4.4, 4.5 Computer systems

Computer Sciense GCSE

Low-level languages

Machine code and assembly language

Machine code

written in binary

Assembly language

written using mnemonics, then converted to binary by an assembler

Advantages of Low-level languages for programs

machine code is run directly by the CPU, uses instructions that correspond directly to the CPU’s hardware, very fast to execute

Disadvantages of Low-level language programs

are hardware dependent, different hardware= different sets of instructions, are difficult to write code for as their syntax is very different to english

High-level languages

Python, C#

Advantages of High-level languages

are not hardware dependent - code can run on different hardware after translation, easier to code, syntax is similar to English

Disadvantages of High-level languages

need to be translated in order to run on a CPU, use instructions that are specific to the language, are slower to execute because they need to be translated

Program translators

software that converts high-level code into machine code

3 common types of translators

compilers, interpreters, assemblers

A compiler

works through high-level code, translating every line into machine code, producing an executable file specific to the hardware. the original code isn’t shared with end user. once compiled, the executable file executes quickly and doesn’t need to be compiled again for the hardware on which it will run

Interpreter

translates one line of high level code then runs it - repeats.

no executable file produced, any hardware can run the code

the original high level code must be shared with end user.

each time a program is executed it must be re-interpreted.

slower to execute

Assembler

needed to convert assembly language into machine code. they have a 1:1 correspondence with each other. one instruction in assembly translates to 1 instruction in machine code

hardware

physical components of a computer

software

the programs that provide instructions for the computer

what 4 things do computers do with data?

inputs, processes, stores, outputs

What hardware processes the data

CPU - the central processing unit

Von Neumann Architecture

data and instructions stored in the same memory location

CPU architecture

the internal logical structure and organisation of the computer hardware

What does the CPU do

carries out a set of instructions that is contained in a computer program

5 CPU components

• ALU

• Control Unit

• Clock

• Registers

  • Buses

Arithmetic Logic Unit

carries out all arithmetic calculations, logical decisions, and comparisons

Control unit

decodes instructions and sends signals to control how data moves around the CPU and memory to execute these instructions

Clock

a vibrating crystal that generates digital pulses at a constant speed of vibration that synchronises the CPU activity

Registers

memory locations within the CPU that hold data that can be accessed very quickly

Buses

communication channels through which data moves, connect the CPU to main memory and any input/output devices

The performance of the CPU is affected by the:

• Clock speed

• cache memory

  • number or processor cores

Clock Speed

the clock controls how quickly the CPU works, the faster the clock the more instructions can be completed per second. clock chips currently operate at speeds of up to 4 billion instructions per second

Cache memory

small in size but can be accessed quickly by CPU, more cache gives CPU faster access to data

Number of Processor cores

each core performs separate fetch-execute cycles, if a cpu has multiple cores it can perform more instructions simultaneously

Fetch-execute cycle

The CPU continually fetches, decodes, and executes instructions

fetch

an instruction in the form of data is retrieved from main memory

decode

the CPU decodes the instruction

Execute

the CPU performs an action according to the instruction

Memory

computer needs this to access data quickly, made up of main memory, cache memory and registers

2 types of main memory

ROM AND RAM

Random Access Memory

volatile - needs constant power to maintain, holds the OS and any apps/data currently in use by the computer

can be accessed by the CPU quickly

the more RAM, the more programs/data it can run at the same time

ram is read/write

Read Only Memory

non-volatile, doesn’t need power to maintain it, provides storage for data and instructions needed to start the computer, ROM is normally written at the manufacturing stage.

Secondary storage

needed to permanently store files and programs, needs to be non volatile, low cost, high capacity, and reliable

What is magnetic storage -Magnetic Hard Disk Drives

They use a stack of rotating magnetic platters, a moving head moves across the surface to read and write data

Advantages of Magnetic storage

larg capacity

portable small drives

reliable for medium term 5-7 years

low cost

Disadvantages of Magnetic storage

slower access speeds

large hard drives aren’t portable

not durable - strong electric/magnetic fields can easily damage

What is solid state storage - solid state drives

uses electronic circuits to store data and flash memory, no moving parts-low power needed

Advantages of solid state storage

SSDs are lightweight and compact

very fast access speed

portable and durable

very reliable long term

Disadvantages of Solid state storage

lower capacity then magnetic, more expensive

Optical storage - CDs/DVDs

These devices use light properties to store data, they reflect laser light onto the surface of the rotating disk and read the reflections as 1s and 0s

Advantages of Optical storage

very portable, very durable, most reliable, very low cost

Disadvantages of Optical storage

Can’t withstand scratches, limited capacity, slowest access times, cost per GB=expensive

Cloud storage

services and apps stored/run on remote servers accessed via the internet, use magnetic or SS tech

Advantages of Cloud storage

data can be accessed from anywhere in the world with internet connection

amount of storage space is flexible, users can buy additional storage

all data is regularly backed up and kept secure

data can easily be shared with anyone anywhere in the world

Disadvantages of cloud storage

internet connection needed, users have little control over security, data can be targeted more easily by hackers, cloud providers can quicky change terms and prices

What is an embedded system

a computer system that has been designed for a dedicated function as a part of a bigger system, often made as a single chip. they are robust and reliable.

Characteristics of an embedded system

• perform a limited set of tasks to reduce size and improve performance

• uploaded at the manufacturing stage

• very limited options to modify

• low power consumption

• small in size

• rugged

• low cost

Exampls of an embedded system

washing machine, microwave oven