Computer Systems

Computer Components

Central Processing Unit

The CPU can be described as the brains of a computer. Its purpose is to carry out instructions from programs and allows a computer and any external devices to perform their tasks. The CPU contains the ALU and registers. It performs basic arithmetic, logic and controls input/output operations.

Arithmetic & Logic Unit

The ALU performs arithmetic & logic operations. It is made up of the Arithmetic Unit (AU) which performs addition, subtraction, multiplication and division, and the Logic Unit (LU) which solves comparisons, mostly in AND, OR and NOT.

Registers

A processor register is a quickly accessible location available to a computer's processor. They usually consist of a very small amount of fast storage. A register may hold an instruction, a storage address, or any kind of data (such as a bit sequence or individual characters).

Program Counter

A program counter is a register in a computer processor that contains the address (location) of the instruction being executed at the current time. It stores addresses for previous instructions and the address for the next instruction to be executed.

Random Access Memory

RAM is a form of short-term memory that stores data currently being used by the processor. It does not store the information long-term, like the hard drive. Once the computer is turned off, all RAM is wiped. For this reason, it is known as volatile memory.

More RAM means more applications can be loaded in RAM at the same time. This means the CPU spends less time loading RAM from secondary storage and/or paging/thrashing. This in

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Computer Systems

This in turn means that the CPU can spend more time running user applications.

Read-Only Memory

ROM is a form of non-volatile memory. It receives data and permanently writes to the CPU. The data is coded to not be overwritten, so it's used for things like your printer software or your start-up programs.

RAM	ROM
Volatile	Non-volatile
Reads & writes	Read only
Stores data currently being used	Stores instructions require during bootstrap
Fast memory	Slow memory
More expensive	Cheaper

Hard Disk Drives & Solid-State Drives

HDD	SSD
Larger, slower & louder	Smaller, faster & quieter
Contains moving parts	Contains no moving parts
Not safe from magnets	Safe from magnets
Mechanical and consumes more power	Completely electronic and consumes less power

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Computer Systems

Primary Storage & Secondary Storage

Primary storage (also known as main memory) is the component of the computer that holds data, programs and instructions that are currently in use.

Primary storage is located on the motherboard. As a result, data can be read from and written to primary storage extremely quickly. This gives the processor fast access to the data and instructions that the primary storage holds.

Types of primary storage

There are four types of primary storage:

• Read only memory (ROM)
• Random access memory (RAM)
• Flash memory
• Cache memory

Secondary storage

Secondary storage (also known as auxiliary storage) refers to the storage of data that is not accessed frequently as the data in primary storage. Generally, secondary storage technology is much cheaper than primary storage.

Types of Secondary storage

• Hard drives
• USB sticks
• Cloud storage

Von Neumann Architecture

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Computer Systems

It is also known as ISA (Instruction set architecture) computer and is having three basic units:

1. The Central Processing Unit (CPU)
2. The Main Memory Unit
3. The Input/Output Device

The Central Processing Unit (CPU)

The central processing unit is defined as the it is an electric circuit used for the executing the instruction of computer program. It has following major components:

• Control Unit (CU)
• Arithmetic and Logic Unit (ALU)
• Variety of Registers

Control Unit

A control unit (CU) handles all processor control signals. It directs all input and output flow, fetches code for instructions, and controls how data moves around the system.

Arithmetic and Logic Unit (ALU)

The arithmetic logic unit is that part of the CPU that handles all the calculations the CPU may need, e.g. Addition, Subtraction, Comparisons. It performs Logical Operations, Bit Shifting Operations, and Arithmetic operations.

Main Memory Unit (Registers)

• Accumulator: Stores the results of calculations made by ALU. It holds the intermediate of arithmetic and logical operations. It act as a temporary storage location or device.

• Program Counter (PC): Keeps track of the memory location of the next instructions to be dealt with. The PC then passes this next address to the Memory Address Register (MAR).

• Memory Address Register (MAR): It stores the memory locations of instructions that need to be fetched from memory or stored in memory.

• Memory Data Register (MDR): It stores instructions fetched from memory or any data that is to be transferred to, and stored in, memory.

• Current Instruction Register (CIR): It stores the most recently fetched instructions while it is waiting to be coded and executed.

• Instruction Buffer Register (IBR): The instruction that is not to be executed immediately is placed in the instruction buffer register IBR.

Buses

Data is transmitted from one part of a computer to another, connecting all major internal components to the CPU and memory, by the means of Buses.

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Computer Systems

Computer Science

Types:

• Data Bus: It carries data among the memory unit, the I/O devices, and the processor.

• Address Bus: It carries the address of data (not the actual data) between memory and processor.

• Control Bus: It carries control commands from the CPU (and status signals from other devices) in order to control and coordinate all the activities within the computer.

Von Neumann Bottleneck

Whatever we do to enhance performance, we cannot get away from the fact that instructions can only be done one at a time and only one person can be carried out sequentially. Both of these factors hold back the competence of the CPU. This is commonly referred to as the 'Von Neumann bottleneck'. We can provide a Von Neumann processor with more cache, more RAM, or faster components but if no original gains are to be made in CPU performance, then an influential inspection needs to take place of CPU configuration.

This architecture is very important and is used in our PCs and even in Super Computers.

Advantages of Von Neumann Architecture

• Not only data but also instructions of programs are stored within the same memory. This makes it easier to re-program the memory.
• Data from memory and devices is accessed in the same way.
• The control unit gets data and instructions in the same way from one memory so simplifies the design and development of the control unit.

Disadvantages of Von Neumann Architecture

• Has only one data bus shared from the transfer of data transfers and instruction fetches; they must be scheduled because they cannot run simultaneously.
• Serial instruction processing does not allow for parallel execution of programs. Paralleled executions must be simulated later by the operating system (i.e. no pipelining)
• Higher chance of corruption or error as the instructions and data are stored and transferred in the same way so may be accidently rewritten by bugs in programs.

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Computer Systems

Harvard Architecture

Advantages of Harvard Architecture

• There is less chance of corruption since data and instructions are transferred via different buses.
• Offers higher performance since Harvard allows for simultaneous fetching of data and instructions - they are kept in separate memory and travel via separate buses.
• Both memories can use different cell sizes making effective use of resources.

Disadvantages of Harvard Architecture

• When there is free data memory it cannot be used for instructions and vice versa. Memory dedicated to each must be carefully balanced in manufacture.
• Production of a computer with two buses is more expensive and takes more time to manufacture.
• Harvard architecture has more pins so more complex for main board manufactures to implement.
• Harvard architecture requires a control unit for two buses that is more complicated and development of which is expensive and needs more time.

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Computer Systems

memory is used to store both instructions and data and they are both transferred over the data bus.

However, the CPU's cache has Harvard architecture. There is a separate cache memory for instructions and data. There is also a separate data bus between these caches.

Fetch – Decode – Execute Cycle

The fetch-decode-execute cycle is the sequence of steps that the CPU follows to process instructions.

1. The processor checks the program counter to see which instruction to run next. The program counter gives an address value in memory of where the next instruction is.
2. The processor fetches the instruction value from this memory location. Once the instruction has been fetched, it needs to be decoded and executed.
3. Once this is complete, the processor goes back to the program counter to find the next instruction.
4. This cycle is repeated until the program ends.

Electronics

Voltage is the pressure from an electrical circuit's power source that pushes charged electrons (current) through a conducting loop, enabling them to do work such as illuminating a light. Measured in volts (V).

Current is measured in amps (small = mA). It is the measure of the flow of electricity i.e., how fast the electricity is moving. Current is measured using an ammeter. To measure the current across a circuit, you must connect the ammeter in series with it i.e., you must break the circuit

Resistors

Resistance is a measure of how hard it is for electrons to move from one point to another. We measure resistance in Ohms or with the symbol Ω. Suppose the pipe joining the two water-containing vessels above had a narrow part in the middle as shown. This would slow down the rate at which the water would flow from A to B. Resistors are used to control the flow of electrons in a circuit, designed to reduce the current. A variable resistor has a resistance that can be changed while a fixed resistor has a resistance that remains the same.

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Computer Systems

Capacitors

A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of a capacitor is known as capacitance.

Transistors

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. Transistors are used to represent binary 1's and 0's.

Integrated Circuits

An integrated circuit (chip) is an electronic component that contains multiple discrete components combined together into logic circuits on a single silicon board. ICs contain resistors, capacitors and inductors.

Moore's Law

Moore's Law is the observation that the number of transistors in a dense integrated circuit doubles about every two years. (Moore's Law is more of an observation than a scientific theory)

However, Moore's Law may come to an end in the 2020s. Transistors will be unable to operate in smaller circuits at higher temperature. Cooling the transistors will take more energy than the energy that passes through the transistor itself.

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Computer Systems

Digital & Analogue

Differences Between Analogue & Digital

• Analogue is continuous, digital is non-continuous
• Analogue has various amplitudes, digital is either on or off (1 or 0)
• Devices such as printers, audio devices and TVs use analogue signal
• Devices such as computers, CDs and DVDs use digital signal

Logic Gates

AND Gate

A	B	Output
1	1	1
1	0	0
0	1	0
0	0	0

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Computer Systems

NAND Gate

A	B	Output
1	1	0
1	0	1
0	1	1
0	0	1

OR Gate

A	B	Output
1	1	1
1	0	1
0	1	1
0	0	0

NOR Gate

A	B	Output
1	1	0
1	0	0
0	1	0
0	0	1

NOT Gate

A	Output
1	0
0	1

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Computer Systems

Layers of an Operation System

I/O Management controls the user interface (handles graphics, screen sizes, resolutions, etc.). It also provides access to peripheral devices (through device drivers).

The File System keeps track of where files are located (e.g. maintains the file allocation table in FAT systems). It controls access to files (e.g. sequential vs direct access) and handles file security.

Memory Management controls the allocation of RAM. It also loads data from secondary storage to primary storage and swaps out data from primary to secondary.

Processor Scheduling decides which job to run next and for how long (e.g. first-come, first-served, shortest job first, round robin). It maintains the state of each process (e.g. ready/waiting/running/blocked). It also handles multitasking and multiple users.

Hardware consists of any physical devices relating to the operating system. Can include the computer it's running on, monitors, keyboards and mice.

An Operating System (OS) consists of a collection of programs that control the operations of a computer. The OS runs any applications and also operates any external devices. An OS is essential for a computer, or any electronic device, to run. Examples of operating systems are Windows (for PCs), Mac OS (for Macs) and iOS (for iPhones).

Functions of an OS:

• Manage data from input and output devices
• Managing how computers interact
• Provide a user interface, allowing a user to interact with the computer
• Manage the computer's file system
• Control access and security

An application are programs that users make use of to perform tasks. This can range from performing an email (Outlook), video calling (Teams, Zoom) to internet browsing (Edge, Safari).

The user is the person using the computer, OS and applications in order to perform tasks.

Embedded Systems

Embedded systems are a combination of hardware and software designed to perform a specific function. They are called 'embedded' because they are often part of a larger system. Many embedded systems use sensors to receive analogue digital inputs. Input data is often supplied in real time and then processed, resulting in an output. This process is normally done quickly within a fixed time constraint.

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Computer Systems

Examples of embedded systems include self-driving cars, calculators, appliances such as fridges or ovens and a camera or GPS on your phone.

'Real time' embedded systems are those that respond to some external stimuli within a stated/minimum time. For example, an alarm will sound immediately (or a light will switch on) when a PIR sensor detects movement.

Advantages of Embedded Systems

• Easily mass produced
• Highly reliable and cheap to make
• Highly reliable and fast
• Low power consumption

Disadvantages of Embedded Systems

• Difficult to backup embedded files
• Task and time specific
• Cannot be upgraded
• Limitations in terms of memory and performance

Potential Impacts of Embedded Systems on Society

Embedded systems have the potential to improve many aspects of society and daily life. They can be used for things such as communication, navigation, as well as healthcare and agriculture.

End users:

Embedded systems give the end user the ability to do things they otherwise couldn't with ease, for example, a calculator can help you solve a complex maths equation in less than a second.

Embedded systems can also be harmful in some cases, for example, hidden cameras are a massive breach of privacy, but can be freely purchased by anyone.

Business owners:

Embedded systems can be good for business owners, as they can make the job of tracking employee's hours much easier, for example a digital clock in system at a workplace. Embedded can also be harmful in a business however as AI becomes more capable of doing human jobs in the future, rendering a large amount of the workforce obsolete.

Information Systems

An information system (IS) is the software used to help organize and analyze business data and streamline the day to day (transactional) operation of a business.

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Computer Systems

Databases

A relational database is a type of database that stores and provides access to data points that are related to one another. Relational databases are based on the relational model, an intuitive, straightforward way of representing data in tables.

A primary key is a special relational database table column (or combination of columns) designated to uniquely identify each table record. A primary key is used as a unique identifier to quickly search through data within the table. A table cannot have more than one primary key.

Validation Check

Verification and validation are independent procedures that are used together for checking that a product, service, or system meets requirements and specifications and that it fulfils its intended purpose.