Unit 2 - Systems software

Operating System (OS)

• An Operating System is a program or set of programs that manages the operations of the computer for the user.
• It acts as a bridge between the user and the computer's hardware.
• The Operating System is held in permanent storage, for example on a hard disk. A small program called the loader is held in ROM. When a computer is switched on the loader in ROM sends instructions to load the OS by copying it from storage into RAM.

Functions of an Operating System

• The OS provides...
• a user interface
• memory management
• interrupt handling
• processor scheduling

User Interface

• The OS hides the complexity of the hardware from the user by providing a user interface.

Memory management

• Programs and their data need to be loaded into RAM
• The OS must manage the allocation of RAM to the different programs
• There may not be sufficient RAM for all desired processes to be completely loaded into RAM at once

Memory management - Paging

• Available memory is divided into fixed sized chunks called pages
• Each page has an address
• A process loaded into RAM is allocated sufficient pages, but those pages may not be contiguous in physical terms
• A page table maps between the logical memory locations and the physical memory locations

Memory management - Segmentation

• Alternatively, memory is divided into segments which can be of different lengths
• Segmentation can relate to parts of a program, for example a particular function or subroutine may occupy a segment.

Virtual memory

• A computer has a fixed amount of RAM, the demands of memory will often exceed this amount
• An area of the hard disk can be designated as virtual memory
• Some of the pages of a current process are stored in virtual memory until they are needed, at which point they are swapped into RAM
• If many processes are running and the computer has insufficient RAM, lots of time is spent swapping pages in and put of virtual memory
• Repeatedly swapping pages can noticeably slow down the computer this is known as disk thrashing

Interrupts

• It is vital that the CPU can be interrupted when necessary
• Interrupts can be sent to the CPU by software, hardware devices or the CPUs internal clock
• Examples of interrupts are:
• ​​​​​​​An input/output device sends an interrupt signal
• The printer runs out of paper
• An error occurs in a program
• A scheduled interrupt from the internal clock
• Power failure
• The CPU checks at the end of each clock cycle whether there are any interrupts to be processed

Interrupts - Using the stack

• When an interrupt is detected, the processor stops fetching instructions and instead pushes the current contents of its registers onto a stack
• The CPU uses an Interrupt service routine to process the interrupt
• When processing has finished the values can be popped from the stack and reloaded into the CPU

Interrupt Priority

• Interrupts have different priorities, and will be processed in order of priority
• Interrupts can themselves be interrupted if the new interrupt is of a higher priority
• If a higher priority interrupt occurs whilst an interrupt is being processed the original interrupt's registers will be pushed onto the stack as well
• The stack is a Last In First Out (LIFO) data structure so the last data to be pushed on the stack will be the first to be retrieved

Processor scheduling

• A single CPU can only process instructions for one application at a time
• The OS must schedule when each application can use the CPU
• This gives the illusion of multi-tasking multiple applications appear to be running simultaneously

Aims of scheduling

• To provide an acceptable response time to all the users
• To maximise the time the CPU is usefully engaged
• To ensure fairness on a multi-user system

Processor scheduling - Round Robin

• Each job is allocated (by FIFO) a time slice during which it can use the CPU's resources
• If the job has not been completed by the end of its time slice, the next job is allocated a time slice
• 
• Caption: Round Robin

First com first served

• The first job to arrive is executed until it is completed

Shortest remaining time

• The time to completion is estimated as each new job arrives
• The job with the shortest remaining time to completion is executed, meaning that a shorter new job can take over from the current process

Shortest job first

• Also known as "shortest process next"
• As with shortest remaining time, the total execution time of each job is estimated by the user
• The waiting job with the shortest total execution time is executed when the current job is completed

Multi-level feedback queues

• Multiple queues are created with different priority levels
• If a job uses too much CPU time it is moved to a lower priority queue
• Processes can also be moved to a higher priority queue if they have waited a long time

Distributed Operating Systems

• A distributed OS can coordinate the processing of a single job across multiple computers
• A program can be run by the user that uses data or resources from any other computer e.g- processor time, memory, I/O devices
• The distribution of tasks is coordinated by the OS passing instructions between computers
• The user can access more computational power with the illusion of working with a single processor
• Advantage - No need for training or writing programs differently
• Disadvantage - The programmer has no control over the task distribution as this is entirely handled by the OS

Multi-tasking Operating Systems

• Enables a user to carry out task simultaneously
• Time slicing is used to switch quickly between programs and applications in memory

Multi-user Operating System

• Multiple users make use of one computer
• A scheduling algorithm is used to allocate processor time fairly between jobs and prevent processor starvation

Multi-user, Multi-tasking system

• Some systems use a very powerful computer called a mainframe
• Lots of users with their own terminals access the mainframe's CPU and each gets a time slice
• Each terminal is also running multiple processes

Mobile Operating Systems

• A smartphone is a computer with a multi-tasking Operating system
• Mobile OSs are linked to specific hardware
• A low level proprietary OS is used for handling the hardware and special features
• Meanwhile the main OS handles the user interface and running applications

Open source Operating Systems

• Open source is used by all major device manufacturers
• It allows for major customisation
• A device manufacturer fine-tine the OS to suit their devices, and add additional features or user interfaces to enhance the users experience
• The interface and the available apps can be a major selling point

Embedded Operating System

• Built to perform a small range of specific tasks
• Catered towards a specific device e.g- a household appliance
• Limited functionality and hard to update
• Consumes less power than other types of OSs

Real time Operating Systems

• Some operating systems must operate in real time
• Must respond extremely quickly to inputs
• May need to cope with many inputs simultaneously
• Real time OSs are usually seen in safety critical environments
• If a hardware component fails the OS must have a failsafe to detect this and respond accordingly
• There is hardware redundancy - crucial components are duplicated in case one fails

Basic Input Output System (BIOS)

• Basic Input Output System is the first program that runs when a computer system is switched on
• Runs a series of tests before the operating system is loaded into memory:
• POST(Power On Self Test) which ensures that all hardware is correctly connected and functional
• Checking the CPU clock, memory and processor
• ​​​​​​​Testing for external memory devices

Device drivers

• A driver is a program that provides an interface for the OS to interact with a device
• Drivers are hardware dependent and OS specific:
• ​​​​​​​Drivers are needed to allow the OS to control hardware devices from your speakers to your graphics card
• The OS does not need to know the specifics of the hardware to be able to interact with it

Virtual Machine

• A theoretical computer and a software implementation of a computer system
• Provides an environment with a translator for intermediate code to run
• Uses of virtual machines include:
• Testing programs
• Protection from malware
• Running software compatible with different versions/types of operating systems

Intermediate code

• Code halfway between machine code and object code is called intermediate code
• Independent of processor architecture so can run across different machines and operating systems
• Takes longer to execute than low-level code

Categories of software

Systems software

• Systems software is low-level software that is responsible for running the computer system smoothly, interacting with hardware and generally providing a platform for applications software to sun.
• The user does not directly interact with systems software but it ensures high performance for the user
• Examples - Library programs, utility programs, operating system, device drivers

Utility programs

• Utility programs optimise the performance of the computer and perform useful background tasks
• Examples - Disk defragmenter, Automatic backup, Automatic updating, Virus checker and compression software

Utility Programs - Disk defragmenter

• Large files stored on a magnetic hard drive may be split up across several physical disk locations
• Disk defragmenter reorganises the hard drive so that files are in sequential blocks where possible
• The result is that files can be read more quickly

Utility programs - Automatic backup

• Data in permanent storage needs to be regularly backed up
• The users may forget so...
• The process can be automated, specifying:
• Where - portable hard drive, cloud, local server...
• When - usually when the computer is not being used
• What - which data should be backed up
• How - should backups be compressed​​​​​​​

Utility programs - Automatic updating

• Runs in the background detecting software update releases and automatically installing them
• Some updates add new software features
• Some important updates fix security issues which could otherwise be exploited by hackers

Utility programs - Virus checker

• Scans permanent storage for viruses by comparing files to known virus definitions
• May also san files "on access" as they are opened
• Must be kept up to date with new virus definitions to be effective
• Antivirus software uses heuristics - it now what types of behaviour are likely to be used by a malicious program

Utility program - Compression software

• Operating systems provide utilities that enable files to be compressed and decompressed
• This is used when compressing large files to be transmitted across the Internet and commonly used to compress scanned files

Libraries and Translators

• Allows programs to use pre-written routines to be translated into executable code

Application software

• Application software is designed to be used by the end-user to perform one specific task
• Application software requires system software in order to run
• Examples - desktop publishing, word processing, spreadsheets, web browsers

Off the shelf or Bespoke software?

• Ready made software available to anyone to purchase is known as off the shelf software
• Software that is custom created for a specific user is known as bespoke software
• Bespoke software is mostly used by businesses rather than individuals

Open source software

• Open source software allows anyone to access its source code
• The software is open source licensed out free to use
• Anyone may modify the software and sell it on the condition that the software produced is also open source
• Different from freeware - software that is free for anyone to use but the source code is not available

Closed source (proprietary) software

• Closed source software does not allow access to the source code
• Users must pay the person or company who owns the copyright for a license to use the software
• There may be restrictions on how the software is used

Assembly code

• Computers execute machine code
• It is difficult for humans to read, write and debug machine code
• Assembly code instructions are equivalent to machine code but easier for humans to work with e.g- LDA 5

Assembler

• Assembly code is a low level language
• Translating assembly code instructions into machine code is done by an assembler
• Each processor has its own instructions set and so the object code produced will be hardware specific

Compiler

• A compiler translates a whole program written in a high level language into executable machine code, going through several stages
• The resulting machine code is called object code
• The object code produced is also hardware specific

Interpreter

• An interpreter also translates code written in a high level language into machine code
• However, the interpreter does this line-by-line rather than translating the whole program before any of it can be executed

Compiler vs Interpreter

• Compiler
• Advantages:
• Program can be run many times without the need to recompile
• Faster to execute than interpreted code
• Compiled code cannot be easily read and copied by others
• Disadvantages:
• Must be recompiled to run on other platforms as it is hardware specific
• The source code must be re-compiled every time the programmer changes the program.
• Interpreter
• Advantages:
• Source code can be run on any machine without the interpreter
• If a small error is found, no need to recompile the entire program
• Disadvantages
• The program may be slower than a compiled program

Bytecode

• Most languages are not solely compiled or interpreted they use a combination of both
• For example - Java is compiled into bytecode which is an intermediate step between source code and machine code
• The bytecode is interpreted by a bytecode interpreter

Stages of compilation

• A compiler goes through several stages to convert source code to object code they are...
• Lexical analysis
• Symbol table
• Syntax analysis
• Semantic analysis
• ​​​​​​​Code generation

Lexical analysis

• All unnecessary spaces and all comments are removed
• Keywords (e.g-print), constants and identifiers are replaced with tokens representing their function in the program

Symbol table

• The lexer will build up a symbol table for every keyword and identifier in the program
• The symbol table helps to keep track of the run-time memory addresses for each identifier

Syntax analysis

• The stream of tokens from the lexing stage is split up into phrases
• Each phrase is parsed which means it is checked against the rules of the language
• If the phrase is not valid, an error will be recorded

Semantic analysis

• It is possible to create a sequence of tokens which is valid syntax but is not valid program
• Semantic analysis checks for this kind of error
• If an identifier has not previously been declared then semantically it is not a valid program

Code generation

• Once the program has been checked the compiler generates the machine code
• It may do this in several "passes" over the code because code optimisation will also take place

Code optimisation

• Sometimes source code is written insufficiently
• Code optimisation aims to...
• remove redundant instructions
• replace inefficient code with code that achieves the same result but in a more effective way

Libraries

• Most languages have sets of pre-written ( and pre-compiled) functions called libraries eg-mathematical operations
• A programmer can also write their own libraries
• Library functions can be called within a program

Linker

• The linker needs to put appropriate memory addresses in place so that the program can call and return from a library function

Loader

• The job of the loader is to copy the program and any linked subroutines into main memory to run
• When the executable code was created it may assume the program will load in memory address 0
• However, memory addresses in the program will need to be relocated by the loader because some memory will already be in use