The computer processor, often called the Central Processing Unit (CPU), is the circuitry that execute the program instructions to process data.  It consists of the Control Unit and the Arithmetic Logic Unit that are connected by an electronic roadway called a bus.  The processor work with main memory (RAM) and registers to turn data into information.

The functions performed by each part of the CPU follows.

• Control Unit.  This unit directs and coordinate processing.  The control unit sequentially retrieve instructions from RAM and places them into an instruction register.  Each instruction is then interpreted to find out what need to be done.  According to the interpretation, the control unit then sends signal to the data bus to fetch data from RAM, and to the ALU to process the data. 
     
• Arithmetic Logic Unit.  This unit perform arithmetic and logical operations such as addition, subtraction, and comparisons.  The ALU contain the circuitry that perform additions and comparisons plus temporary storage areas called registers and accumulator.  The registers hold data that is being processed.  The accumulator hold the result of an arithmetic or logical operation.  

 The Machine Cycle

The machine cycle comprises a series of operations performed to execute a single program instruction.  The time required to complete the cycle is dependent on processor speed, which is measured by the system clock rate.  The machine cycle consist of two parts: an instruction cycle, which fetches and decodes, and an execution cycle which executes and stores.

Instruction Set

A computer perform a complex task by performing a series of very simple steps, that are referred to as instructions.  The list of instructions that a CPU can perform is known by the instruction set.

The following example illustrate how the CPU process instructions.

Performance

Computer performance are affected by the following factors:

• RAM Capacity
    
• Processing Speed
  • For microcomputers is measured by the system clock.  For example, a 500MHZ processor means a system clock rate of 500 million tick per second.
  • For Mid-Size Computers and Mainframes is measured by millions of instructions per seconds (MIPS).
  • For Supercomputers is measured by floating points operations per second (flops)
      
• Word Size refer to the number of bits the processor can hold in its registers and transfer over the bus at one time.
    
• Instruction Set
  • Complex Instruction Set Computer (CSIC)
  • Reduced Instruction Set Computer (RISC)

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