Computer Systems

Computer Architecture

Von Neumann Architecture

Most modern computers share a common high-level architecture known as the von Neumann architecture.

The key components of a von Neumann architecture include :

1. The Central Processing Unit (CPU)
2. Memory
3. Bus System
4. Input / Output devices

Central Processing Unit

The CPU contains the control unit, arithmetic/logic unit and registers.

Control Unit

The control unit (CU) is responsible for fetching instructions from memory and directs other components of the CPU to execute those instructions.

Arithmetic / Logic Unit

The arithmetic/logic unit (ALU) performs basic arithmetic and logical operations to operate of data according to the instructions it is given.

Registers

Registers temporarily hold data and instructions that are being processed. There are several types of registers :

• Program Counter : Stores and keeps track of where the CPU is in the program at any given time. When executing a program, the program counter is updated to indicate what the CPU must do next.
• Instruction Register: Contains the currently executing instruction.
• Data Register : Used to hold data that will be stored or fetched from the computer memory
• Address Register : Used to store the memory address from which data will be fetched to the CPU registers.

Fetch-Execute Cycle

Fetch-Execute Cycle

5. The cycle begins with PC holding the memory address address of the next instruction to be executed.
6. The CPU sends a read signal to the memory to fetch the instruction located at the address held in PC.
7. The fetched instruction is placed into the instruction register.
8. The PC is incremented to point to the address of the next instruction in the sequence.
9. The control unit then decodes the instruction held in the instruction register.
10. The CU then directs the appropriate parts of the CPU to carry out the instruction.
11. The output of the operation is then stored in the appropriate location, such as a register or memory.

Clock

The clock signal is a continuous sequence of oscillating electrical pulses generated by an oscillator circuit within the computer.

The frequency of the clock speed is measured is hertz and indicates how many cycles per second the CPU can perform.

Cores

Cache

Memory

Memory on a computer is essential for storing data and instructions that the CPU needs to access quickly.

It can be categorised into two types : primary memory and secondary memory.

Primary Memory

Volatility : RAM is volatile memory, meaning once the computer is turned off, all data stored in RAM is lost.

Speed : RAM is significantly faster than secondary storage mediums, which allows the CPU is quickly access the data needed for processing.

Random Access : Unlike sequential memory, such as tapes, RAM allows data to be read or written in any order, allowing for computer tasks to be more efficient.

Non-Volatility : ROM retains the data is stores even when the power is switched off, making it suitable to store boot-up instructions.

Speed : The CPU can read data from ROM during the boot process or when executing specific system functions. The access time for reading ROM is generally slower compared to RAM, but this is acceptable since ROM is not accessed as frequently.

Secondary Memory

Types of Secondary Memory

• HDDs : Hard Disk Drives use magnetic storage to read and write data on spinning disks with a read / write head. They are common in desktops and servers for storing large volumes of data.
• SSDs : Solid-State Drives use NAND flash memory to store data with no moving data unlike HDDs. They are increasingly used in desktops and servers since they are more durable, consume less power and have a higher cost per gigabyte than HDDs.

Bus

Input / Output Devices

Input Devices

• Keyboard
• Mouse
• Scanner
• Microphone
• Touchscreen

Output Devices

• Monitor
• Speaker
• Projector
• Printer

Electricity

Computers use binary code to represents information, which is composed of a sequence of bits.

Conceptually, a bit is a binary value containing one of two values (0 or 1). Physically, a bit is represented by the presence or absence of an electricity signal (on or off).

Electrons are subatomic particles with a negative charge. The flows of these electrons creates an electric current, which is harnessed to perform computational tasks.

A voltage of 3.3 to 5 volts represents an on state while a voltage or 0 or close to 0 represents an state.

If the resistance to an electron's movement is low, it will move freely when voltage is applied to it.

Computers manipulate the flow of electrons in order to represent this binary information. This is done using several electronic devices.

Transistors

Resistors

Capacitor

Logic Gates

Transistors are arranged in different ways to carry out logical operations. The arrangement of transistors to circuit is known as a logic gate.

The CPU is composed of many combinations of these logic gates with each gate being made up of transistors.

For each logic gate we will use $A$ and $B$ as to represent Boolean variables, either $1$ or $0$. These are analogous for true and false respectively.

Different logic gates act as operations on $A$ and $B$ in the same way we have addition or subtraction act as operations for arithmetic variables.

Assume $Y$ is the output of the logic gate.

NOT Gate

The NOT gate has one input and one output. Given an input of $A$, the gate will output the inverse of $A$.

OR Gate

The OR gate has two inputs and one output. Given an input of $A$ and $B$, the gate will output the $1$ if either $A$ or $B$ or both are $1$.

AND Gate

The AND gate has two inputs and one output. Given an input of $A$ and $B$, the gate will output the $1$ only if $A$ and $B$ are $1$.

NOR Gate

The NOR is the same as as an OR gate followed by a NOT gate. The NOR gate is the inverse of the output of a regular OR gate.

NAND Gate

The NAND is the same as as an AND gate followed by a NOT gate. The NAND gate is the inverse of the output of a regular AND gate.

Operating Systems

Functions of an Operating System

• Resource Management : The OS manages the CPU by scheduling processes, handling interrupts, and ensuring efficient execution of tasks through mechanisms like multitasking and multiprocessing.
• Memory Management: The OS manages the computer's memory, allocating space for applications and ensuring that each process has sufficient memory while preventing conflicts and memory leaks.
• Graphical User Interface (GUI): Most modern operating systems provide a GUI that allows users to interact with the computer through visual elements like windows, icons, and menus.
• File System Management : The OS manages files and directories on storage devices, providing a hierarchical structure for organising data and supporting operations such as file creation, deletion, copying, and access control.
• Security & Access Control : The OS provides security features to protect data and resources from unauthorised access and threats. This includes user authentication, permissions, encryption, and other security measures