3.1 - Computer architecture

Central Processing Unit (CPU): Processes instructions and data that are input into the computer so that the result can be output

Microprocessor: A type of integrated circuit on a single chip

Von Neumann Machine

• Data and programs are indistinguishable, allowing them to share the same memory
• Operates with a single processor which rely on registers
• Follows a linear sequence of fetch, decode, and execute operations for processing instruction

Registers: Smallest unit of storage within the CPU used to hold data or instructions temporarily during processing

CPU Components

Units

• Arithmetic Logic Unit (ALU): Carries out arithmetic calculations & logical decisions
• Control Unit (CU): Controls the operation of the memory, processor and input/output devices by using the fetch-decode-execute cycle

Registers

• Program Counter (PC): Holds the memory address of next instruction to be executed
• Memory Address Register (MAR): holds the address from which data will be fetched or to which data will be sent.
• Memory Data Register (MDR): Temporarily stores data that is being transferred to CIR by the MAR.
• Current Instruction Register (CIR): Holds the instruction currently being executed by the CPU
• Accumulator (ACC): Stores intermediate arithmetic and logic results during processing

Buses

A set of parallel wires that connects multiple components and facilitate communication among them

• Address Bus: Unidirectional bus that carries memory addresses from the CPU to other memory, input or output locations.
• Data Bus: Bidirectional bus that carries data between the CPU, memory, and other peripherals.
• Control Bus: Bidirectional bus that transmits control signals from the CPU to coordinate and manage the operations of various components.

System Clock
A timing device connected to the processor that synchronizes the execution of the fetch-execute cycle

The Fetch-Decode-Execute Cycle

Fetch Stage

• The Program Counter (PC) holds the address of the next instruction to be fetched from main memory.

• The address stored in the PC is copied to the Memory Address Register (MAR) using the address bus.
  MAR ← [PC]

• The address in MAR is sent to main memory through the address bus.

• The Control Unit (CU) sends a read signal through the control bus to tell main memory to read from that address.

• The instruction stored at the address in MAR is copied from main memory to the Memory Data Register (MDR) using the data bus.
  MDR ← [[MAR]]

• The instruction in MDR is copied to the Current Instruction Register (CIR).
  CIR ← [MDR]

• The Program Counter (PC) is incremented so that it points to the next instruction.
  PC ← [PC] + 1

Decode Stage

• The CU works out what is required from the instruction

Execute Stage

• CPU will carry out the decoded instruction
  • ALU: Performs the calculation
  • CU: Coordinates with the appropriate components to store data or results back into main memory or retrieves data from a different memory location

Key Points

• This process cycles which happens billions of times per second
• [ ] denote value currently in that register
• [[-]] means the CPU must do a logical operation then copy this value

Performance of Computer System Factors

Bus Width

• Determines the number of bits that can be simultaneously transferred
• Increasing width increases data transfer rate

Clock speed

• Determine the clock cycle that synchronizes all computer operations
• Increasing clock speed increases the number of operations per second
• This doesn’t necessarily lead to better performance
• Overclocking: Using a higher clock speed than designed. This may lead to operations becoming unsynchronized (causing crashes) or cause overheating of CPU

Cache

• Small, high-speed memory
• Stores frequently accessed data and instructions
• Located inside the CPU
• Larger cache memory size leads to better CPU performance

Number of Cores

• Most CPU chips are multi-core
• Each core processes different instructions using multithreading
• Higher core number improves performance

Instruction Set

List of all the commands that can be processed by a CPU, and the commands are machine code

Embedded System

• A combination of hardware and software designed to carry out a dedicated function
• This is different to a general purpose computer that is used to perform many different functions
• May contain
  • Microcontrollers - CPU, RAM, ROM and other peripherals on a single chip
  • Microprocessor - Integrated circuit with CPU only
  • System on Chips (SoC) - Microprocessor with I/O ports, storage and memory
• Process
  • User gives input that is sent to microprocessor
  • ADC may be required to convert the input
  • Data from the user interface is also transmitted to the microprocessor
  • The microprocessor processes the data and sends signals to actuators which are the output
• Non-programmable devices needs replacing for software update
• Programmable devices may be updated by
  • Connecting the device to a computer to download the update
  • Automatic updates via a satellite, cellular or Wi-Fi
• Applications
  • Domestic appliances
  • Cars
  • Security Systems
  • Lighting Systems
  • Vending machines.
  • GPS systems
  • Washing Machines
  • ATM (Automated Teller Machine)
  • Microwave