The CPU

The CPU

1. CONTROL UNIT (CU)

   • PROGRAM COUNTER
     • The program counter (PC) holds the memory address of the the instruction for each cycle

2. Arithmetic & Logic Unit (ALU)

   • Accumulator
     • The accumulator secures intermediate results of calculations in the ALU

3. Registers

   • Memory Address Register (MAR)
     • The memory address register (MAR) holds any memory address about to be used by the CPU. The address might point to data or a CPU instruction
   • Memory Data Register (MDR)
     • The memory data register (MDR) holds the actual data or instruction. This may have been fetched from memory, or be waiting to be written to memory

4. CPUs follow the Fetch-Execute Cycle

   • Fetch Instruction: 1) Copy the memory address from the program counter to the MAR 2) Copy the instruction stored in the MAR address to the MDR 3) Increment (increase) the prgram counter to point to the address of the next instruction, ready for the next cycle.
     • Decode Instruction: The instruction in the MDR is decoded by the Cu. The CU may then prepare for the next step, e.g. by loading values in to the MAR or MDR
       • Execute Instruction: The instruction id performed. This could be: load data from memory, write data so memory, do a calculation or logic operation (using the ALU), change the address in the PC, or halt the program.

5. CLOCK SPEED

   • This is the number instructions a single processor core can carry out per second (Hz). For most desktop computers, this will be somewhere around 3.5(Hz0
   • The higher the clock speed, the greater the numbers of instructions that cab carried out per second.
   • Some CPUs can be overclocked to make them run at a higher clock speed than the factory-set rate. But it's risky if not done properly- It can make them CPUs overheat, causing crashes or permanent damage to the system. High performance cooling systems (e.g. water cooling) are usually needed