the instruction to completin without interruption using register for storage. The program counter is then updated.

Question 12

What factors influence the speed of the CPU?

Answer 12

1. The data width of the bus.
2. The size of the data the CPU can operate on.
3. CPU clock speed.
4. Speed and size of various parts of memory.

Question 13

What does the system bus do ?

Answer 13

It allows the CPU to communicate with memory and with devices through their respective device controllers.

Question 14

Name the three software interfaces that link a user program to a device controller.

Answer 14

1. The interface between code and runtime environment (library calls).
2. Between runtime environment and OS (system calls)
3. Between OS and device controllers (device calls)

Question 15

What is the point of a device driver?

Answer 15

The device driver resides in the OS, it translates high level requests from the OS into lower level requests to the device controller.

Question 16

What is the purpose of a device controller?

Answer 16

The device controller is independent from the CPU , it has its own registers, memory and simple processes to enable it to ‘speak’ to the device it controls.

Question 17

What is an interrupt?

Answer 17

An interrupt is a means for a device or software to signal to the processor that something of interest (an event) has happened.

Question 18

What happens in the CPU when an interrupt is received?

Answer 18

On receipt of an interrupt the CPU can halt the code being processed. If the interrupt is of high enough priority the CPU performs a context switch. It executes the next necessary routine then performs another context switch and returns to the original process.

Question 19

What is polling?

Answer 19

Polling is the act of checking to see if an event has occurred or a device or process is ready. Also known as busy-waiting.

Question 20

What happens in the CPU if a user program attempts to execute a privileged instruction via a system call?

Answer 20

A supervisor call interrupt is generated. A context switch takes place to put the CPU into supervisor mode and execute the interrupt handler. At this point the interrupt handler may decide to execute the code or not. Subsequently the processor returns to user mode via another context switch.