OCR GCSE Computer Science Revision Guide: Systems Architecture and Memory

Systems Architecture

• The Central Processing Unit (CPU) is often described as the brain of the computer system, responsible for processing data and instructions through the execution of programs. It is composed of several critical internal components that coordinate to perform all computational tasks.

• ALU (Arithmetic Logic Unit)

  • The ALU is the component within the CPU responsible for performing all mathematical and logical operations.
  • Arithmetic operations include basic calculations such as addition, subtraction, multiplication, and division.
  • Logical operations involve comparisons between data values such as equal to ($=$), less than ($<$, greater than ($>$, and logical operators like AND, OR, and NOT.

• Control Unit (CU)

  • The Control Unit acts as the coordinator of the CPU, managing the execution of instructions.
  • It sends control signals to other parts of the CPU and external hardware to direct the flow of data within the system.
  • The CU is responsible for the overall management of the Fetch-Decode-Execute cycle.

• Registers

  • Registers are extremely small, high-speed storage locations within the CPU itself. They are used to hold specific pieces of data needed during processing.
  • Key registers include:
  • Program Counter (PC): Holds the memory address of the next instruction to be fetched from RAM.
  • Memory Address Register (MAR): Holds the address of the current instruction or data being read from or written to memory.
  • Memory Data Register (MDR): Holds the actual data or instruction that has just been fetched from memory or is waiting to be written to memory.
  • Accumulator (ACC): Stores the immediate results of calculations and operations performed by the ALU.

• Cache

  • Cache is a small amount of very high-speed Random Access Memory (RAM) located inside or very close to the CPU.
  • Its primary purpose is to store frequently used instructions and data so the CPU can access them much faster than fetching them from the main RAM.
  • Cache is organized into levels: Level $1$ (fastest and smallest), Level $2$, and Level $3$ (slowest but largest).

The Fetch-Decode-Execute Cycle

• The Fetch-Decode-Execute (FDE) cycle is the continuous process by which the CPU handles instructions. It repeats billions of times per second in modern computer systems.

• Fetch Phase

  • The memory address of the next instruction is copied from the Program Counter (PC) to the Memory Address Register (MAR).
  • The CPU sends a signal along the address bus to the location in RAM identified by the MAR.
  • The data/instruction at that address is then copied from RAM into the Memory Data Register (MDR).
  • Simultaneously, the Program Counter (PC) is incremented by $1$ to point to the next instruction in sequence.

• Decode Phase

  • The instruction stored in the MDR is interpreted by the Control Unit (CU).
  • The CU translates the instruction into a series of control signals that the CPU can understand.

• Execute Phase

  • The Control Unit directs the appropriate component (such as the ALU) to carry out the instruction.
  • This might involve performing a calculation, moving data from one register to another, or jumping to a different part of the program.

CPU Performance Factors

• The speed and efficiency of a system are determined by specific hardware characteristics of the CPU.

• Clock Speed

  • The clock speed refers to the number of cycles the CPU can perform per second, measured in Hertz ($Hz$).
  • A modern processor typically has a clock speed in Gigahertz ($GHz$), where $1\,GHz$ equals $1,000,000,000$ cycles per second.
  • Higher clock speeds generally result in a faster CPU capable of processing more instructions in a given timeframe.

• Number of Cores

  • A "core" is essentially a complete processing unit within the CPU.
  • Multi-core processors (such as dual-core with $2$ cores or quad-core with $4$ cores) allow for parallel processing.
  • Parallel processing enables multiple instructions to be executed simultaneously across different cores, or for one core to process a background task while another focuses on a primary application.

• Cache Size

  • A larger cache allows the CPU to store more frequently used data locally.
  • This significantly improves performance because the CPU does not have to wait for the relatively slow data transfer from the main RAM.

Embedded Systems

• An embedded system is a dedicated computer system designed to perform one specific, pre-defined task within a larger mechanical or electrical system.

• Key Characteristics

  • They are often part of a larger device (e.g., the controller inside a washing machine).
  • They usually have a simple user interface and are optimized for low power consumption and high reliability.
  • They are typically stored in Read-Only Memory (ROM) because their software (firmware) rarely needs changing.

• Examples of Embedded Systems

  • Household appliances like microwave ovens and dishwashers.
  • Control units in vehicles, such as Engine Management Systems (EMS) or Anti-lock Braking Systems (ABS).
  • Wearable technology like digital watches and fitness trackers.
  • Industrial equipment such as traffic light controllers and manufacturing robots.

Memory and Storage

• Primary Storage

  • Random Access Memory (RAM): Volatile memory that stores data and programs currently in use. When the power is turned off, the data in RAM is lost.
  • Read-Only Memory (ROM): Non-volatile memory that contains the boot-up instructions (BIOS) for the computer. Data in ROM is permanent and is not lost when power is removed.

• Secondary Storage

  • Secondary storage is non-volatile and used for the long-term storage of files, software, and the operating system.
  • Magnetic Storage: Uses magnetic patterns to store data. Examples include Hard Disk Drives (HDD). These offer high capacity and low cost but contain moving parts.
  • Solid State Storage: Uses flash memory to store data. Examples include Solid State Drives (SSD) and USB sticks. These are faster and more durable (no moving parts) but generally more expensive per Gigabyte ($GB$).
  • Optical Storage: Uses lasers to read/write data from discs. Examples include CDs, DVDs, and Blu-ray discs. These are portable and cheap but have much lower capacities and slower access speeds compared to other types.