Subtopic Notes

4.1 Central Processing Unit Architecture

4. Processor Fundamentals

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. Registers are of two types

General purpose registers
- Holds data temporarily within the microprocessors
- Assembly language instructions can use it
Special purpose registers
- Dedicated for certain functions like CPU operation and interrupt
- Holds either address or data but not both
- Some are accessible by assembly language instructions
- Includes: PC, MDR, MAR, ACC, IX, CIR, Status Register

CPU Components

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
Immediate Access Store (IAS): A memory unit that the processor accesses directly. For example when typing, the keys are stored here for processing
System Clock: A timing device connected to the processor that synchronizes the execution of the fetch-execute cycle

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
Index Register (IX): Holds an index value to modify addresses for efficient data access in memory, especially in array processing and loops
Status Register: Holds flags indicating the outcome of operations, such as zero, carry, overflow, error, comparisons, or negative
Interrupt Register: A special register in the CPU

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.

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

The CU works out what is required from the instruction
Instruction is of two parts:
- Opcode - What the instruction is
- Operand - Data or addresses required to complete the instruction

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

Bus Width

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

Number of Cores

Hardware connecting CPU and a peripheral device which cannot be directly connected to CPU
Universal Serial Bus (USB): Connects input and output devices to the processor
- A voltage change occurs when the drive is plugged in
- The computer detects the change
- The code of the device is transferred to the computer
- The OS finds the code of the device in the list of devices and loads the appropriate device driver
High Definition Multimedia Interface (HDMI)
- Transmits high-quality audio and video signals between devices
- Connects to output devices like LCD and LED screens
Video Graphics Array (VGA)
- Allows only video transmission
- Outdated technology
- Connects to output device like monitor

Purpose
- Used to attend to certain tasks/issues
- Used to make sure that vital tasks are dealt with immediately
- A signal telling the CPU that its attention is required
- Enables multi-tasking
Operation
- CPU checks the interrupt register after each F-D-E cycle
- If flag is true in interrupt register, interrupt source is detected
- If priority is low, it is ignored
- If it is high priority
  - Contents of registers in CPU are stored in stack
  - PC is loaded with Interrupt Service Routine (ISR)
  - ISR executed
  - CPU restores the registers’ contents,
  - Old program continues
  - Interrupts are again re-enabled
  - F-D-E cycle begins again
Examples
- A hardware interrupt - Paper jam in a printer, Pressing a key, moving mouse
- A software error – File not found, division by zero, two processes trying to access the same memory location

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