Computer organisation and architecture

Question 1

What is a computer system?

Answer 1

A network of connected hardware and software that makes up a functional computer.
Every system consists of an input, a process, and an output.
A computer system is any device that can take a set of inputs and process them into useful outputs.

Question 2

What is hardware?

Answer 2

Hardware is the term given to the physical components of the computer system.
Hardware includes the internal components of a computer system like the hard drive and the sound card as well as external components like printers and speakers.

Question 3

What is software?

Answer 3

Software is the name given to program code (sequences of instructions which are
executed to perform a task).

Question 4

What are the internal components of a computer system?

Answer 4

The internal components of a computer system consist of the hardware required to process data and to allow the processor to communicate with other devices such as secondary storage, display screens, and printers.
The main internal components of a computer system are:
- Processor (CPU)
- Main memory
- Input/output (I/O) controllers
These components are connected together by high-speed communication buses.

Question 5

Which three buses is the system bus made up of?

Answer 5

Address bus, data bus, and control bus.

Question 6

What is the role of the CPU within a computer system?

Answer 6

The processor, sometimes referred to as the CPU (central processing unit), is the part of the computer that processes data by executing program instructions. At processor level, these will be low-level instructions in the form of machine code that the processor has been designed to handle, based on a specific processor instruction set.

Question 7

What needs to be done in order for the CPU to be able to execute a program?

Answer 7

For the processor to be able to execute a program, the program instructions need to be transferred from secondary storage into main memory from where they can be fetched, decoded, and executed. The data that needs to be processed is also loaded into main memory (from secondary storage) or provided by the input and output devices via the I/O controllers.

Question 8

What is main memory?

Answer 8

Main memory is memory that can be accessed directly by the processor. Each memory location, where instructions or data are stored as binary sequences, has a physical address, which is a number used to locate that memory location and access its contents.

Question 9

What are the two types of main memory?

Answer 9

There are two main types of main memory:
RAM, which is the working memory that is used by the processor during the fetch-decode-execute cycle
ROM, which is used in the boot process for the computer system

Question 10

How is main memory different from secondary storage?

Answer 10

Main memory is distinct from secondary storage, which the processor can’t work with directly. Secondary storage must be accessed through I/O controllers.

Question 11

What are input/output (I/O) controllers?

Answer 11

I/O controllers are a series of microchips which help in the communication of data between the central processing unit and the motherboard. The main purpose of this system is to help in the interaction of peripheral devices with the control units (CUs).
All external (peripheral) devices are connected to the processor through I/O controllers. These provide the mechanism for:
- input data to be received for processing from input devices, such as keyboards
- the results of computation to be output from the system to output devices, such as display screens

Question 12

What are the tasks performed by I/O controllers?

Answer 12

The controller converts the signals received from a peripheral device into a format the computer can process, and vice versa.
It receives I/O requests from the CPU, and then sends device-specific control signals to the device it is controlling.
It also manages the data flow to and from the device, freeing the CPU to get on with other tasks.

Question 13

What is a device driver?

Answer 13

The software that interacts directly with the I/O devices is known as the device driver.
When you install a new device (e.g. a printer), you must install the device driver to go with it.

Question 14

What is a bus?

Answer 14

A bus is a high-speed internal connection. Buses are used to send control signals and data between the processor and other components.
They typically consist of 8, 16, 32 or 64 lines/wires.

Question 15

What is the role of the address bus?

Answer 15

The address bus carries memory addresses from the processor to other components such as primary storage and input/output devices.
Addresses travel only one way along the address bus: the processor sends the address of an instruction or a datum to be stored or retrieved to memory or an I/O controller.
So the address bus is unidirectional.

Question 16

What is the role of the data bus?

Answer 16

The data bus carries data between the processor and other components. It is bidirectional.

Question 17

What is the role of the control bus?

Answer 17

The control bus carries control signals between each I/O controller and the processor, as well as between the processor and memory. The control bus also carries the clock’s pulses. It is bidirectional.

Question 18

What are the different types of control signals that can be sent along the control bus?

Answer 18

Memory read: causes data from the addressed location to be placed on the data bus
Memory write: causes data on the data bus to be written into the addressed location
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

Question 19

What is the system bus?

Answer 19

The system bus is a set of parallel connections that allow internal components to communicate with each other and exchange data.

Question 20

What are words?

Answer 20

Memory is divided up into equal units called words.
Word length is usually 8, 16, 32 or 64 bits.
Each word has a separate memory address.

Question 21

What are external buses?

Answer 21

External buses are buses that are used to connect the peripherals to the processor. These can be serial or parallel connections.

Question 22

What does the width of the data bus refer to?

Answer 22

The width of the date bus refers to the number if bits that can be transferred in one operation. For example, how many bits can be transferred in one go between the memory and the processor. It is typically a multiple of a byte (e.g. 8, 16, 32, or 64 bits).
In general, if the width of the data bus is expressed as n bits, then n bits can be transferred at one time. The amount of data that can be fetched at one time can affect the processor performance.

Question 23

What does the width of the address bus refer to?

Answer 23

The width of the address bus refers to its number of parallel lines, which determines the number of bits that can be used to form an address of a memory location. It is typically a multiple of a byte (e.g. 8, 16, 32, or 64 bits).
In general, if the width of the address bus is expressed as n bits, then there are 2^n numbers that can be used to address memory locations.
Therefore, the width of the address bus determines the maximum number of addressable memory locations, i.e. the maximum memory capacity of a computer system. This is called the address space and it can affect processor performance.

Question 24

What is the stored program concept?

Answer 24

Machine code instructions are loaded into main memory to be executed by the processor.
The instructions are fetched one at a time and executed immediately by the processor in a sequential order.

Question 25

What is von Neumann architecture?

Answer 25

The most common implementation of the stored program concept is the von Neumann architecture.
Instructions and data are stored in a common main memory and transferred using a single shared bus.
Used in everyday, general purpose computers.

Question 26

What is Harvard architecture?

Answer 26

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.
Used for specialist embedded systems such as digital signal processing where speed takes priority over the complexities of the design.

Question 27

What is a general-purpose computer?

Answer 27

Early computers were able to calculate an output using fixed instructions. They could perform only one set of instructions.
A general purpose computer can perform many different tasks at different times.
Their programming is not fixed.
To achieve this, computers are designed to allow data and instructions to be stored – this is called the stored program concept.

Question 28

What are the main components of a CPU?

Answer 28

Arithmetic-Logic Unit (ALU)
Control Unit
Clock
General Purpose Registers
Dedicated registers

Question 29

What’s the role of the control unit in the CPU?

Answer 29

The CU is the part of the processor that coordinates the activity of all the other components.
Each instruction is accepted and decoded.
Separate steps such as fetching the address of the data and fetching the data itself from memory are identified.
Each step is synchronised with a regular pulse from the system clock.
The control unit operates by repeating three operations:
Fetch – causes the next instruction to be fetched from memory
Decode – decodes the instruction
Execute – causes the instruction to be executed

Question 30

What’s the role of the Arithmetic Logic Unit in the CPU?

Answer 30

The ALU is the problem-solving part of the processor.
This component performs arithmetic, logical and shift operations on data.
Arithmetic operations: add, subtract, multiply, divide.
Logical operations: Consist of comparing one data item to another to determine whether the first data item is smaller than, equal to, or greater than the second data item.
Bitwise logical operations: AND, OR , NOT, XOR
Shift operations: Move bits to the left or right within a register

Question 31

What’s the role of general purpose registers in the CPU?

Answer 31

Results from the ALU need to be stored somewhere.
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.
The processor is then able to access and re-use these results is subsequent calculations, e.g. Add 2+3/4
Some processors have a single general purpose register called an Accumulator.

Question 32

What is the role of the system clock in the CPU?

Answer 32

The system clock is a series of regular ON/OFF signals that are used to synchronise the operations of the processor components.
Actions are usually carried out on the rising edge of the clock.

Question 33

What is the role of the Memory Data Register (MDR) in the CPU?

Answer 33

The MDR temporarily holds the data (data values or instructions) that are read from or written to the main memory.

Question 34

What is the role of the Current Instruction Register (CIR) in the CPU?

Answer 34

The CIR holds the current instruction that the processor is executing.

Question 35

What happens in the fetch stage of the FDE cycle?

Answer 35

Address of next instruction is transferred from the Program Counter to the Memory Address Register.
The Program Counter is incremented by 1.
Instruction stored in location addressed by the Memory Address Register are transferred to the Memory Data Register.
Instruction transferred from the Memory Data Register to the Current Instruction Register.

Question 36

What happens in the decode stage of the FDE cycle?

Answer 36

The instruction in the Current Instruction Register is decoded.
Additional data, if required from the instruction, is fetched from memory., and passed to the registers.

Question 37

What happens in the execute stage of the FDE cycle?

Answer 37

The registers in the CPU now contain everything that the ALU needs to carry out the instruction.
The instruction is executed by the ALU.
Registers are used to store immediate data or results.
The result is stored in the accumulator, a general purpose register or main memory.

Question 38

What are some factors that affect the performance of the CPU?

Answer 38

Number of cores - more processors can be linked together on a single chip allowing more instructions to be executed simultaneously.
Word length - the amount of data that the CPU can process simultaneously.
Address and bus width.
Amount of cache memory.

Question 39

What is cache memory?

Answer 39

Cache is a small amount of super-fast memory that stores data that is frequently used by the processor.
Larger and slower than a than a register, faster and smaller than RAM.
Larger amounts of cache memory can improve processing speed.

Question 40

What is the processor instruction set?

Answer 40

Computer processors have different ways of expressing instructions.
An instruction set determines the commands a processor can perform.
Different types of processors have their own instruction sets, but they may perform similar or identical operations.

Question 41

What are some examples of instruction that could be in the processor instruction set?

Answer 41

Data transfer such as LOAD, STORE
Arithmetic operations such as ADD, SUBTRACT
Comparison operations that compare two values
Logical operations such as AND, OR, NOT
Branch – conditional and unconditional
Shift operations – shift bits left or right in register

Question 42

What are the two components of an instruction?

Answer 42

Operation code (opcode)
Operand(s)

Question 43

What is the operation code (opcode) in an instruction?

Answer 43

The operation code (opcode) includes:
The actual command the processor needs to carry out, e.g. ADD, SUB etc.
The addressing mode – specifies whether the operand is the actual data, the memory address where the data is held, or a register.

Question 44

What is the operand in an instruction?

Answer 44

One or more items of data (which can be values, memory addresses or registers), that are to be used in the instruction.

Question 45

What are addressing modes within opcodes?

Answer 45

The operand contains a reference to the data that is to be used in the instruction.
This data may be either:
- An actual value that can be used as presented
- The address in memory where the data to be used is held
- The register where the data is held
The addressing mode (typically two bits) specifies which of the above describes the data.

Question 46

What is immediate addressing?

Answer 46

In immediate addressing, the operand is the actual value to be used in the instruction.
The addressing mode 00 specifies that the data is a value (in this case 12), and not an address.

Question 47

What is direct addressing?

Answer 47

In direct addressing, the operand is the address of the location in memory of the data to be used.
The addressing mode 01 specifies that the data is an address, not a value.

Question 48

What are branch instructions in assembly language?

Answer 48

Assembly language has no IF, FOR, or WHILE statements.
Instead, COMPARE and BRANCH instructions are used.

Question 49

How does the Operating System allocate processor time?

Answer 49

The standard operation of an OS is to divide processing time between the processes that are running. Processing time is divided between tasks based on factors including:
- When the task was requested
- The importance of the task
- The length of the task
The more processes a computer is running, the longer it may take for a process to be allocated processing time.

Question 50

What are interrupts used for?

Answer 50

To allocate time to processes that are deemed urgent, devices use interrupts. An interrupt is a signal that is sent to the processor to request immediate attention. When the processor receives this request, it suspends what it is doing and runs the process associated with the interrupt.

Question 51

What might interrupts be used for?

Answer 51

The processor can be interrupted for a number of reasons, including:
- A hardware device has signaled that it has data to process
- A hardware device has completed a task that it was asked to do
- A software process needs a service to be provided or OS function to be performed
- An allotted amount of time has expired and an action needs to be performed
- A hardware failure has occurred and needs to be addressed

Question 52

What is an Interrupt Service Routine (ISR)?

Answer 52

The operating system will have interrupt service routines (ISRs) (or interrupt handlers) designed to process each interrupt. An ISR is a mini program that has been designed to respond to an interrupt’s request. ISRs can be built into an operating system or provided via device drivers.
In the example of a key being pressed, the ISR will determine which key this was by polling the keyboard, then it will store the relevant character in the keyboard buffer. A process that is waiting for keyboard input can later check the keyboard buffer to find out which key was pressed.

Question 53

How does an interrupt affect the execution of an FDE cycle?

Answer 53

1. The processor receives the interrupt.
2. The processor completes the fetch-decode-execute cycle of the instruction that it was running when it received the interrupt.
3. The current contents of the processor registers (including the program counter) are saved to memory.
4. The origin of the interrupt is identified so that the appropriate ISR is called.
5. All other lower-priority interrupts are put on hold to allow the ISR to finish running.
6. The program counter is updated with the address of the first instruction of the ISR.
7. The ISR completes its execution.
8. The processor registers are reloaded with the values that were saved to memory.
9. The lower-priority interrupts that were put on hold are re-established.
10. The program counter is set to point to the address of the next instruction that needs to be executed in the program that the processor was running when it received the interrupt.

Question 54

How does the number of cores affect processor performance?

Answer 54

Generally speaking, the more cores a computer has, the more instructions it can execute at the same time. As a result, the computer will perform more efficiently than computers with the same type of processor but fewer cores.
Having a quad-core instead of a dual-core processor (both running at the same speed) does not mean that the amount instructions that can be processed in the same time frame will double. However, the quad core will still achieve a significant improvement because data and instructions need to be fed to the cores appropriately and so the computer system will need to spend time organising which cores receive which data and instructions. Also, the efficiency of a multicore processor depends on the nature of the required task, i.e. if it is possible to divide a computation into subtasks that can be processed in parallel (one task per core at the same time). This is known as parallel processing and it is only possible on multicore systems.

Question 55

What are the different levels of cache?

Answer 55

Level 1: L1 cache, being part of the circuitry of each core, is the smallest and fastest cache. A quad-core CPU would have four L1 caches.
Level 2: L2 cache is often shared by cores. It is slower than L1 cache and tends to be larger. In new systems, the L2 cache is usually part of the core in the same way as the L1 cache.
Level 3: L3 cache is slower than L2 cache but larger. It sits on the processor or near it on the motherboard.
Level 4: Newer systems incorporate an L4 cache. As with L3, this is placed on or near the processor.

Question 56

What are input devices?

Answer 56

Input devices are peripherals used to provide data and control signals to a computer system. Data comes in many different forms, and there are a wide range of devices that can be used. The input process might be manual, such as through a keyboard or mouse, or automated, such as through a sensor.

Question 57

What is a barcode?

Answer 57

A barcode is a means of representing data in a machine-readable form. Traditional barcodes represent data by using a set of parallel lines of varied width and spacing. Although it is hard for you to see, each character in a barcode is represented by seven equal-sized bars. These are coloured in either black or white (sometimes a different secondary colour is used) to represent the characters in the code.
Many systems use codes that include a check digit for validation at the end of the code.
The code is usually printed below the barcode in case the barcode cannot be read. This allows the code to be entered manually.

Question 58

What are some uses of barcodes?

Answer 58

• Airline baggage tracking
• Product labelling
• Parcel delivery and shipment
• Ticketing and identification

Question 59

How do barcode readers work?

Answer 59

Barcode readers typically work on the principle of reflected light.
Light from a laser is directed at a pattern and a sensor detects the intensity of light that bounces back.
A black bar will absorb more light and be less reflective giving a binary reading of 1.
A white bar will be more reflective with a binary reading of 0.
The pattern of 0s and 1s creates a unique identity.

Question 60

What are Universal Product Code version A (UPC-A)/European Article Number (EAN) barcodes usually used for?

Answer 60

Retail and warehousing.

Question 61

What are code 128 barcodes used for?

Answer 61

Transport and shipment tracking.

Question 62

What is the difference between UPC-A or EAN, and code 128 barcodes?

Answer 62

Code 128 can represent letters and numbers whilst UPC/EAN can only represent numeric digits.

Question 63

How do UPC-A/EAN barcode systems work?

Answer 63

They use long guard bars to show the start and end of the barcode and also central guard bars to distinguish left uniquely from right.
UPC-A/EAN uses seven bar elements to form four alternating dark and light bars of varying thickness to represent each data item between the guard bars.

Question 64

What does the ‘QR’ stand for in QR code?

Answer 64

Quick response

Question 65

What are QR codes typically used for?

Answer 65

QR codes are typically used to provide a link to a particular page on a website or to provide a link to an email or app that holds, for example, purchase details for concert tickets or travel information such as train times and seat numbers for a journey.

Question 66

What are QR codes?

Answer 66

QR codes are 2D barcodes.
A QR code can store up to 7,089 numbers or 4,296 characters. Because of the higher storage capacity, some businesses are using QR codes to replace barcodes in their business systems.
QR code scanners do not use lasers; the codes are captured and read by taking an image of the QR code, either with a dedicated scanner, or by using a camera and app on a mobile device.

Question 67

How do laser printers work?

Answer 67

To print a page, a bitmap image of the page is created.
A negative charge is applied to the print drum and then the laser is used to change the polarity on the drum, in line with the bitmap image.
When this stage is complete, the drum is exposed to positively charged toner, which is attracted to the negatively charged areas on the drum.
A sheet of paper is passed under the drum and the toner is transferred onto the paper.
When the toner application process is complete, it passes to a fusing (heating) stage to ensure that the toner sticks to the paper.
If the laser can produce colour printouts, it will have four different toner cartridges: Cyan, Magenta, Yellow, and Key(Black). Each toner is applied in turn to achieve the correct colour print.

Question 68

How do digital cameras work?

Answer 68

When digital cameras capture an image, it breaks up what it sees through its lens into a grid of pixels.
The shutter opens to let light into a Charge-Coupled Device (CCD) or Complementary metal Oxide Semiconductor (CMOS), sensor at the back of the lens.
The intensity of the light is measured by millions of tiny sensors (one per pixel) arranged in a grid on the sensor.
Light levels for each pixel are represented as binary values.
This information is now stored as an image in the camera’s memory.

Question 69

How do Radio Frequency ID (RFID) tags work?

Answer 69

RFID systems use a transponder and a receiver.
The powered receiver emits radio frequency energy.
The transponder antenna in the bank card , mobile code, or tag becomes energized by radio waves.
The transponder can then send data to the receiver.

Question 70

What are some example uses of RFID tags?

Answer 70

Security control points or identification of people, animals, goods or valuables.
Shipping and supply chain tracking for goods.
Banking and fast-payment systems.
As a potential replacement for barcode systems.

Question 71

How do passive RFID tags work?

Answer 71

Passive devices do not contain a power supply. Instead, the device is powered by radio energy transmitted by the reader. The antenna collects the energy from the transmitter to activate the chip. The range of a passive system is up to 1 metre, and some systems require the reader to be very close to the tag.

Question 72

How do active RFID tags work?

Answer 72

Active systems have a small battery within the tag, and the device will transmit its identifier at regular intervals. Depending on the power supply, the range can be up to 200 metres. Active RFID devices are commonplace in shops, where battery-powered tags are secured onto items. The tags are deactivated or removed when the goods are paid for. A reader is situated at the shop’s exit and an alarm is triggered if it detects an active tag when a customer leaves the shop.

Question 73

What is the difference between primary and secondary storage?

Answer 73

Primary storage is volatile and usually refers to RAM.
Secondary storage refers to non-volatile storage, including:
- Magnetic storage such as the hard disk
- Optical storage that uses laser light such as CD-ROM
- Solid state devices using Flash memory

Question 74

How do you evaluate the advantages and disadvantages of a particular secondary storage device?

Answer 74

Different technologies have evolved for saving data.
Each of these have their own advantages and disadvantages.
- Durability
- Read/write speed
- Capacity
- Portability
- Cost

Question 75

What is capacity in terms of secondary storage and why is it important to consider?

Answer 75

How much storage does it provide? Some users, such as professional photographers, sound engineers, and videographers create a lot of very large files.

Question 76

What is speed in terms of secondary storage and why is it important to consider?

Answer 76

How quick is it to access files? Device manufacturers and reviewers will often publish performance data for read-write access. Sometimes, additional data will be provided for small and large files.

Question 77

What is cost in terms of secondary storage and why is it important to consider?

Answer 77

How much the secondary storage would cost - cost per gigabyte can be a useful measure.

Question 78

What is portability in terms of secondary storage and why is it important to consider?

Answer 78

Is it important that the disk can be transported? Does it need to be secured safely away from the computer, or transported between locations?

Question 79

What is compatibility in terms of secondary storage and why is it important to consider?

Answer 79

Compatibility is always an issue. If the device is designed to be installed inside a system case, it must be the right size for the slot(s) available and it must have the right kind of connector. External devices are usually connected by USB, but other connections are available.

Question 80

What is longevity in terms of secondary storage and why is it important to consider?

Answer 80

Technology is changing at a rapid rate. For example, optical storage devices (CD/DVD) that were commonly found on computers ten years ago are rarely seen in new devices. If you store your photos on a CD (for example), will you have a means of accessing them in a few years time?

Question 81

What is reliability in terms of secondary storage and why is it important to consider?

Answer 81

As far as reliability is concerned, any storage mechanism can fail, and data can be corrupted. Disk reliability is usually measured in terms of the mean time to failure (MTTF). However, the failures cannot be predicted and will be distributed over the entire time period; this means that theoretically, your hard disk could fail at any moment. Backing up important data is imperative, and cloud storage provides an easy, managed solution for most users.

Question 82

How does magnetic storage (e.g. hard disk drives) work?

Answer 82

Concentric tracks are created on a magnetic disk.
Disk spins at high speed (3600-7200rpm).
Spinning platters are each read by drive heads.
Data is read or written as sector moves under the head.
Positive or negative polarisation of magnetic particles creates a binary pattern on the disk.
Changes from negative to positive, or vice versa, creates electromagnetic pulses.
Each pulse (or change) is read as a 1. Anything else is a 0.
Fitting more data into the same physical space has required technological changes.
- More densely packed platters.
- Smaller magnetic parts and read/write heads.
- Perpendicular over longitudinal recording.

Question 83

What are some advantages and disadvantages of magnetic storage? What are they most suited for?

Answer 83

The main advantages of a magnetic hard disk are its capacity and cost. This traditional format still offers greater capacity at a much lower unit cost than SSDs. However, the gap between the technologies is narrowing fast.
Compared to solid state drives, they are less robust due to having moving parts.
Magnetic hard disks are best suited to desktop and laptop devices where low purchase cost is the most important selection criterion.

Question 84

What are some examples of optical discs?

Answer 84

Optical discs are a large family of different disc types, the most common of which are CD (compact disc), DVD (digital versatile disc), and Blu-ray Disc. When discussing optical storage, the word ‘disc’ is usually spelt like this (‘disc’), in contrast to other types of disk storage.

Question 85

How do optical discs work?

Answer 85

A high-powered laser “burns” pits into the disc’s surface.
A low-powered laser detects the reflection from pits and lands.
Only a pit end reflects the laser light and is read as a binary 1.

Question 86

What are the different formats of optical discs?

Answer 86

Optical disks are available as:
- Read only
- Recordable
- Re-writeable
Each format uses slightly different techniques to achieve a differential between a ‘pit’ and a ‘land’.
Recordable formats use a transparent dye that becomes opaque when heated by a laser.
Re-writeable formats use a laser to change the state of a phase-change a lot and a magnet to set the new state.

Question 87

Why are the capacities of CDs, DVDs and BluRays different?

Answer 87

Different laser wavelengths ‘burn’ smaller parts.
The spiral track can therefore be more tightly wound, creating a longer track (so more data can be stored).

Question 88

What are some possible uses of optical storage?

Answer 88

Software distribution: CDs, DVDs, and Blu-ray discs have been used as a medium for distributing software applications, operating systems, and video games to consumers.
Backup and archiving: Optical storage media are suited for long-term storage and backup of important digital data, including documents, photos, videos, and databases.
Audio and video storage: As a high-capacity storage option, optical media have been used to archive and distribute various forms of media content, such as music (CDs), movies (DVDs and Blu-ray discs), and other multimedia files.
Auxiliary storage: Optical discs can be used as auxiliary storage devices by computer systems, enabling users to store and access additional data without filling up the primary storage.

Question 89

What are some advantages and disadvantages of optical storage?

Answer 89

High storage capacity.
Fast data transfer rates.
Good portability, however, the physical size may be seen as a disadvantage compared to smaller devices e.g. USB sticks.
Very cheap.
Slower data access times compared to SSDs and hard disk drives.
Compatibility: As the optical storage landscape includes a variety of different disc formats and standards, compatibility issues may arise for both drives and media.
Scratch and dust sensitivity.

Question 90

What are SSDs?

Answer 90

The abbreviation SSD stands for solid-state disk, but it is also sometimes interpreted as solid-state drive or solid-state device. An SSD is an electronic device that uses flash memory, and has no moving parts.

Question 91

How do SSDs work?

Answer 91

Solid-state disks are made up of a controller and a bank of millions of NAND ‘flash’ memory cells.
Floating gate transistors trap and store a charge.
The charge is retained without a power.
Cells are combined in blocks.
An electron trapped inside the middle layer reads as a 0, outside the middle is read as a 1.
Data must be read, deleted, or written in blocks.
Data cannot be overwritten without being erased first.

Question 92

What are the advantages of SSDs compared to hard disks?

Answer 92

Faster access speed as there is no moving read/write head.
Lower power consumption:
Extended battery life in portable devices.
Devices stay cooler.
Less susceptible to damage.
Silent in operation.
Lighter in weight.

Question 93

What are the disadvantages of SSDs?

Answer 93

SSDs become slower as they fill up. A new empty drive will be full of pages that can be written to immediately. When the disk is nearly full, writing and updating will become relatively slow operations as a result of having to copy out, then rewrite existing pages within a block.