System Unit

Working of CPU

The CPU

CPU

The CPU• CPU stands for central processing unit. it is brain of computer

• It is most important component of the computer. it is also called processor.

• Converts data into information

• It continually receives instructions to execute. Each instruction tells CPU to process data.

• CPU performs all operations on data according to the given instructions.

• It executes instructions and tells other parts of computers what to do.

• Most of the work consists of calculations and data transfer.

• Two parts– Control Unit (CU)– Arithmetic and Logic Unit (ALU)

Control Unit CU

• Control Unit is an important component of CPU. It acts Like a supervisor of the computer.

• It controls all activities of the computer system

• All the computers resources are managed from the control unit.

• Think of the control unit as a traffic signal directing the flow of data through the CPU.

• The CPU´s instruction for carrying out commands are built into the control unit.

• The instruction or instruction set, list all the operations that the CPU can perform.

• Each instruction in the instruction set is a series of basic directions that tell the CPU how to execute more complex operations.

Control Unit CU

It performs the following operations:

1. It fetches or retrieve instruction from main memory.

2. It interprets the instruction to find what operation is to be performed.

3. It controls the execution of instruction. The control unit determines the sequence in which computer programs and instructions are executed

System Clock• In computers, sequence is everything. The system clock

synchronizes the tasks in a computer, like loading data before manipulating it, etc.

• The system clock is a circuit that emits a continuous stream of precise high and low pulses that are all exactly the same length.

• One clock cycle is the time that passes from the start of one high pulse, until the start of the next.

• If several events are supposed to happen in one clock cycle, the cycle is subdivided by inserting a circuit with a known delay in it, thus providing more highs and more lows.

Arithmetic / Logic UnitALU

ALU is a part of CPU. Actual Execution of instructions takes place in this part. All Arithmetic and Logical operations are performed in ALU.

It consists of two units

• Arithmetic Unit

Performs arithmetic operations

• Logic Unit

Performs logical operations

Arithmetic Operations

Addition

Subtraction

Multiplication

Division

Logical Operations

• Evaluates conditions

• Makes comparisons

• Can compare– Numbers– Letters– Special characters

ALU

• When the control unit encounters an instruction that involves arithmetic or logic , however, it passes that instruction to the second component of the CPU, the ALU.

• The ALU performs these operations.

• ALU includes a group of registers--- high speed memory locations built directly into the CPU that are used to hold the data currently being processed.

ALU

• ALU will use the register to hold the data currently being used for calculation.

For example

• CU might load two numbers from memory into the registers in the ALU.

• Then it might tell the ALU divide the two numbers (an arithmetic operation) or to see whether the numbers are equal (a logical operation).

• The answer to this calculation will be stored in another register before being sent out of the CPU.

Registers

• A location used for temporarily hold data or instructions at the moment of execution

• The storage capacity of registers measures in bytes (can be 1,2,4 or 8 bytes registers).

Registers in ALU

• Accumulator (Acc)– storing intermediate results of computations

• Status Register (SR)– to store the status of the result

Registers in CU• Program Counter (PC)

– to contain the address of next instruction to be executed

– keep a record of the sequence of execution.

• Instruction Register (IR)– to contain instruction that is being executed.

Registers in CU

• Memory Address Register (MAR)– to store the address of the data or instruction

to be processed

• Memory Data Register (MDR)– to store data or instruction to be processed.

Registers in CPU

CPUCU ALU

PC

MAR

MDR

IR

PSR

Acc