Computer Architecture

1. Introduction to Computer Architecture

• Computer Architecture refers to the internal components of a computer and the relationships between them.
• Almost all computers share a similar foundational structure.
• A computer consists of three main components:
  1. Input/Output Unit
  2. Central Processing Unit (CPU)
  3. Memory Unit

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2. Input and Output Units

Input Unit

• Definition: Hardware devices used to send data, instructions, or programs into the computer.
• Without an input unit, a computer would just be a display device like a TV, and the user could not perform any tasks.
• Key Function: It translates the data or commands given in human language into Binary Code that the computer (CPU) can understand.
• Primary Tasks:
  • Accepts and reads instructions and data provided by the user.
  • Converts this data into a format accepted by the computer.
  • Sends the converted data to the CPU for further processing.

Output Unit

• Definition: Devices used to display the final results of data and instructions.
• Key Function: It takes the processed results from the computer (which are in binary code) and translates them into appropriate signals, languages, or images that humans can understand.
• Primary Tasks:
  • Accepts the final results generated by the computer in binary form.
  • Converts these binary results into a human-readable and understandable format.
  • Presents or prints the converted results for the user.

Comparison Table: Input vs. Output Unit

Feature	Input Unit	Output Unit
Basic Function	Sends data and commands into the computer.	Displays processed results from the computer.
Data Conversion	Human Language $\rightarrow$ Binary Code.	Binary Code $\rightarrow$ Human Readable Format.
Next Step in Flow	Sends data to the CPU for processing.	Presents or prints final data for the user.

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3. Central Processing Unit (CPU)

• Definition: The CPU is the primary processing unit and is commonly known as the "Brain of the Computer".
• In microcomputers, the CPU is also called a Microprocessor.
• Core Responsibilities:
  • Controls all internal and external devices of the computer.
  • Performs all arithmetic and logical operations.
  • Decodes and executes instructions.
• Major Functions:
  1. Transfers data and instructions from the Main Memory into registers.
  2. Executes instructions sequentially.
  3. Transfers output data from registers back to the main memory when required.

Three Main Components of the CPU:

A. Arithmetic and Logical Unit (ALU)

• Function: Performs all arithmetic operations (addition, subtraction, multiplication, division) and logical comparisons (identifying if numbers are smaller, larger, or equal).
• Mechanism: Operates using the binary system. It is made up of electronic circuits that take two numbers and produce their sum, difference, product, or quotient.
• Contains special bytes called Registers to temporarily store incoming numbers and operational results.

B. Registers

• Definition: The fastest, temporary storage devices located at the highest level of the computer's memory hierarchy.
• Purpose: They provide the CPU with the fastest access path to use data, ensuring high-speed execution of programs.

C. Control Unit (CU)

• Definition: The most crucial part of the CPU, acting as a supervisor that monitors and coordinates all parts of the computer.
• Functions:
  • Reads programs from memory sequentially, analyzes them, and ensures their execution.
  • Issues exact commands to other parts of the computer (e.g., instructing the memory to store data or send it to the ALU).
  • Handles the flow of input and routes final results/messages to the output unit.

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4. Microprocessor

• Definition: A programmable digital electronic component built on a semiconductor integrated circuit (IC) that performs all CPU functions.
• It acts as the heart and brain of microcomputers and only understands Machine Language.
• Math Processor: Sometimes, a separate math processor is added to handle complex calculations.
• Important Examples:
  • Intel 4004: The very first microprocessor that combined all CPU components onto a single chip.
  • Modern Examples: Intel Dual Core, Pentium IV.

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5. Memory Unit

• Definition: The part of the computer that stores data and instructions.
• It provides the facility of "Information Retention," which is a fundamental function of modern computers.
• Classification: Memory is divided into two parts:
  1. Primary Memory
  2. Secondary Memory

Primary Memory (Main/Internal Memory)

• Connection: Directly connected to the CPU. The CPU continuously reads and executes instructions from it.
• Nature: Very fast processing speed but has a limited storage size.
• Function: Temporarily stores active programs, their input data, and outputs. Once the task is finished, the old data is removed to make space for new programs.
• What it Stores:
  • All data to be processed and input instructions.
  • Intermediate processing results.
  • Final results (kept safely until sent to the output device).
• Types of Primary Memory:
  1. Random Access Memory (RAM)
  2. Read Only Memory (ROM)

Random Access Memory (RAM)

• Composition: Located on a chip made of Metal-Oxide Semiconductor (MOS). It consists of registers and connected circuits that allow data transfer.
• Functionality: Users can select any memory location and directly access it to either read data from it or store data into it.
• Addressing: Every memory location in RAM has a specific address, allowing the CPU to quickly locate and access required data.

• Volatility: RAM is temporary (volatile) memory; all stored information is lost as soon as the computer's power is turned off.
• Functionality: Data and programs are sectioned systematically so the CPU can fetch and store information via the data bus using specific memory addresses.

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2. Instruction Format

• Computers only understand instructions in the form of 0s and 1s, which is called Machine Language.
• A computer program is a set of instructions detailing the exact steps needed to complete a task.
• Any processor requires two types of inputs to function:
  1. Data
  2. Instruction (Tells the computer what action to perform)
• Parts of an Instruction: An instruction is divided into two main parts:
  • Op-code (Operation): The actual action or operation to be performed.
  • Operand: The data or variables on which the operation is performed.
  • Example: In "$A + B$", '$A$' and '$B$' are the operands, and '$+$' is the op-code.

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3. The Instruction Cycle

• The Control Unit (CU) is known as the "Nervous System" of the computer because all commands pass through it.
• The sequence of processing an instruction is called the Instruction Cycle, which consists of four main steps:

Step	Phase Name	Description	Cycle Type
1	Fetching	Retrieves the instruction from memory and brings it to the Instruction Register.	Fetch Cycle (Same for all instructions)
2	Decoding	Interprets and decodes the fetched instruction.	Fetch Cycle (Same for all instructions)
3	Read Effective Address	If the instruction uses an indirect address, it reads the actual address from memory.	Execute Cycle (Varies per instruction)
4	Execution	Executes the operation defined by the instruction.	Execute Cycle (Varies per instruction)

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4. Read Only Memory (ROM)

• Definition: Pre-written memory that can only be read, not erased or modified by standard computer operations.
• Nature: It is Permanent (Non-Volatile); data remains safe even when the power is turned off.
• Applications: Extensively used in electronic devices like calculators, video games, and digital cameras.
• BIOS (Basic Input-Output System): Older personal computers stored the BIOS in ROM. It acted as a translator between the PC's hardware and the Operating System and contained the boot program.

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5. Secondary (External) Memory

• Why is it needed?: Main memory (RAM) is very expensive, has limited capacity, and is volatile. Secondary memory solves these issues.
• Characteristics:
  • Located outside the CPU (External Memory).
  • Highly cost-effective with massive storage capacity.
  • Permanent storage (Non-volatile).
  • Drawback: Read and write times are much slower compared to main memory.
• Usage: Primarily used for long-term storage and Backups of data that is not immediately needed, freeing up active space.
• Examples: Punched cards, paper tape, and magnetic tapes were used historically. Today, Magnetic Disks (like Hard Drives) are predominantly used due to convenience.

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6. The Motherboard

• Definition: The central, most important printed circuit board (PCB) in a computer, originating around 1974 for microcomputers.
• Other Names: Mainboard, Baseboard, System Board, or Logic Board.
• Function: Provides the essential electronic and logical connections to link all system components together.
• Connected Components: It houses the Microprocessor (CPU) and Main Memory, and connects to external storage, video/sound controllers, and I/O devices via ports and cables.

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7. System Bus (Data & Signal Transmission)

• Definition: A bus is a collection of wires or connections that act as a transmission medium to carry signals, data, and instructions between the CPU, memory, and peripheral devices.
• Types of Buses:

A. Internal Bus (System Bus)

• Connects internal motherboard components (like CPU and System Memory).
• Key Internal Buses include:
  • Control Bus: Carries control instructions to memory and I/O devices.
  • Address Bus: Carries the specific memory addresses of I/O devices or memory locations.
  • Data Bus: Responsible for safely transferring the actual data.

B. External Bus

• Connects external components to the motherboard, such as peripherals, ports, and expansion slots.

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8. Important Terminology (For Quick Reference)

• Machine Cycle: The time it takes to bring two operands from registers, perform an ALU operation on them, and store the resulting data back into the register.
• Buffer: A temporary storage area within RAM used to hold data while it is being transferred from one place to another. Input data lands here first.
• System Performance: A computer's execution power depends heavily on the size/speed of its Registers, RAM, Cache Memory, and the System Clock speed.
• Sockets: The physical connecting points on the motherboard where microchips are installed.
• Clock: Often refers to the control unit / timing mechanism of a digital computer that synchronizes operations.
• Instruction Code: A specific grouping of bits that commands the computer to perform a targeted task.
• Processing Cycle: The total time taken by a device to locate data/instructions and deliver them to the CPU.

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Quick Revision Summary (Key Takeaways)

• Computer Architecture = Input/Output Unit + CPU + Memory.
• Input Unit: Human readable $\rightarrow$ Binary.
• Output Unit: Binary $\rightarrow$ Human readable.
• CPU (Brain): Contains ALU (Math/Logic), Registers (Fast temporary storage), and Control Unit (Coordinator).
• Registers: Highest and fastest in the memory hierarchy.
• Microprocessor: A CPU on a single semiconductor IC. First ever was the Intel 4004.
• Primary Memory: Directly linked to CPU. Fast but temporary. Holds active data, instructions, and results.
• RAM: Random Access Memory made of MOS, allows direct reading and writing to any location.
• Instruction parts: Op-code (action) + Operand (data).
• Instruction Cycle: Fetch $\rightarrow$ Decode $\rightarrow$ Read Address $\rightarrow$ Execute.
• ROM vs. RAM: RAM is volatile and read/write; ROM is permanent (stores BIOS/boot programs) and read-only.
• Secondary Memory: High capacity, cheap, slow, and permanent. Great for backups (e.g., Magnetic Disks).
• Motherboard: The main circuit board connecting the CPU, memory, and all peripherals.
• System Bus: The wire pathways for data. Address Bus (locates data), Data Bus (moves data), and Control Bus (sends commands).
• Buffer: Temporary RAM storage for data in transit.