UNIT 1: Components of a computer and their uses Flashcards
1
Q
what does CPU stand for
A
Central processing unit
2
Q
What is the Central Processing Unit (CPU)
A
the processor (also called the central processing unit or cpu) has a number of different components each with their own role to perform:
control unit
buses
arithmetic logic unit (ALU)
dedicated registers
3
Q
What is the Control Unit (CU)
A
the part of the processor that coordinates the activity of all other components
control signals are sent along the control bus between the control unit and the other componenets of the computer
4
Q
what is a bus?
A
Buses in a computer consist of a series of connectors that transfer signals between internal components
they typicaly consist of 8,16,32 or 64 lines
The components of a computer system are connected together using buses. A bus is a communication system that allows internal components to communicate with each other and exchange data.
Each bus consists of a set of parallel — not single — lines along which data can travel. Multiple lines means that multiple bits can travel at the same time allowing data to be transferred quickly between components.
5
Q
System bus
A
the system bus consists of three separate buses carrying control signals, address and data (the big 3)
6
Q
name the BIG 3 (buses)
A
address
data
control
7
Q
control signals can include
A
memory read - data from addressed location in RAM to be placed on the data bus
memeory write - data on the data bus to be written into the addressed location in RAM
bus request - indicates that a device is requesting use of the data bus
bus grant - indicates that the CPU has granted access to the data bus
clock - used to synchronise operations
8
Q
Address bus purpose
A
Carries address locations of stored data from the processor to main memory and input/output controllers
9
Q
Data bus purpose
A
Carries data to and from the processor, main memory, and input/output controllers
10
Q
Control Bus purpose
A
Carries signals that coordinate the operation of the components
11
Q
Arithmetic Logic Unit (ALU)
A
the problem solving part of the processer
- perofmrs arithmetic logical and shift operations on data
- arithmetic operations - Add, subtract, multiply, divide
- logical operations - ADD OR NOT XOR
- shift operations: move bits to the left or right within a register
12
Q
the accumulator
A
results from the ALU are stored here
- rather than writing working data back to slow memory , processors have several locations of super fast memory called registers that are used to temporarily store results
- processor is then able it immediately access and re0use these results in subsequent calcs (add 2+3+4)
- for simplicity we will assume the processor has a single general purpose register called an accumulator
13
Q
executing instructions (where info is stored)
A
- carrying out a sequence of programming instructions requires many different pieces of information to be held
- the processor has to temporarily hold the current instruction being executed
- it has to hold the address of the data that it needs and also the data itself
- it has to keep track of the address of the next instruction to be executed
14
Q
dedicated registers
A
PC
CIR
MAR
MDR
ACCUMULATOR
15
Q
Program counter (PC)
A
holds the memory address of the next instruction to be executed
16
Q
Current Instruction Register (CIR)
A
holds the current instruction which is split into opcode and operand
17
Q
Memory address register (MAR)
A
holds the address in memory where the processor is required to fetch or store data from or to
18
Q
Memory data register (MDR)
A
temporarily holds data moving between the processor and main memory
19
Q
accumulator
A
to hold intermediate results of an instruction
20
Q
Fetch Decode Execute Cycle
A
The fetch-decode-execute cycle describes the basic operation of modern computer systems.
To run a program these instructions must be fetched, decoded, and executed.
They are repeated over and over again for every instruction of every program that is run inside a computer.
21
Q
Fetch: Steps 1- 4 NAME STEP 1
- the address….
- The instruction held at that address is copied to the Memory Data Register (MDR)
- Simultaneously, the contents of the Program Counter (PC) are incremented
- The contents of the MDR are copied to the Current Instruction Register (CIR)
A
- the address of the next instruction is copied from the PC to the memory address register (MAR)
- The instruction held at that address is copied to the Memory Data Register (MDR)
- Simultaneously, the contents of the Program Counter (PC) are incremented
- The contents of the MDR are copied to the Current Instruction Register (CIR)
22
Q
Fetch: Steps 1- 4 NAME STEP 2
- the address of the next instruction is copied from the PC to the memory address register (MAR)
- The instruction…
- Simultaneously, the contents of the Program Counter (PC) are incremented
- The contents of the MDR are copied to the Current Instruction Register (CIR)
A
- the address of the next instruction is copied from the PC to the memory address register (MAR)
- The instruction held at that address is copied to the Memory Data Register (MDR)
- Simultaneously, the contents of the Program Counter (PC) are incremented
- The contents of the MDR are copied to the Current Instruction Register (CIR)
23
Q
Fetch: Steps 1- 4 NAME STEP 3
- the address of the next instruction is copied from the PC to the memory address register (MAR)
- The instruction held at that address is copied to the Memory Data Register (MDR)
- Simultaneously, the contents…
- The contents of the MDR are copied to the Current Instruction Register (CIR)
A
- the address of the next instruction is copied from the PC to the memory address register (MAR)
- The instruction held at that address is copied to the Memory Data Register (MDR)
- Simultaneously, the contents of the Program Counter (PC) are incremented
- The contents of the MDR are copied to the Current Instruction Register (CIR)
24
Q
Fetch: Steps 1- 4 NAME STEP 4
- the address of the next instruction is copied from the PC to the memory address register (MAR)
- The instruction held at that address is copied to the Memory Data Register (MDR)
- Simultaneously, the contents…
- The contents…
A
- the address of the next instruction is copied from the PC to the memory address register (MAR)
- The instruction held at that address is copied to the Memory Data Register (MDR)
- Simultaneously, the contents of the Program Counter (PC) are incremented
- The contents of the MDR are copied to the Current Instruction Register (CIR)
25
Decode: steps 5 - 7 NAME STEP 5
5. The instruction....
6. It is split into Operand and Opcode to determine the type of instruction it is. Additional data, if required is fetched from memory
7. and passed to the accumulator
- the opcode specifies the operation that is to be carried out
- the operand holds either:
address of the data to be used, which is then copied to the MAR or
the actual data to be operated on which is then passed to the MDR
Decode: steps 5 - 7
5. The instruction held in the CIR is decoded
6. It is split into Operand and Opcode to determine the type of instruction it is. Additional data, if required is fetched from memory
7. and passed to the accumulator
- the opcode specifies the operation that is to be carried out
- the operand holds either:
address of the data to be used, which is then copied to the MAR or
the actual data to be operated on which is then passed to the MDR
26
Decode: steps 5 - 7 NAME STEP 6
5. The instruction held in the CIR is decoded
6. It is split into..
7. and passed to the accumulator
- the opcode specifies the operation that is to be carried out
- the operand holds either:
address of the data to be used, which is then copied to the MAR or
the actual data to be operated on which is then passed to the MDR
Decode: steps 5 - 7
5. The instruction held in the CIR is decoded
6. It is split into Operand and Opcode to determine the type of instruction it is. Additional data, if required is fetched from memory
7. and passed to the accumulator
- the opcode specifies the operation that is to be carried out
- the operand holds either:
address of the data to be used, which is then copied to the MAR or
the actual data to be operated on which is then passed to the MDR
27
Decode: steps 5 - 7 NAME STEP 7
5. The instruction held in the CIR is decoded
6. It is split into Operand and Opcode to determine the type of instruction it is. Additional data, if required is fetched from memory
7. and passed to the accumulator:
- the opcode specifies ....
- the operand holds either...
Decode: steps 5 - 7
5. The instruction held in the CIR is decoded
6. It is split into Operand and Opcode to determine the type of instruction it is. Additional data, if required is fetched from memory
7. and passed to the accumulator
- the opcode specifies the operation that is to be carried out
- the operand holds either:
address of the data to be used, which is then copied to the MAR or
the actual data to be operated on which is then passed to the MDR
28
Execute: step 8 NAME STEP 8
The instruction is executed and the result is held in the accumulator or stored in memory
29
Words
memory is divided up in equal units called words
word length is usually 8,16, 32 or 64 bits
each word has a separate memory addressa
30
address bus in more detail
width of the address bus determines the maximum possible memory addresses of the system
with an 8 bit address bus, the max number of addresses is 2^8 (256)
an average PC has a memory capacity of 4 GiB (gibi bytes) which 2^32 bytes
therefore it must have a 32 bit address bus
basically the width of the bus determiens how much memory it has
if the pc has 4 GiB memory capacity which is 2^32 bytes it must have a 32 bit address bus for some reason
31
Data bus in more detail
the data bus is bi directional as data can be sent both ways along the bus
- the width of the data is defined by the number of wires or lines it contains
if the data bus is the same width as a computer word, data can be transferred to and from memory in a single operation
32
format of instructions (assembly and machine stuff)
assembly language is very closely related to machine code
- generally a one to one correspondance
the architecture of a computer including
- the word size and the width of the address bus
determine the format of machine code instruction for a particular type of processor
33
A machine code instruction
the basic structure of a machine code:
operation code. |. Operand(s)
_______________________________________________
basic machine operation | addressing mode|operand
______________________________________________
10101001010010101010101010101010101010
maximum size of the operand will depend on the width of the address bus
34
factors affecting performance (3)
clock speed
number of cores in the processor
amount and type of cache memory
35
Clock (factors affecting performance)
FDE cycle is triggered by the clock pulses of the system clock
faster the clock speed the faster a computer can fetch, decode and execute instructions
36
The System clock
series of regular ON/OFF signals are used to synchronise the operations of the processor components
- actions are usually carried out on the rising edge of the clock
- actions each take a fixed number of cycles to complete
The time taken between signal changes is called a clock cycle. The speed of the clock is measured by the number of clock cycles in one second. One clock cycle per second is 1Hz.
Hertz | Measurement
1 hertz (Hz) | One clock cycle per second
1 megahertz (MHz) | One million clock cycles per second
1 gigahertz (GHz) | One billion clock cycles per second
37
Number of cores (Factors affecting performance)
many computers today including personal computers, have multiple cores
- dual core computer has 2 processors linked together in the same integrated circuit
- a quad core computer has 4 linked processors
each core is theoretically able to process a different instruction at the same tie with its own fetch execute cycle, making a quad core computer two or even four times faster than a single core computer
However the software may not always be able to make full advantage of all four processors
38
parallel processing
using several processor cores working at the same time is known as parallel or concurrent processing
- in systems designed for parallel processing, each core can work concurrently on different parts of the same task
- since instructions are processed sequentially, this is not always possible
39
cache memory (1)
cache is a small amount of superfast (but expensive) memory that stores data and instructions that have recently been used by the processor
level 2 cache is larger but not as fast as level 1 cahce
both type are held on the processor chip
CPU registers
Level 1
Level 2
RAM
(pyramid shape)
40
cache memory (2)
level 1 cache memory is split into instruction cache and data cache so that data and instructions can be fetched simultaneously
- the more cache memory a computer has, the more likely it is that it will not have to fetch the next instruction or data from RAM, as it will already have been loaded into the super fast cache memory from which it can be retrieved much more quickly
41
pipeling
technique used to improve performance, for example by overlapping stages in the fetch-execute cycle, or by breaking down the stages in an arithmetic instruction
- an insturction enters the pipeline and as soon as one stage has been completed another insturction enters the pipeline
- a third instruction then enters before either of the othes is completed
There may be 10 or 12 stages in the pipeline with some stages taking longer than others
42
multipurpose machines
Early computers were able to calculate an output using fixed instructions
- they could perform only one set of instructions
in the 1940s, John von Neumann and Alan Turing both proposed the stored program concept
43
stored program concept
a program must be loaded into main memory to be executed by the processor
the instructions are fetched one at a time, decoded and executed sequentially by the processor
the sequence of instructions can only be changed by a conditional or unconditional jump instruction
44
John Von Neumann
In the 1940s, John von Neumann and his team developed the concept of the stored program computer. The von Neumann systems architecture used the idea of storing program instructions and data in main memory and moving them between memory and the processor when they were needed by the processor. Von Neumann architecture is used in many modern-day computer systems.
The von Neumann architecture consists of
- a Processor
- a Memory unit that can communicate directly with the processor
- connections for input and output devices
- Secondary storage for saving/backing up data
The processor can access the instructions and data in the main memory as required to execute the program. It does this by using dedicated connections called buses:
an address bus is used to identify the addressed location
a data bus is used to transfer the contents to/from that location
This means that the same address and data buses are used in the process of transferring instructions and data between main memory and the processor. A third bus, the control bus, is used to synchronise and control operations.
45
Harvard architecture
On the other hand, Harvard architecture keeps instructions and data in separate memories. The processor accesses these memories using separate data and address buses; the processor is connected to the ‘instructions memory’ using a dedicated set of address and data buses, and is connected to the ‘data memory’ using a different set of address and data buses. The Harvard architecture is used extensively in embedded systems, for example in digital signal processing (DSP) systems.
Many microcontroller devices use a Harvard-like architecture. Some types of smartphones use a modified Harvard architecture — although distinctions between modern processors are difficult to categorise in relation to such 'pure' theoretical models.
an alternative model separates the data and instructions into separate memories using different buses
program instructions and data are no longer competing for the same bus
46
use of Harvard architecture
different sized memories and word lengths can be used for data and instructions
harvard principles are used with specialist embedded systems and digital signal processing (DSP) where speed takes priority over the complexities of design
47
advantages of Von Neumann architecture
owing primarily to cost and programming complexity, almost all general purpose computers are based on von neumann's principles
it simplifies the design of the control unit
data from memory and from devices are accessed in the same way
48
Von Neumann (4 points)
- used in PC's, laptops, servers and high performance computers
- data and instructions share the same memory. Both use the same word length
- One bus for data and instructions is a bottle neck
- One bus is simpler for control unit design
49
Harvard architecture (4 points)
- used in digital signal processing, microcontrollers and in embedded systems such as microwave ovens and watches
- instructions and data are held in separate memories which may have different word lengths. Free data memory can't be used for instructions, and vice versa
- separate buses allow parallel access to data and instructions
- control unit for two buses is more complicated and expensive
50
contemporary processor architectures
- modern CPU chips often incorporate aspects of both von Neumann and Harvard architecture
- in desktop computers, there is on main memory for holding both data and instructions, but cache memory is divided into an instruction cache and a data cache so data and instructions are retrieved using Harvard architecture
- some digital signal processors have multiple parallel data buses (two write, three read) and one instruction bus
51
what does CISC stand for
Complex instruction set computers
52
CISC
in CISC a large instruction set is used to accomplish tasks in as few lines of assembly language as possible
- a CISC instruction combines a load/ store instruction with the instruction that carries out the actual calculation
CISC (complex instruction set computer) processors are used in desktop and laptop computers. This type of processor can process more complex instructions. For example, a single instruction can involve loading two values, calculating their sum, and storing the result back in memory.
53
what does RISC stand for?
Reduced instruction Set Computers
54
RISC
RISC take an opposite approach
a minimum number of very simple instructions, each taking one clock cycle, are used to accomplish all the required operations in multiple general purpose registers
RISC (reduced instruction set computer) processors are used in modern smartphones and tablets. This type of processor can carry out simple instructions quickly. Several instructions are needed to carry out basic tasks, such as adding together two numbers.
55
Advantages of CISC (3 points)
-quicker to code programs
- the complier has very little work to do to translate a high-level language statement into machine code
- because the code is relatively short very little RAM is required to store the instructions
56
Advantages of RISC (3 points)
- Hardware is simplier to build with fewer circuits needed for carrying out complex insturctions
- Because each instruction tasks the same amount of time, i.e. one clock cycle, pipelining is possible
- RAM is now cheap, and RISC use of RAM and software allows better performance processors at less cost.
57
Multi-core and parallel systems
multicore processors are able to distribute workload across multiple processor cores, thus achieving significantly higher performance by performing several tasks in parallel
these are therefor known as parallel systems
many personal computers and mobile devices are dual-core or quad-core, meaning they have two or four processing chips
supercomputers have thousands of cores
58
using parallel processing
software has to be written to take advantage of multiple cores
for exmaple browers such as chrome, firefox can run several concurrent processes (tabs and stuff)
59
co-processor systems
a co-processor is an extra processor used to supplement the functions of the primary processor (the CPU)
- may be used to perform floating point arithmetic, graphics processing, digital signal processing and other functions
- it generally carries out only a limited range of functions
60
GPU
A computer running applications that require video- or graphic-intensive applications needs a separate, dedicated processor or card. This design frees up the main processor to handle other tasks. Graphics cards contain their own RAM as well as a processor. The processor on a graphics card is referred to as a graphics processing unit (GPU).
GPUs are designed for parallel processing and are used in a wide range of applications, including graphics and video rendering. It is not unusual for a GPU to contain 1,000+ cores. Operations such as 3D graphics rendering are divided across multiple cores, as each core performs a small part of the whole operation.
GPUs are a good example for how multicore systems with parallel processing have already achieved widespread use. See the earlier material on multiple cores for further information on the benefits and possible limitations of such architectures.
simple way of explaining:
its a speicialised electronic unit which is very efficient at manipulating computer graphics and image processing
- consists of thousands of small efiicient cores deisgned for parallel processing
- it can process large blocks of visual data simultaneously
- in a PC, a GPU may be present on a graphics card
61
function of a GPU
Although they’re best known for their use in gaming, GPUs are also used for other specialised purposes, such as machine learning applications. GPUs with thousands of cores are optimised to parallel process calculations, which are required for machine learning algorithms (for example, when training a neural network).
scientific, engineering and other applications
used in mobile phones, tables, cars, drones and robots
62
RAM
volatile
Main memory (sometimes called primary storage) refers to storage locations that are directly accessible by the processor. The most common types of main memory are commonly referred to a RAM and ROM.
RAM is used to hold the data and instructions that are currently being processed.
In order for any program to be executed, its instructions must first be loaded into RAM from Secondary storage. The processor will fetch instructions from RAM before decoding and executing them.
Data that is being processed is also stored in RAM. The processor will fetch data from RAM as it is needed and will write data back into RAM once it has been processed. For data to be saved permanently, it must be transferred to secondary storage (such as a hard disk).
RAM is made up of many separate locations, each of which is given a unique memory address. The processor will use these addresses to access the correct location. Any of the locations in RAM can be accessed directly in the same amount of time. This is why it is called random access: any location can be accessed "at random" with no impact on speed.
63
ROM
ROM is read-only memory which means that it cannot be written to. It is typically used to store the boot sequence (BIOS) for a computer. The contents of ROM are set by the computer manufacturer.
The BIOS (Basic Input/Output System) is a limited sequence of instructions that checks that the core components of the computer system (RAM, key input/output devices, secondary storage) are connected and responding correctly.
Once done, the boot sequence loads the essential parts of the main operating system (the "kernel") from secondary storage into RAM. From this point, the main operating system will oversee the operation of the computer, managing memory, storage, and requests for input/output.
Modern computer systems usually store the BIOS on flash memory so that the BIOS can be updated to add new features or fix bugs. This must be done with care, as any changes you make could stop the computer booting up properly.
64
Manual input devices examples
keyboards and keypads
pointing devices
microphones
touch screens
interactive whiteboards
all input devices transfer data from the source in the outside world to the computer
65
types of sensor
temp
moisture
light
infra red
pressure
acoustic
gas
pH
magnetic field
66
monitoring and control
monitoring- computer or microprocessor will make no changes to the actual prcoess it will simply report the values
control - thr output from the computer or microprocessor can alter how the process is operating
- it can change the value of the next input recieved by, for exmaple, oepnign a vlave, switiching off a heater or changing the speed of a pump
- the output from the computer or micro processor can affect the next input it receives
67
detection systems
pressur, acousitc, infrared
68
Output devices
take data produced by a computer and turn it into a human-readale form (such as a printed doc or an image on a screen)
- speaker or smth, printer screen, projector
69
3d printers
medicine, medicine, manufcaturing and art
dangers - parts for a gun
70
radio frequency ID (RFID
input/output devices
they use a transponder and a receiver
- the powered reciever emits radio frequency energy
- trasnponder antenna in the bank card, mobile phone or tag becomes energiesed by radio waves
- the transponder can then send data to the receiver
RFID tags
- security control points
- indfenification of people, animals, goods or valuables
- shipping, supply chain for tracking goods
- bnking and fast payment systems
71
passive and active tags
passive transponders, used in bank cards for exmaple, have no power source themselves and rely on the radio waves from the receiver for their energy
- transponders need to be places very close to the receiver
active tags user a larger battery powered beacon which can broadcast its own signal to recievers up to 300m
- they are useful for larger items that are not places on a reciever by hand for exmaple in shipping, toll stations, warehosuing and control points
72
actuators
motors used in conjunction with sensors to control a mechanism
73
what does LCD stand for
liquid crystal display
red green and blue diodes to form each pixel
74
what does CCFL stand for
cold cathode fluorescent lamps
75
what does LED stand for
light emitting diode
76
LEDs vs CCFLs
LEDs
reach mx brightness almost immeditaley
the image is sharper and more realsitic with vivid colours
they produce a brighter light, better picture definition
LED's are very small, this allows the screens to be much thinner in construction
they last almost indefinitely which makes the screens much more reliable
the consume very little power and therefore produce very little heat as well and reduce running costs
77
OLED
organic LED screens
plastic rather than glass, much brighter (more thinner and lighter)
generate their own light, no need for back lighting
less power consumed, good for battey operated devices
little heat is produced
larger field of view than LCD screens
enhanced when the screen is curved
78
multimedia light projectors
compact, high res full colour projectors
takes a video signal, converts it into a viewable image and projects it on a screen
79
inkjet printer
most common form of printer (common sense)
80
laser printer
use dry powedered ink called toner
usually business use as they are fast and reliable
print quality is excellent
81
dot matrix printers
impact printers
like a typewriter they strike an inked ribbon which imprints dots to form letters on the page
82
primary storage/ memory
RAM AND ROM
83
what does ROM stand for
read only memory
84
what does RAM stand for
read only memory
85
whats often stored in RAM
- operating system
- running programs
- data currently being used
86
What is often stored in ROM
Computing bootup instructions (Bootstrap)
87
what happens to its contents when u turn the power off
they are lost
its volatile
88
what happens to the contents of ROM when power turns off
they are retained
non volatile
89
can u read and write data to RAM
read and write
90
can u read and write data to ROM
Read only
91
is access to RAM fast?
yes
92
at one time RAM will normally contain what?
OS
software in use
data which the software is using
93
What makes ROM so special?
Some data needs to be permanently held in memory, even when a computer has no power!
94
if ur computer is off is RAM empty??
computer stores running programs and data in RAM when your computer is turned on
when ur computer is off RAM is empty as it is volatile
95
RAM memory
when apps or programs are loaded , they are copied into RAM from sceondary storage (eg hard disk)
docs (data) that are used with those programs are also opened by copying them into RAM as the working memory
RAM starts to fill up
96
running out of space in RAM (VM)
virtual memory
vm is part of the hard drive used as an extension to RAM
97
primary and secondary storage
primary storage is volatile and usually refers to RAM
secondary storage refers to non-volatile storage
magnetic storage - hard disk
optical storage - CD-ROM (laser light)
solid state devices using flash memory
98
secondary storage devices
different technologies have evolved for saving data
each of these have their own advantages and disadvantages in terms of
durability
read / write
capacity
portability and cost
99
HARD DISK
tracks, sectors and platters:
- concentric tracks are created on a magnetic disk
- disk spins at high speed: 3,600 - 7,200 rpm
- spinning platters are read by drive heads
- data is read or written as sector moves under the head
100
hard disks have very high capacity
fast read and write speeds
relatively cheap storage per TB
101
Cloud storage
Cloud storage is a method of storing digital data on remote servers, accessed over a network like the internet, instead of on a physical device
102
solid state drives
solid state media commonly uses electrically programmably non-volatile flash memory
103
advantages and disadvantages of solid state media
more durrable (SSD's dont have any moving parts) - extreme heat, cold, pressure and drops
typically less capacity than a hard disk
faster than hard disk drives
consume less power
more portable
more expensive than hard disk drives
104
USB flash drives
can store up to 128gb of data (2,500 photos)
prices start at under 5.00
can use password protection
useful for transferring data between computers
105
CD's, DVD's and BluRay
capacites of these discs are different even tho they are the same physically
- different laser wavelengths burn smaller pits
- the spiral track can therefore be more tightly wound, creating a longer track
106
optical disk formats
read only
recordable and
re-writable formats
107
CD formats (3)
CD-ROM (read only)
CD-R (recordable)
CD-RW (Rewriteable)
108
CD-ROM is --------- at the time of manufacture
pressed
109
CD-R can be written to (how many times)
once
110
CD-RW can be ....
CD-RW can be written, read many times, erased and written again
111
uses for optical disks
CD-ROM usually for software distrbutiojn
CD-R for copying a game, sfotware, audio, video files, docs - easily stored offline
CD-RW used for medium term backup or transferring files
DVD- BlueRAY - high capacity discs usually for films
112
capacity and access speed of a hard disk
c - 512 GB - 6 TB
as - Fast
113
capacity and access speed of CD-ROM
c - 700MB
as - Medium
114
capacity and access speed of DVD
c - 4.7 - 8.5 GB
as - Medium
115
capacity and access speed of BluRay Disk
c - 25 - 50 GB
as - Medium
116
capacity and access speed of SSD
c- 4GB - 2TB
as - very fast
