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What is a computer?
A computer is a machine that processes data
What is the purpose of a computer?
To take data, process it and produce an output
What is hardware? 2 examples
The physical things that make up a computer. Examples: Monitor and CPU
What is software? 2 examples
The programs and applications that a computer system runs. Examples: Operating System and Word Processor
What is an embedded system?
Computers that are built into other devices that are often used to control and monitor machinery
Pros of embedded systems
Easier to design, cheaper to produce and more efficient as they’re dedicated to a single task
Cons of embedded systems
Less functionality, less variability in the things it can do
What does CPU stand for?
Central Processing Unit
What does the CPU do?
It processes all of the data and instructions in order to make the computer functional
What affects the CPUs processing power?
Clock speed, number of cores, cache size
What is the main computer architecture?
Von Neumann
What are the three main parts of the CPU?
Control Unit, Arithmetic Logic Unit, Cache
What does the Control Unit do?
It controls the flow of data inside the CPU (to registers, etc) and outside the CPU (to main memory, etc) and manages the fetch-execute cycle
What does the ALU do?
All the calculations, logic operations and binary shifts
Which register does the ALU house?
The accumulator
What is cache?
On board memory for the CPU, faster than the RAM but slower than registers
Why is cache useful?
It stores regularly used data to allow faster access times when the data needs to be retrieved
What are the downsides to cache memory?
They are low capacity and expensive compared to slower memory forms (RAM, secondary storage)
What are the levels to cache and their characteristics?
L1, L2 and L3, L1 has the lowest capacity but fastest access times whereas L3 has the highest capacity but slowest access times
What are registers and what do they do?
They temporarily hold data needed by the CPU, they are extremely fast and small pieces of onboard memory, crucial in the fetch-execute cycle
Example of registers
Program Counter, Accumulator, Memory Address Register, Memory Data Register
Program Counter - where is it and what does it do?
In the Control Unit, it holds the address in memory of the instruction for each cycle
Accumulator - where is it and what does it do?
In the ALU, it stores the results of calculations performed in the fetch-execute cycle
Memory Address Register - what does it do?
It holds any memory address (for data or an instruction) about to be read by the CPU
Memory Data Register - what does it do?
It holds the data or instruction after being read from or about to be written to memory
How does the CPU carry out the fetch-execute cycle? (common 6 mark question)
FETCH
Read and copy memory address from the Program Counter to the MAR
Read and copy the instruction stored at the address in the MAR to the MDR
Increment the Program Counter to point to the memory address of the next instruction
DECODE
The instruction in the MDR is decoded by the Control Unit
The Control Unit may then prepare for the next stage by loading certain values into the MAR/MDR
EXECUTE
The instruction is performed, which could be loading data from the memory, writing data to the memory, perform a calculation or logic operation, change the address in the PC or halt the program altogether
What is volatile memory? (1 Example)
It is temporary memory, it needs power to retain the data inside. Example: RAM
What is non-volatile memory? (1 Example)
It is permanent memory, keeps its contents even without power. Example: ROM
RAM - what is it and what is it used for?
Random Access Memory, it is high speed volatile memory often used as the main memory in a computer system. It can be read and written to and is where all data, files and programs are stored while in use.
What is virtual memory and when is it used?
A place in secondary storage where data can be moved if the RAM fills to capacity, it is used when too many programs/data/files are open at once or one particularly memory intensive program is being run
How does virtual memory work?
The computer moves data that hasn’t been recently used from the RAM into a place in secondary storage. When the CPU needs to read the data stored in the virtual memory, it has to move it back to RAM
Disadvantage of virtual memory and why
Can make a computer slow to respond when switching between applications or running a memory intensive program, this is because the access time for secondary storage vs RAM is significantly slower which in turn increases the process time for each instruction
What happens when a computer boots up?
The CPU reads the BIOS instructions from ROM which tells the CPU to perform self-checks such as test memory, hardware checks, etc
The OS is copied from secondary storage to RAM allowing the user interface to load and the computer to start running
ROM - what is it and what is it used for?
Read Only Memory, it is a small factory-made chip built into the motherboard of the computer that contains the instructions the computer needs to boot up (BIOS). As it is read-only, the chip cannot be written to but it is possible to update (flash) the BIOS
what are the 3 factors that affect CPU performance?
clock speed, number of cores and cache size
Clock speed - what is it and how does it affect the CPU performance?
The number of instructions a single processor core can carry out per second. Higher clock speed means instructions are carried out faster
What is overclocking a CPU and how can it be dangerous?
When a CPU is made to run at a higher clock speed than the factory set rate, if not done properly the CPU can overheat causing permanent damages to the system, to prevent this high performance cooling systems are usually needed
Number of cores - what is a core and how does this affect CPU performance?
They are each individual processing unit in a CPU that can process instructions independently. The more cores in a CPU the more instructions that can be carried out at once. Most PCs and smartphones have 4 or more cores.
why doesn’t increasing the number of cores automatically increase the performance?
software needs to be designed to use multicore processing meaning some data and instructions won’t benefit from more cores. also, instructions that are dependent on one another will stall a core, waiting for the other data to be processed which reduces efficiency
Cache Size - how does it affect CPU performance?
A larger CPU cache gives the CPU faster access to more data needed to process instructions
RAM - how does increasing it affect the overall performance of the computer?
The more RAM available, the more programs or more memory intensive programs can be run smoothly, leading to better performance. However, if a computer already has enough RAM to run everything the user wants, increasing RAM will make no difference to the performance
GPUs - what are they and what are they used for?
Graphics Processing Unit, they are specialised circuits for handling graphics and image processing in a CPU, which frees the CPU to process other data. Dedicated GPUs (graphics cards) can be used to increase the graphics quality for purposes such as PC gaming and design software
what is primary storage?
The memory areas that the CPU can access very quickly, they are often volatile eg. register, cache, RAM
what is secondary storage?
non-volatile storage where all data is stored when not in use eg. SSDs, HDDs, CDs
Hard Disk Drives - what are they?
A stack of magnetised metal disks that spin thousands of times a second, data is stored magnetically in small areas on the disks’ circular tracks, a movable arm can access these areas to read or write data
Advantages of HDDs (versus SSDs)
Cheaper
High Capacity
Long lasting + reliable
Disadvantages of HDDs
Easily damaged by impact
Slower access times
Need defragmenting
Susceptible to strong magnets corrupting data
Noisy
Solid State Drives - what are they?
A storage device that uses flash memory (non-volatile RAM) to store data, other common types of flash memory are USB sticks and SD cards
Advantages of SSDs (versus HDDs)
Faster read/write times
Don’t need defragmenting
More durable
Not susceptible to magnets
Silent
Disadvantages of SSDs
Expensive
Shorter life
Optical Discs - what do they include and how do they work?
Include CDs, DVDs and Blu-Ray discs, each with increasing capacity. Optical devices (that read/write data to the discs) use a laser to scan the surface of the discs where there are tracks, each with areas of lands and pits indicating binary values 1 and 0 based on the reflection of light from the laser to the sensor, lands reflect the light back (1) where pits scatter the light (0)
what are the different forms of optical discs and what are they used for?
ROM media has data pre-written onto it, and cannot be overwritten. Used for games, music and film
R media has data written on it only once and cannot be written to again. Used for copies of data
RW media can be written to over and over. Used for anything
Advantages of Optical Discs
Very cheap per GB
Portable
Can’t be damaged by water or shocks
DIsadvantages of Optical Discs
Relatively low capacity
Slowest read/write speed
Can be easily scratched
Magnetic Tapes - what are they and what are they used for?
Reels of tape that can be magnetised to store data, used by large organisations for archives of large amounts of data
Advantages of Magnetic Tape
Massive storage capacity
Extremely low cost per GB
Fast read/write speeds
Disadvantages of Magnetic Tape
Needs specialist equipment to read/write data
Slow searching for specific data
Order of secondary storage for read/write speed (fastest to slowest)
SSD
HDD
Magnetic Tape
Memory Cards
Optical Discs
Order of secondary storage for average cost per GB (lowest to highest)
Magnetic Tape
Optical Discs
HDD
Memory Card
SSD
Order of secondary storage for average capacity per unit (highest to lowest)
Magnetic Tape
HDD
SSD
Memory Card
Optical Discs
OS - what is it and what does it do?
A complex piece of software found on most computer systems. They exist to:
communicate with internal and external hardware (via device drivers)
provide a user interface
provide a platform for applications to run on
allow the computer to multitask (through memory management and controlling the CPU)
deal with file and disk management and organisation
manage system security and user accounts on the device
Device Drivers - what are they and what do they do?
Pieces of software that act as translators for the signals between the OS and the hardware. The OS chooses the right device drivers in accordance with the hardware detected, if a new piece of hardware is detected, the relative device driver is installed
UI - what is it and what does it do?
User Interface, allows the user to interact with a computer system
what are the two types of UI
GUI - Graphical User Interface
CLI - Command Line Interface
GUIs - what are they?
They are user interfaces that rely on visual stimulus to prompt the user's input, they have been WIMP-based in the past, but Android and iOS have adapted that for touchscreens
WIMP
Windows, Icons, Menus, Pointers
A guide to key elements of GUIs
Advantages of GUIs
Easy to understand for everyday users
Intuitive
Interactive
Less prone to errors
CLIs - what are they?
A text based interface where the user enters specific commands to complete tasks
Advantages of CLIs
More efficient for advanced users
Less resource heavy
Can be used to automate processes
How does the OS allow for multitasking?
By managing memory and CPU processing time:
when an application is opened the OS will decide when and which parts need to be moved to memory and will continue throughout the programs runtime
when multiple applications are opened, the OS allocates certain memory addresses to certain applications to ensure data is not overwritten
it also organises instructions into the most efficient order to be processed (by the CPU) in
what is file management?
the organisation of data into a usable hierarchical structure, deals with the naming, saving, movement, editing and deletion of data
how does the OS manage the hard disk?
it splits the disk into storage sectors, writes data to the disk in given sectors and keeps track of free space on the disk. it also organises and maintains the hard disk with utility software
what is a single user OS?
an operating system that only allows one user to use the computer at once eg. Windows
what is a multi user OS?
an operating system that allows several users to use the computer at the same time, these are often on mainframes and give simultaneous access eg. ATMs allow many people access to the bank's mainframe at the same time
how does the OS ensure data is kept private for each user on a system?
it allows each user to have access to only specific data that has been approved and their own personal data
often anti-theft measures are in place to prevent other users from accessing data they shouldn’t do such as password protected user accounts
Defragmentation - what is it?
putting broken up files stored in memory back together
why does defragmentation need to happen?
as files are moved, deleted and change size, gaps appear in the memory
these gaps are then filled by new data being written to the disk but that can further fragment the overall data on the hard disk causing the time needed to read/write data to the disk to increase
what does defragmentation software do?
reorganises data on the hard drive to put fragmented files back together, it also puts the free space together to reduce further fragmentation
why does defragmentation only need to happen on hard disks (rather than SSDs)?
SSDs have no moving parts and use flash memory so data can be found quickly wherever it is stored, defragmentation can sometimes shorten the lifespan of SSDs
Compression Software - what is it and what does it do?
a piece of software that reduces the size of files by permanently or temporarily removing data from them, these compressed files are quicker to download and take up less disk space
Encryption Software - what is it and what does it do?
a piece of software that encrypts and decrypts data to stop unwanted third parties from accessing it
to decrypt the data, the computer uses a unique key and a set of instructions to unscramble the data so it can be understood
Note 
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