A Level Computer Science Paper 1: 1.2.1 System Software

A collection of programs that interact to provide an interface between a user and the computer

Only via applications

User interface, memory management, interrupt management, processor scheduling

Hides the complexity of the hardware by providing an interface for the user to interact with

Graphical User Interface
Command Line Interface
Menu-based Interface
Voice Interface

A computer interacts with a series of graphics
e.g. iPhones
Convenient, easy to use, intuitive

The user has a command line they can use
e.g. Command window in windows OS
Text-based, more power to use more commands, can use batch processing

The user can interact with a series of menus
e.g. ATM/PIN machine
Secure, limited functions, easy to use

You interact using your voice
e.g. Amazon Alexa/ google home
Can interact anywhere, easy, hands-free

1. Memory is divided into fixed-size chunks (pages)

2. Each process is allocated x amount of pages - still 1 page if less than page size but may not be contiguous

3. A page table maps logical memory and physical memory location

Where all the memory for a process is next to each other

+ Improves efficiency of space as data can be stored in all available space

• A whole page may not be full with a process

• Can increase reading time as data is separated

Memory is divided into segments which can be of different lengths
All segments of the same process are contiguous
Could relate to part of a program

+ All memory for a process is together

• The remaining space may be smaller than the size of the segment required

• Swapping is more difficult

An area of the hard disk drive is designated as virtual memory
Some current pages are stored there if RAM is full and are swapped back in when needed

Moving pages between RAM and Virtual Memory

Repeatedly swapping pages can slow down the computer as more time is spent swapping pages than running code

• A signal to the CPU that something needs dealing with

• Can be sent by software, hardware or internal clock

• Stored in a priority queue

An error caused by irregular changes in data - caused by electromagnetism

Where the answer to the calculation is bigger than the memory space available e.g. /0

A region of memory to temporarily store data

1. F/D/E

2. Are there any higher priority interrupts to be processed? No- F/D/E again

3. Push registers on memory stack

4. Load interrupt service routine into program counter and sets a flag to show the ISR has begun

5. F/D/E on interrupt

6. Are there any higher priority interrupts to be processed? No- pop register data off of the stack

They have different priorities and are processed in order of priority
Interrupts can be interrupted if a new interrupt has a higher priority

The registers for the interrupt are also pushed onto the stack
They are retrieved on a last-in first-out basis

A single-core CPU can only process instructions for one application at a time so the OS must schedule when each process uses the CPU

The job that is running can be interrupted

a) Provide an acceptable response time to all users
b) Maximise the time the CPU is fully engaged
c) To ensure fairness on a multi-user system

Each process is allocated a maximum amount of processor time, at which stage it is put back on the ready queue
Very inefficient for long jobs
Pre-emptive

The first job to enter the ready queue is the first to enter the running state
Not pre-emptive

The ready queue is sorted based on how long each time each process has left, with the shortest running
The time for each process only has to be estimated when it enters the queue
Pre-emptive

The user estimates how long each job will take and the shortest is run to completion
Suited to batch systems
Non pre-emptive

• Multiple queues are created with different priority levels - jobs move between as their priority changes

• Jobs move down if they have uses up a lot of CPU time or up if they haven't been recently used

• Pre-emptive

Where a job has to wait a long time before it gets running on the CPU

When memory or storage is used inefficiently, reducing capacity or performance

Code between machine code and object code, independent of the processor architecture. Used to create a development environment to test programs