Scheduling is the operating system's decision about which job or process receives a resource next. The aim is to keep the system busy and to treat users fairly.

How does the scheduler allocate the processor?

The scheduler picks the highest priority ready process and gives it the processor. The process runs until it blocks, finishes or uses up its time slice.

How does the operating system allocate devices to jobs?

The operating system keeps a queue of requests for each device. A device already in use is not given to a second job until the first releases it.

Why must a device be allocated to only one job at a time?

Two jobs writing to one device would interleave their output. The results of both jobs would then be unusable.

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Data Structures: Arrays, Records and Hash Tables

Topic overview One-dimensional arrays and indexing Two- and three-dimensional arrays Manipulating arrays: insertion, deletion and searching Records and fields Arrays of records and record manipulation Hash tables, hashing and collisions Choosing and justifying a data structure

Stacks, Queues, Linked Lists and Trees

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Boolean Logic and Simplification

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Variables, Scope, Parameters and Identifiers

Topic overview Variables and constants Self-documenting identifiers Annotation and program layout Scope of variables: local and global Lifetime of variables Procedure calling, parameters and return values Call by value and call by reference

Recursion and Modular Programming

Topic overview Recursion and the base case How recursive calls use the stack Recursive algorithms: factorial and Fibonacci Recursion compared with iteration Integer division with DIV and MOD Standard functions and standard modules User defined subprograms and modular design

Sorting Algorithms

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Searching and Shortest-Path Algorithms

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Big O Notation and Algorithm Efficiency

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Validation, Verification and Testing

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Data Compression

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Programming Paradigms and Computer Languages

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System Design and Human-Computer Interaction

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Software Engineering Tools and Translators

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Computing, Society and the Law

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Computer Architecture, the Processor and Assembly Language

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Input, Output and Secondary Storage

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Networks, Protocols and the Internet

Topic overview Networks, LANs and WANs Network topologies and connection hardware Networking standards and the layered model IP addressing, TCP and UDP Application protocols: HTTP, FTP, SMTP, IMAP and DHCP Handshaking and establishing a connection Wireless hardware and wireless technologies The internet and connected mobile devices

Data Transmission and Network Performance

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Number Systems and Binary Arithmetic

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Data Types, Characters and Floating Point

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Files, Records and File Organisation

Topic overview The purpose of files in data processing Files, records and fields Fixed and variable length fields and records Master files and transaction files Serial and sequential file access Indexed sequential access and multi-level indexes Direct access and hashing algorithms Overflow handling and file reorganisation File security, backups, generations and archiving

Databases, Normalisation and Entity Modelling

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SQL, Big Data and Distributed Systems

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The Operating System and Its Modes

Topic overview The need for an operating system and the role of the kernel Managing peripherals, processes and memory protection The user interface and the layers between user and hardware Directory structures and file attributes Utility software Batch processing, real time control and real time transaction systems Types of operating system: single-user, multi-user, multi-tasking and multi-programming Designing an interface for different users and environments

Interrupts, Memory Management and Scheduling

Topic overview What causes an interrupt Interrupt handling and the interrupt service routine Interrupt priorities and how they are allocated Partitioning main memory Methods of data transfer between processor and devices Buffers and double buffering High level scheduling, processor and device allocation Process states, time-slicing, polling and threading

Types of Software System

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Data Security, Cryptography and Attack

Topic overview Online updating and the integrity of shared files Dangers of holding personal data Clerical procedures, access levels, passwords and write protection Cryptography: its purpose, and symmetric and asymmetric keys Hashing, digital signatures and certificates Caesar and Vernam ciphers and comparing cipher strength Biometric technologies and their limitations Threats, damage and contingency planning Malicious software, hacking and penetration testing

Interrupts, Memory Management and Scheduling

High level scheduling, processor and device allocation

3 things to know
Component 2

Overview

High level scheduling, three aims of a scheduler and job priority.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is scheduling?

Key term

What is high level scheduling?

Key term

State three aims of a scheduler.

Questions from this subtopic

Each one with the model answer underneath.

1
What is scheduling?
Model answer
Scheduling is the operating system's decision about which job or process receives a resource next. The aim is to keep the system busy and to treat users fairly.
2
What is high level scheduling?
Model answer
High level scheduling decides which jobs are admitted into the system to compete for resources. The choice controls how many processes are resident at once.
3
State three aims of a scheduler.
Model answer
A scheduler aims to maximise use of the processor and to give every user a fair share. A scheduler also aims to keep response times acceptable.
4
How does the scheduler allocate the processor?
Model answer
The scheduler picks the highest priority ready process and gives it the processor. The process runs until it blocks, finishes or uses up its time slice.
5
How does the operating system allocate devices to jobs?
Model answer
The operating system keeps a queue of requests for each device. A device already in use is not given to a second job until the first releases it.
6
Why must a device be allocated to only one job at a time?
Model answer
Two jobs writing to one device would interleave their output. The results of both jobs would then be unusable.

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