1 Central Processing Unit (CPU) Consists of complex set of electronic circuitry Executes stored program instructions Three components –Registers –Control unit –Arithmetic/Logic Unit (ALU) Control Unit ALU Registers Memory CPU

2 ALU Executes all arithmetic and logical operations –Arithmetic operations Addition, subtraction, multiplication, division –Logical operations Compare numbers, letters, or special characters Tests for one of three conditions –Equal-to condition, Less-than condition, Greater-than condition –Branch to different instructions

3 Registers Registers: temporary place for instructions and data. All instructions and data must be placed in registers before being executed and processed High-speed temporary storage areas Work under direction of control unit A special register, called program counter, keeps the track of where the next instruction to be executed or needed data is stored

4 Control unit 1.Direct the computer system to execute stored program instructions 2.Tell ALU what to do 3.Storage operations –Moving data/instructions between registers and memory –Moving data/instructions secondary storage and memory as needed

5 Memory Primary storage –Often expressed as random-access memory (RAM) Holds data and instructions for processing Stores instructions and data of a running program

6 CPU and memory CPU cannot process data from disk or input device Items sent to ALU for processing Data and instructions held in memory until sent to an output or storage device or program is shut down

7 CPU and Instructions An instruction is the basic operation a CPU can execute. Each CPU is designed to implement a set of instructions Types of instructions

8 How the CPU executes instructions Four steps performed for each instruction 1.Fetch 2.Decode 3.Compute 4.Save

9 Machine cycles Machine cycle: the amount of time needed to execute an instruction Personal computers execute in less than one millionth of a second Supercomputers execute in less than one trillionth of a second

10 Instruction time Also called I-time Control unit gets instruction from memory and puts it into a register Control unit decodes instruction and determines the memory location of needed data

11 Execution time Control unit moves data from memory to registers in ALU –ALU executes instruction on the data Control unit stores result of operation in memory or in a register

12 Computer processing speeds Instruction speeds measured in fractions of seconds –Millisecond: one thousandth of a second –Microsecond: one millionth of a second –Nanosecond: one billionth of a second Modern computers have reached this speed –Picoseconds: one trillionth of a second Measure of system clock speed

13 The machine cycle and system clock System clock synchronizes operations One instruction takes one or more clock cycle Hz Clock cycle time: