Unit 1.1 - System Architecture

Unit 1.1 - System Architecture

1. Purpose of CPU and Components

   • The purpose of a CPU is to Fetch, Decode & Execute instructions in a computer system
   • PC - Program Counter - Holds the location of the next instructions/data address in main memory
   • MAR - Memory Address Register - The contents of the PC are copied here and then transferred along the address bus
   • MDR - Memory Data Register - Once data/instructions are brought from the MAR in Main Memory, they are placed in the MDR
   • CIR - Current Instruction Register - The instructions stored in the MDR are copied here
   • ALU - Arithmetic Logic Unit - Performs arithmetic & logic operations
   • Accumulator - Results of calculations are placed into the Accumulator
   • Cache - Stores frequently used instructions & data. Acts as an intermediary between the processor and the RAM

2. Buses

   • A set of parallel wires connecting two or more independent components in a computer system in order to pass signals between them
   • Data Bus
     • Carries Data/Instructions from main memory to the processor
     • Bi-Directional (Two way)
     • Data can be read/written
   • Address Bus
     • Carries Addresses from the processor to main memory or other I/O devices
     • Uni-Directional (One way)
     • The processor generates an address
   • Control Bus
     • Instructs which way data will be travelling to/from memory

3. Factors Affecting CPU Performance

   • Factors affecting CPU performance include the clock speed (GHz), Cache Size (MB) and the amount of cores
   • A CPU with more cores can execute more operations similtaneously
     • The number of cores multiplies the number of instructions which can be executed
       • E.G A 2GHz Single core processor can operate 2 billion instructions a second. A 2 GHz dual core processor can operate 2*2 billion instructions a second, so 4 billion instructions a second
   • The more cache you have, the more frequently used instructions are stored and the CPU relies on the RAM less
   • Clock Speed - Number of cycles per second measured in Hertz (Hz)
     • The faster the clock speed the more operations are executed per second

4. Computer Types /Embedded Systems

   • An Embedded System is a computer system built into another device
   • The purpose of an embedded System is to provide specific predefined functions
   • Examples of embedded systems include Sat Navs, Mobile Phones, Dishwashers, TVs, Microwaves etc