🧠 CPU, Motherboard & System Unit

The System Unit — The Computer's Body

The system unit is the rectangular box that houses all the core components of a computer. Everything important lives inside it — the processor, memory, storage, and circuit boards. The keyboard, monitor, and mouse connect to it from outside.

There are two main types:

• Desktop type — Lies flat, monitor placed on top
• Tower type — Stands vertically beside the monitor (most common today)

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CPU — The Brain of the Computer

The CPU (Central Processing Unit) is the most important component. It is called the "brain" of the computer because it performs all calculations, makes decisions, and controls every operation.

The CPU has three main parts:

1. ALU (Arithmetic Logic Unit)

The ALU performs two types of operations:

• Arithmetic Operations — Addition (+), Subtraction (−), Multiplication (×), Division (÷)
• Logical Operations — Comparisons using AND, OR, NOT, greater than (>), less than (<), equal to (=)

Every calculation your computer does — from adding numbers in Excel to rendering a video — goes through the ALU.

2. CU (Control Unit)

The Control Unit is the manager of the CPU. It does not perform calculations itself, but:

• Fetches instructions from memory
• Decodes what each instruction means
• Directs other components to execute the instruction
• Controls the flow of data between CPU, memory, and devices

Think of CU as the traffic police — it doesn't drive the cars, but directs all traffic flow.

3. Registers

Registers are tiny, ultra-fast storage locations inside the CPU that hold data currently being processed. They are the fastest memory in any computer.

Register	Purpose
Accumulator	Stores intermediate arithmetic results
Program Counter	Holds the address of the next instruction
Instruction Register	Holds the current instruction being executed
Memory Address Register	Holds the address of memory to read/write

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How the CPU Processes Instructions

The CPU follows a cycle for every instruction called the Fetch-Decode-Execute cycle:

1. Fetch — CU retrieves the next instruction from RAM
2. Decode — CU interprets what the instruction means
3. Execute — ALU performs the calculation or operation
4. Store — Result is saved back to memory or register

This cycle repeats billions of times per second in modern processors.

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Motherboard at a Glance

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CPU Speed and Performance

Measure	What it Means
Clock Speed	Number of cycles per second (measured in GHz)
Cores	Independent processing units (dual-core = 2, quad-core = 4, octa-core = 8)
Cache	Small, fast memory inside CPU for frequently used data
MIPS	Million Instructions Per Second
FLOPS	Floating Point Operations Per Second (for supercomputers)

Major CPU Manufacturers: Intel (Core i3, i5, i7, i9) and AMD (Ryzen series)

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How the Motherboard Connects Everything

The motherboard (also called mainboard or system board) is the main circuit board of the computer. Every component connects to it — CPU, RAM, storage, graphics card, and all ports.

Key Components on the Motherboard

Component	Function
CPU Socket/Slot	Where the processor is installed
RAM Slots	Where memory modules are inserted (DIMM slots)
Expansion Slots	For adding graphics cards, sound cards, network cards (PCI, PCIe)
BIOS/UEFI Chip	Stores basic startup instructions (BIOS = Basic Input Output System)
CMOS Battery	Powers the BIOS clock and settings when computer is off
Chipset	Controls communication between CPU, RAM, and peripherals
Power Connector	Receives power from the power supply (SMPS)

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Ports and Connectors

Ports are sockets on the back/side of the system unit where external devices plug in:

Port	Full Form	Used For
USB	Universal Serial Bus	Mouse, keyboard, pen drive, printer, phone charging
HDMI	High Definition Multimedia Interface	Monitor, projector, TV (video + audio)
VGA	Video Graphics Array	Older monitors (video only)
Ethernet (RJ-45)	—	Wired internet connection (LAN cable)
Audio Jack (3.5mm)	—	Headphones, speakers, microphone
PS/2	—	Older keyboard/mouse (green = mouse, purple = keyboard)
Serial Port	—	Older devices, modems
Parallel Port	—	Older printers

USB is the most widely used port today — USB 2.0, 3.0, 3.1, and USB-C are progressively faster versions.

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Buses — The Computer's Highway System

A bus is a set of electrical pathways that carry data between components inside the computer. Think of it as a highway system connecting different parts of a city.

Bus Type	What it Carries	Direction
Address Bus	Memory addresses (where to read or write)	CPU to memory, so it is unidirectional
Data Bus	Actual data between CPU, RAM, and devices	Bidirectional
Control Bus	Control signals such as read, write, interrupt, and clock	Bidirectional

Bus Width: The number of wires — a 32-bit bus carries 32 bits at a time; a 64-bit bus carries 64 bits (wider = faster).

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SMPS — Power Supply

SMPS (Switched Mode Power Supply) converts AC power from the wall outlet into DC power that computer components need. It supplies different voltages to different parts:

• 3.3V — RAM, chipset
• 5V — USB ports, drives
• 12V — CPU, graphics card, fans

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CPU Performance in More Detail

Clock Speed

Clock speed is measured in GHz (Gigahertz) — it indicates how many billions of cycles the CPU performs per second. A 3.5 GHz processor executes 3.5 billion cycles per second. Higher clock speed generally means faster processing.

Multi-Core Processors

Modern CPUs have multiple independent processing units called cores:

Type	Cores	Advantage
Single-core	1	One task at a time
Dual-core	2	Two tasks simultaneously
Quad-core	4	Four tasks simultaneously
Octa-core	8	Eight tasks simultaneously

More cores usually improve multitasking, while clock speed describes how quickly each core can execute its cycles.

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Motherboard — Additional Components

Chipset

The chipset manages data flow between the CPU, memory, and all peripherals. In older systems, it was divided into two chips:

Chip	Function
Northbridge	Connects CPU to high-speed components — RAM and GPU
Southbridge	Connects CPU to slower components — USB, audio, BIOS, hard drives

In modern systems, Northbridge functions are integrated into the CPU itself.

CMOS Battery

The CMOS battery (CR2032, coin-shaped) powers the BIOS/UEFI settings and the real-time clock (RTC) when the computer is turned off. If this battery dies, the computer loses its date/time settings and BIOS configuration.

Heat Sink and Fan

The heat sink is a metal block (usually aluminium/copper) placed on top of the CPU to absorb heat. A fan blows air over the heat sink to dissipate heat. Without proper cooling, the CPU can overheat and throttle or shut down.

BIOS Chip

The BIOS (Basic Input Output System) chip stores firmware — the first program that runs when you power on the computer. It performs the POST (Power-On Self Test) and then loads the operating system from the hard drive.

Modern systems use UEFI (Unified Extensible Firmware Interface) as a replacement for traditional BIOS — it supports larger drives, faster boot, and a graphical interface.

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Expansion Slots

Expansion slots on the motherboard allow you to add extra capability:

Slot Type	Full Form	Purpose
PCI	Peripheral Component Interconnect	Sound cards, network cards, older devices
PCIe x1	PCI Express x1	Small cards — Wi-Fi adapters, USB expansion
PCIe x4	PCI Express x4	NVMe SSDs, RAID controllers
PCIe x16	PCI Express x16	Graphics cards (GPU) — fastest slot
AGP	Accelerated Graphics Port	Old graphics card slot (obsolete, replaced by PCIe)

PCIe x16 is the standard slot for modern graphics cards, while AGP belongs to an older generation of hardware.

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Motherboard Form Factors

The form factor defines the size, shape, and layout of the motherboard:

Form Factor	Size	Expansion Slots	Used In
ATX	305 x 244 mm	7 slots	Standard desktops, gaming PCs
Micro-ATX	244 x 244 mm	4 slots	Budget desktops, compact builds
Mini-ITX	170 x 170 mm	1 slot	Small form factor PCs, HTPCs

ATX (Advanced Technology Extended) is the most common form factor for desktop motherboards.

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Summary Points

Concept	Key Details
CPU	Brain of computer — ALU + CU + Registers
ALU	Arithmetic (+−×÷) and Logic (AND, OR, NOT, >, <, =) operations
Control Unit	Fetches, decodes, and directs instruction execution
Registers	Fastest memory, inside CPU — Accumulator, Program Counter, IR, MAR
Fetch-Decode-Execute	CPU instruction cycle — repeats billions of times/sec
Clock Speed	Measured in GHz (Gigahertz) — 3.5 GHz = 3.5 billion cycles/sec
Multi-core	Dual (2), Quad (4), Octa (8) — more cores = better multitasking
CPU Makers	Intel (Core i3/i5/i7/i9), AMD (Ryzen)
MIPS / FLOPS	Million Instructions Per Second / Floating Point Operations Per Second
Motherboard	Main circuit board (mainboard/system board) — connects all components
Address Bus	Unidirectional (CPU→Memory) — carries memory addresses
Data Bus	Bidirectional — carries actual data
Control Bus	Bidirectional — carries control signals (read, write, interrupt)
Bus Width	32-bit or 64-bit — wider = faster data transfer
Northbridge	Connects CPU to RAM and GPU (high-speed) — now integrated into CPU
Southbridge	Connects CPU to USB, audio, BIOS, hard drives (low-speed)
CMOS Battery	CR2032 coin cell — powers BIOS clock/settings when PC is off
Heat Sink + Fan	CPU cooling — metal block absorbs heat, fan dissipates it
BIOS Chip	Basic Input Output System — firmware, runs POST at startup
POST	Power-On Self Test — BIOS checks hardware at startup
UEFI	Modern BIOS replacement — faster boot, graphical interface, larger drives
PCIe x16	Fastest expansion slot — used for graphics cards (GPU)
PCIe x4	NVMe SSDs, RAID controllers
PCIe x1	Small cards — Wi-Fi adapters, USB expansion
AGP	Old graphics slot — obsolete, replaced by PCIe
ATX	Standard motherboard form factor (305x244mm, 7 slots)
Micro-ATX	Compact form factor (244x244mm, 4 slots)
Mini-ITX	Smallest form factor (170x170mm, 1 slot)
SMPS	Switched Mode Power Supply — AC→DC (3.3V, 5V, 12V)
USB	Universal Serial Bus — most common port for peripherals