Components of the CPU

• What is the CPU? 

  • The CPU is responsible for processing all data within the computer 
  • It is made up of a number of components including: 
  • Arithmetic and Logic Unit (ALU) 
  • Control Unit (CU) 
  • Registers are temporary storage/memory locations inside the CPU which are used for a single specific purpose. They have a faster access speed than RAM / secondary storage. There are a number of registers in the CPU: 
  • Program Counter (PC) 
  • Accumulator (ACC) 
  • Memory Address Register (MAR) 
  • Memory Data Register (MDR) 
  • Current Instruction Register (CIR) 
• Components of the CPU

  
• Arithmetic and Logic Unit (ALU)

  - This performs any arithmetic calculations (e.g. adding binary) or any logic comparisons (using AND, OR, NOT) 

  - The ALU is made up of several components 
• Control Unit 

  This is where instructions are decoded. The CU also controls the data within the CPU and how it moves around 
• Program Counter (PC)

  This stores the address in memory of the next instruction to be fetched 
• Accumulator (ACC) 

  This is where values are stored temporarily, either after they’ve been inputted or loaded, or after being calculated in the ALU 
• Memory Address Register (MAR) 

  This is where addresses are stored, either for where data is being sent in memory, or where it is being fetched from 
• Memory Data Register (MDR) 

  This is where data/instructions are stored, either before it sent to memory, or after being fetched 
• Current Instruction Register (CIR) 

  When an instruction has been fetched from memory it is loaded here before being split into opcode and operand. After this, it will be decoded. 
• The ALU is made up of several components: 

  • Arithmetic circuit 
  • This carries out any arithmetic (addition, subtraction, multiplication or division) 
  • Logic circuit 
  • This carries out operations like AND, OR, NOT, XOR 
  • Registers 
  • These are additional registers to those mentioned above and can store data 
  • Status flags 
  • This includes overflow flags (if the value is too large for the register) or could include a zero flag (to tell if the answer is 0 easily) 
  • Buses 
  • These are used to transport data around the ALU and to other parts of the CPU 
• Buses 

  There are 3 buses which connect the CPU with the main memory (RAM): 

  • Data bus 
  • Address bus 
  • Control bus 

  There are also a number of other buses within the CPU which transport data between the different areas
• Buses and there purpose