Sat 24 October 2020

System Unit | CPU | ALU


The system unit is a box that contains different components of the computer system. It is also called a chassis. It is made of plastic or metal to protect the components inside it. Most storage devices reside inside the system unit. Other devices like keyboard, mouse, speakers, and printers reside outside the system unit.

There are different types of system units. Some system units sit on the top of a desk. A type called tower model stands vertically on the floor. A type called all-in-one computer contains the system unit and the monitor in the same box. In laptop computers, keyboard and pointing devices are built on the lop of the system unit.



The motherboard is also called the system board. It is the most important circuit board in the system unit. It contains different chips. A chip is a small semiconductor that contains integrated circuits on it. An integrated circuit (IC) is a microscopic path that carries electrical current. An IC may contain millions of transistors. A transistor works as an electronic gate that opens or closes the circuit for electronic signals.

Chips are attached to a circuit board like a motherboard, memory module, or card. Three are different types of chip packages that are as follows:

  1. DIP: DIP stands for a dual-line package. It consists of two parallel rows and downward-pointing thin metal pins. The pins attach the chip package to the circuit board.
  2. PGA: PGA stands for pin grid array. It holds a larger number of pins as the pins are mounted on the surface of the package.
  3. FC- PGA: FC- PGA stands for flip chip-VGA. It is a higher performance PGA. It laces the chip on the upper side of the pins.
  4. SEC. SEC stands for single edge contact. It connects lo motherboard on one of its edges.


Central Processing Unit

CPU stands for central processing unit It is the brain of the computer. It is the most important element of a computer system. It is also called processor

CPU is located on the motherboard.CPU carries out most of the work of a computer. Data passes through CPU continually. Data come from RAM and other units such as keyboard and drives etc. CPU processes the data and sends it back to RAM and other units.

CPU continually receives instructions to execute. Each instruction tells the CPU to process data. CPU performs all operations on data according to the given instructions. It executes instructions and tells other parts of the computer what to do. Most of the work consists of calculations and data transfer. CPU consists of two main units:

CPU consists of two main units:

  1. ALU (Arithmetic and Logic Unit)
  2. CU (Control unit)


ALU (Arithmetic and Logic Unit)

ALU is a part of the CPU. The actual execution of the instructions takes place in this part. All arithmetic and logical operations are performed in ALU. It consists of two units:

  1. Arithmetic unit
  2. Logic unit.


1. Arithmetic Unit

Arithmetic unit of the ALU performs basic arithmetic functions such as addition, subtraction, multiplication, and division

2. Logic Unit

Logic unit of the ALU performs logical Operations like comparing two data items to find which data item is greater than, equal to, or less than the other.


Control Unit

Control Unit is an important component of CPU. It controls all activities of the computer system. It is also called the supervisor of the computer.

CU performs the following operations:

  1. It fetches instructions from the main memory.
  2. It interprets that instruction to find what operation is to be performed.
  3. It controls the execution of the instruction.

Machine Cycle

CPU performs the following steps to execute an instruction:

Fetch: Instruction fetch is a process of getting instruction from the memory to execute it. This process is performed by the control unit.

Interpret Instruction: Interpret instruction is a process to examine the nature of instruction to determine what further operations should be taken. If any operation is needed, it should be carried out before the execution of that instruction.

Execute Instruction: After interpreting the instruction and getting the required data, CPU finally executes that instruction. The instruction is executed by ALU.

Storing: The prom of writing the result to the memory is called storing.

These four operations collectively are called a machine cycle of the instruction cycle. The time required to fetch and decode instruction is called Instruction time or i-time. The time required to execute and store is called Execution time or e-time.


Instruction Set

A set of all instructions that as CPU can perform is called an instruction set. The instruction set consists of the following types of instructions:

  1. Arithmetic instructions
  2. Logical Instructions
  3. Data Transfer Instructions
  4. Control Transfer Instructions

1. Arithmetic Instructions

The instructions used to perform arithmetic operations are called arithmetic instructions. Different arithmetic operations are addition, subtraction. multiplication and division. These instructions are executed by a file arithmetic & logic unit of CPU.

2. Logical Instructions

The instructions used to perform logical operations are called logical instructions. A logical operation is the comparison of two data values. Possible comparisons are:

  1.  Greater than (>)
  2.  Equal to (=)
  3.  Less than (<)

These instructions are also executed by the Arithmetic & Logic unit of CPU.

3. Data Transfer Instructions

The instructions used to transfer data from one component to another during the program execution are called data transfer instructions.

4. Control Transfer Instructions

The instructions used to change the sequence of instructions of a program are called control transfer instructions. These instructions transfer the execution control to a certain part of the program instead of the next instruction.


Types of Processors

The speed of the processor is also measured as millions of instructions per second (MIPS). Computers can process instruction with a speed of more than 300 MIPS.

Two designs of the CPU are as follows: 

CISC: CISC stands for complex instruction set computing. It supports a large number of instructions. It executes complex instruction more quickly. It is mostly used in PC and conventional mainframe computers.

RISC: RISC stands for reduced instruction set computing. It reduces the instructions to only those used more frequently. It executes simple instructions more quickly than CISC CPU. It is mostly used in workstations. The workstations work up to 10 times faster than most PCs due to RISC. It is used in many Macintosh computers.


In some computers, the processor fetches, decodes, executes and stores only instruction at a time. The CPU waits until an instruction completes its all four stages and then executes the next instruction.

Pipelining is a technique in which CPU fetches the next instruction before the completion of the first instruction. It results in faster processing. Most modern computers support pipelining.



The register is a small high-speed memory inside the CPU. It is used to store temporary results. CPU contains a number of registers. Each register has a predefined function. 

Register size determines how much information it can store. The size of registers is in bytes. Each byte can store one character of data. A register can be of one, two, four, or eight bytes. The bigger size of the register increases the performance of the CPU.

Different registers in Intel-8086 family processor are in follows:


1. Accumulative Registers

Accumulative registers are used in mathematical and logical operations. Time registers are part of ALU. There are four types of accumulative registers called EAX, EBX, ECX, and EDX.

The size-of these registers can be 1 to 4 bytes. The two-byte data registers (AX, BX, cx, DX) contain two parts of 8—bit size. These parts are separately accessible. The lower-order bytes are designated as AL, BL, CL, and DL. The high-order bytes are designated as AH, BH, CH, and DH.


2. Stack Control Register

A stack is a set of memory locations in which data is stored and retrieved in order. This order is called Last-ln-First-Out (LIFO). The data item stored at the top of the slack is retrieved before retrieving the item below it.

Stack control registers are used to manage stack in computer. Two special registers SP and BP are used for this purpose. DI, SI, SP & BP can either be used as 2-byte register or 4-byte registers. The names of 4-byte stack control registers are EDI, ESI, ESP, and EBP.


3. Instruction Pointer Register

The instruction pointer register is used to store the memory location of the next instruction to be executed. It works along with the segment registers.


4. Address or Segment Registers

Address or segment registers are used to store the memory location of the instruction that is being executed. This is a group of 4 or 6 registers of two bytes named CS, DS, ES & SS, FG, GS. These registers are used with either the IP register or two index registers DI & SI.

5. Flag Registers

Flag registers are used to indicate a particular condition. The size of the flag register is one or two-byte. Each byte of flag registers is further divided into eight bits. The data in the flag register is stored in 8 distinct bits. Each bit of the flag-register indicates a flag or condition. Some flag registers are Zero flags. Carry flag, Parity flag, Sigh flag, and Overflow flag.


System Clock 

The system clock is an electronic component. It generates electric signals at a fast speed. It controls all functions of the computer using clock ticks. The computer clock can tick from 100 million to 1000 million times in one second.

The speed at which the CPU executes instructions is called clock speed or clock rate. This speed depends on the model of the computer. Clock speed is measured in MHz.

Clock speed tells us how much time a CPU takes to executes each instruction. CPU takes from 1 to 6 clock ticks to execute one instruction.

Heat Sinks & Heat Pipes

The heat sink is a small ceramic or metal component used to absorb and ventilate the heat produced by electrical components of the computer. In some computers, it is part of the processor chip. It can also be installed on top or the side of the chip. The size of the heat sink is large and it occupies more space. So heat pipes are used to absorb heat in notebook computers.



A coprocessor is a special-purpose chip mounted on a processor board that can be installed on a computer. It is designed to handle common functions quickly and efficiently, and so expand on doing some of the processing of the chip. This frees up the CPU for other processing.