computer science paper 1

CPU (central processing unit)

the most important component in any computer system, attached to the motherboard

what is the purpose of the CPU

process data and instructions by constantly repeated the fetch execute cycle

control unit (CU)

sends control signals to direct the operation of the CPU

function of the control unit (2)

control signals and timing signals are sent to the ALU and other components such as RAM
also decodes instructions as part of the F-E cycle

ALU (arithmetic logic unit)

performs simple calculations and logical operations

register

temporary storage space for one instruction or address
different registers are used during the FE cyccle

cache memory

used to temporarily store data that is frequently accessed
split into different levels

cache memory (2)

slower to access than the registers but much faster than RAM

computer architecture

the way a computer is designed and structured is known as its architecture
the most common type of computer architecture is Von Neumann

John von Neumann

developed the concept of a stored program computer in the 1940s
before being able to store programs in memory, instructions had to be manually input by a human

component of von Neumann architecture

stores both program instructions and data in the same memory (RAM)
data is transferred between components on pathways called buses

other computer architecture

others like Harvard have two separate memories: one for program instructions and another for data

F-E cycle

the fetch execute cycle is performed by the CPU millions of times every second
this is how the CPU processes data and instructions for each program or service

program counter (PC)

tracks the RAM address of the next instruction to be fetched

memory address register (MAR)

stores the RAM address of the current instruction or data the CPU needs to access

memory data register (MDR)

stores the instruction that has been transferred from the RAM to the CPU

current instruction register (CIR)

stores the instruction that has been fetched from the RAM and is about the be decoded or executed

accumulator (ACC)

stores the result of mathematical or logical calculations

what is the essential idea of the F-E cycle

instructions are fetched from the RAM to be decoded and executed by the CPU

F-E cycle (1)

the PC register displays the address in RAM of the next instruction to be processed
this is copied into the MAR

F-E cycle (2)

the PC register is increased by 1 to prepare the CPU for the next instruction to be fetched

F-E cycle (3)

the CPU checks the address in RAM which matches the address held in the MAR

F-E cycle (4)

the instruction in RAM is transferred to the MDR

F-E cycle (5)

the instruction in the MDR is copied into the CIR

F-E cycle (6)

the instruction in the CIR is decoded and executed
any result of an execution is stored in the ACC

F-E cycle (7)

the cycle repeats by returning to the first step and checking the program counter for the address for the next instruction

how does cache memory improve performance (1)

closer to the CPU than ram meaning that it can provide data and instructions to the CPU at a faster rate

how can cache memory improve performance (2)

a computer with more cache memory should have a higher performance
repeatedly used instructions can be stored and accessed faster

what is the limitation of cache memory

cache memory is costly so computers only have a small amount

what is clock speed

measure of how quickly can process instructions (GHz)
a typical desktop may have a clock speed of 3.5GHz
this means it can perform 3.5 billion cycles per second

how can clock speed improve performance

the faster the clock speed the faster the computer can perform the FDE cycle resulting in better performance
more instructions can be processed each second

how can overclocking affect performance

overclocking is when the computer’s clock speed is increased higher than the recommended rate
this will make the computer perform faster but can lead to overheating and could damage the machine

how can underclocking affect performance

underclocking is when the computer’s clock speed is decreased lower than the recommended rate
this will make the computer perform slower but will increase the lifespan of the machine

what is a core

complete set of CPU components, each core is able to perform its own FDE cycle

what is a multi-core CPU

has more than one set of components within the same CPU

how does the number of cores improve performance (1)

a single core processor can execute one instruction at a time, a dual core processor can execute two instructions at a time and a quad core can execute four instructions simultaneously

how does the number of cores improve performance (2)

a computer with more cores will have a higher performance because it can process more instructions at once

what are the limitations of having more cores (1)

if one core is waiting for another to finish processing, performance may not increase at all

what are the limitations of having more cores (2)

some software is not written to make use of multiple cores so it will not run any quicker on a multi core computer

embedded system

computer system built into a larger machine to provide means of control
perform a specific pre programmed tasked stored in ROM

function of embedded system

uses a combination of hardware and software
only perform specific tasks and often have a small amount of storage and low processing power

examples of embedded systems (1)

a washing machine has a control chip that manages different program cycles

examples of embedded systems (2)

a traffic light has a control chip that determines when to change to a green or red light

storage in a computer system: primage storage

very quick because it is directly accessed by the CPU
typically smaller in storage size
sometimes called main memory
includes RAM and ROM

volatile storage

temporary data is lost whenever the power is turned off e.g RAM

non volatile storage

saves the data even when not being powered
data can be stored long term and accessed when the computer is switched on

why do computers need primary storage? (1)

it is low capacity, internal storage that can be directly accessed by the CPU

why do computers need primary storage? (2)

program instructions and data must be copied from the hard drive into the RAM to be processed by the CPU

why do computers need primary storage? (3)

primary storage access speeds are much faster than secondary storage devices like the hard drive

types of primary storage (RAM)

volatile storage that stores all programs that are currently running
also stores parts of the operating system needed to be accessed by the CPU

types of primary storage (RAM) (2)

made up of large number of storage locations, each can be identified by a unique address

types of primary storage (ROM)

non volatile storage than cannot be changed

types of primary storage (ROM) (2)

stores the boot program/BIOS for when the computer is switched on
BIOS then loads up the operating system to take over managing the computer

virtual memory

programs must be stored in RAM to be processed by the CPU
even if there is insufficient space in RAM for all programs the computer can use the HDD as an extension of RAM
this is called virtual memory

virtual memory (2) what happens if the original data is required again?

it can be moved backed from virtual memory into RAM
using virtual memory is beneficial because it allows more programs to be run at the same time with less system slow down

storage in a computer system: secondary storage

slower as it is not directly accessed by the CPU
typically larger in storage size
used for long term storage of data and files because it is non volatile
includes magnetic, optical and solid state

what is secondary storage

non volatile storage used to save and store data that can be accessed repeatedly

function of secondary storage

not directly embedded on the motherboard or external and therefore further away from the CPU so it is slower to access than primary storage

storage characteristics: capacity

the maximum amount of data that can be stored on the device

storage characteristics: durability

the strength of the deice to last without breaking

storage characteristics: portability

how easy it is to carry the device around

storage characteristics: access speed

how quickly data on the device can be read or edited

storage characteristics: cost

the average price it costs to purchase the storage device

storage characteristics: reliability

the likely hood of the device continuing to perform well over time

magnetic storage: HDD

most common form of secondary storage within desktop computers
read/write head moves nanometres above the disk platter and uses the magnetic field of the platter to read or edit data

magnetic storage: floppy disk

obsolete and replaced by solid state devices like USB sticks which are much faster and have a much higher capacity

magnetic storage: magnetic tape

high storage capacity but data has to be accessed in order so it is generally only used by companies to back up or archive large amounts of data

magnetic storage characteristics (HDD) capacity and cost

large capacity and cheaper cost per gigabyte than solid state

magnetic storage characteristics (HDD) durability and portability

modern external HDDS are small and well protected
due to moving parts, they should not be moved when powered on as it can damage the devices

magnetics storage characteristics (HDD) access speed

slower access speed than solid state but faster than optical storage

optical storage

uses a laser to project beams of light onto a spinning disc, allowing it to read data from a CD, DVD, or Blu-Ray

what is the result of optical storage’ way of functioning (2)

this makes optical storage the slowest of four types of secondary storage

optical storage: disc drives

traditionally and internal but external disc drives can be bought for devices like laptops

optical storage characteristics capacity

low capacity: 700MB (CD) 4.7GB (DVD) 25GB (Blu-Ray)

optical storage characteristics durability

not durable because discs are very fragile and can break or scratch easily

optical storage characteristics portability and cost

discs are thin and very portable and also very cheap to buy in bulk

optical storage characteristics access speed

optical discs have the slowest access speed

solid state storage

no moving parts

solid state storage: SSDs

replacing magnetic HDDs in modern computers and video game consoles because they are generally, quieter, faster and use less power

solid state storage: USB flash drive/USB stick

another type of solid state storage that is used to transport files easily because of its small size

solid state storage: memory cards

e.g SD card in a digital camera or a micro SD card in a phone are another example

solid state characteristics cost

more expensive cost per gigabyte than magnetic

solid state characteristics durability

usually durable but cheap USB sticks can snap or break

solid state characteristics portability

the small size of USB sticks and memory cards mean they are very portable and can fit easily in a bag or pocket

solid state characteristics capacity and access speed

have a high capacity and the fastest access speed as they contain no moving parts

binary

all computer systems communicate, process and store data using binary
binary is a number system consisting of entirely 0s and 1s

why do computers use binary

computer systems consist of billions of tiny transistors which are switches that only have two values
all data must be represented and processed in this way

why do computers use binary (2)

everything that a computer needs to process must be converted into a binary format including text, images, video and audio

units of data storage (bits and nibbles)

all data in a computer system is made up of bits
a single bit is 0 or a 1
4 bits is called a nibble

units of data storage (bytes and kilobytes)

8 bits is called a byte which can store a single character (A)
1000 bytes is called a kilobyte which can store a short email

units of data storage (megabytes and gigabytes)

1000 kilobytes is called a megabyte which can store about a minute of music
1000 megabytes is called a gigabyte which can store about 500 photos

units of data storage (terabytes and petabytes)

1000 gigabytes is called a terabyte which can store about 500 hours of films
1000 terabytes is called a petabyte which can store about 1.5 million CDs

denary and hexadecimal

denary is a base 10 number system
hexadecimal is a base 16 number system

why is hexadecimal used as a shorthand for binary

it uses fewer characters to write the same value making hexadecimal less prone to errors when reading or writing it

hexadecimal A-F meanings

nice

what is an overflow error

occurs when a binary value is too large to store the bits available

how do overflow errors occur

with a byte the largest number that can be held is 255
any some of two binary numbers that is greater than 255 will result in an overflow as it is too large to be held in 8 bits

binary shift

used to multiple and divide binary numbers
shift left = multiply
shift right = divide
shift 1 has an effect of 2, shift 2 has an effect of 4, shift 3 has an effect of 8

what is character set

table that matches together a character and a binary value
each character in a character set has a unique binary number matched with it

why are character sets used

they allow computers to exchange data and humans to input characters