1.2 Software and software development

Systems software

Manages computer resources, maximises performance.

Operating systems

Provide an interface between the user and computer. Provide many features such as management and security.

Interupts

Signals generated by software or hardware to tell the CPU that a process needs attention.

Scheduling

Ensures all sections of programs being run get a fair amount of processing time.

Distributed OS

Runs across multiple devices, the load of the task is spread across multiple computer processors.

Multi-tasking OS

Tasks can be carried out simultaneously, time slices used to switch between programs and applications in memory.

BIOS (basic input output system)

Runs tests to ensure all hardware is correctly connected and functional.

Device drivers

Allows OS to interact with hardware.

Virtual machines

Theoretical computer and a software implementation of a computer system. Provides an environment with a translator.

Application software

Used by end user, performs one specific task.

Utilities

Maintain a high performing OS, each utility has a specific functions. Example: compression, disk defrag.

Translator

Converts source code into object code.

Stages of compliation

Lexical analysis
Syntax analysis
Code generation and optimisation

Lexical analysis

Removes spaces and comments while key wards and identifiers are replaced with tokens.

Software development life cycle

Analysis
Design
Development
Testing
Implementation
Evaluation
Maintenance

Agile methodologies

Flexible, adapt to changes quickly. High quality, flexible, regular user input. Poor documentation, requires consistent interaction between user and programmer.

Waterfall lifecycle

Only does it once doesn't go back. Straightforward, clearly documented. Lack of flexibility, no risk analysis, limited user involvement.

Spiral model

Manage heavy risk projects. Good risk analysis, caters to changes, produces prototypes. Expensive, lack of focus on efficiency.

Extreme programming

Produce high quality code, constant user involvement means high usability. High cost, teamwork is essential, end-user may not be able to be present.

Polymorphism

Objects behave differently depending on their class. Overloading(different parameters) + overriding(redefining a method).

Paging

Memory is split up into equal-sized sections known as pages. Pages are swapped between main memory and the hard disk as needed

Segmentation

Memory is split up into logical sized divisions, they represent the structure and logical flow of the program.

Virtual memory

Happens when RAM is full and it uses a section of the hard drive, this frees up space in RAM for other programs.

Disk thrashing

Pages swapped too frequently between hard disk and RAM.

Interrupt service routine (ISR)

Checks for interrupts at the end of each FEDEX cycle. If there is an interrupt the contents of the registers are transferred into a stack until the interrupt is solved. This is the same if an even higher interrupt interrupts the interrupt.

Round robin

Each job is given a section of processor time which it runs.

First come first served

Jobs are processed in the order they are added to the queue.

Multilevel feedback queues

Multiple queues that is ordered based on different prioritys.

Shortest job first

The queue is process and the shortest job will be completed first and the longest job will be completed last.

Shortest remaining time

Jobs with shortest amount of time remaining to be processed are processed first.

Embedded OS

Performs a small range of specific tasks suck as household appliances. They consume less power.

Multi-user OS

Multiple users use one computer.

Real time OS

Designed to perform a task within a guaranteed time frame.

Open source

Free and distributed with the source code.

Closed source

Costly, requires a licence and the user does not have access to the source code.

Compiler

Translates all code in one go, if no changes have been made it can be ran without having to be compiled.

Interpreter

Translates line by line and stops at an error and outputs it.

Assembler

Translates assembly code into machine code.

Bytecode

Mix of compiler and interpreter.

Syntax analysis

Tokens analysed against rules, syntax errors are flagged.

Code generation and optimisation

Code is produced, and it optimises by getting rid of unnecessary code.

Linkers

Links external modules and libraries included within the code.

Loaders

Fetches the library from the given memory location.

Rapid application development

Fast and incomplete version are given to the user to trial. Caters to change, usable end product, focus on core features. Low quality documentation, fast pace and late changes may reduce quality.

Structured programming

Sequence - Line after line
Selection - One things happens based of another thing
Iteration - Repeats
Recursion - Calls itself, requires a stopping condition that will be reached

Add

ADD - Adds the value at the given memory address to the value within the accumulator

Subtract

SUB - Subtract the value at the given memory address from the value in the Accumulator

Store

STA - Store the value in the Accumulator at the given memory address

Load

LDA - Load the value at the given memory address into the Accumulator

Input

INP - Allows the user to input a value which will be held in the Accumulator

Output

OUT - Prints the value currently held in the Accumulator

End

HLT - Stops the program at that line, preventing the rest of the code from executing.

Where to store data

DAT - Creates a flag with a label at which data is stored.

Branch if zero

BRZ - Branches to a given address if the value in the Accumulator is zero. This is a conditional branch.

Branch if positive

BRP - Branches to a given address if the value in the Accumulator is positive. This is a conditional branch.

Branch always

BRA - Branches to a given address no matter the value in the Accumulator. This is an unconditional branch.

Opcode

Specifies the instruction to be performed and the addressing mode.

Operand

Data on which the instruction is to be performed.

Immediate addressing

The operand is the actual value upon which the instruction is to be performed

Direct Addressing

The operand gives the address which holds the value upon which the instruction is to be performed

Indirect Addressing

The operand gives the address of a register which holds another address, where the data is located

Indexed Addressing

An index register is used, which stores a certain value. The address of the operand is determined by adding the operand to the index register