Elements of Computer - Computer Fundamentals, Computer Awareness

Table of Contents
1. Computer Architecture
2. Components of Computer
3. Microprocessor
4. Motherboard
5. Interconnection of Units
6. Instruction Cycle
7. Tid-Bits
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Computer Architecture

• Computer architecture describes the organisation and behaviour of the parts of a computer system as visible to a programmer. It specifies the instruction set, data formats, addressing techniques and the way the processor interfaces with memory and I/O devices.
• It also refers to the physical and logical design of the system unit that houses the electronic components which perform computation and control.
• John von Neumann introduced the stored-program concept (commonly called the von Neumann architecture) in the mid-1940s (report on EDVAC, 1945). This became the basis for most modern general-purpose computers: program instructions and data are stored in the same memory and treated uniformly.

Components of Computer

1. Input Unit
2. Output Unit
3. Central Processing Unit (CPU)
4. Memory Unit

Note: The system unit is the metal or plastic case that contains most of the electronic parts of a computer (motherboard, processor, memory, expansion cards, power supply, etc.). The components that perform processing are located inside this case.

Input Unit

The input unit accepts instructions and data from users or other systems and converts them into a coded form that the computer can process. Common input devices include the keyboard, mouse, scanner, microphone and sensors.

Functions of an input unit:

• Accepts instructions and data from the user or other systems.
• Converts the received information into binary coded form that the computer understands.
• Transfers the converted data and instructions into main memory or registers for further processing.

Output Unit

The output unit presents processed results to the user or to other systems. Typical output devices are the monitor, printer, plotter, speakers and actuators.

Functions of an output unit:

• Receives coded results from the CPU or memory.
• Converts these coded results into human-readable or machine-usable form (text, images, sound, control signals).
• Presents or transmits the converted results to the user or other devices.

Central Processing Unit (CPU)

The Central Processing Unit (CPU) is the principal part of a computer that executes instructions. It is often called the brain of the computer. Modern CPUs are implemented as one or more integrated circuits (microprocessors) mounted on the motherboard.

The CPU contains groups of registers, an Arithmetic Logic Unit (ALU) and a Control Unit (CU). These subunits work together to fetch, decode and execute program instructions.

The main responsibilities of the CPU are:

• Move instructions and input data from main memory into appropriate registers.
• Interpret and execute instructions in the sequence determined by the program.
• Move results from registers back to main memory or to output devices as required.
• Coordinate and control internal and external devices, and perform arithmetic and logical operations.

The CPU comprises the following primary sub-systems:

• Registers
• Arithmetic Logic Unit (ALU)
• Control Unit (CU)

Arithmetic Logic Unit (ALU)

The ALU contains the electronic circuits that perform arithmetic operations (for example, addition, subtraction, multiplication, division) and logical operations (for example, AND, OR, NOT, XOR). The ALU operates on data supplied from registers and returns results to registers.

Typical tasks performed by an ALU:

• Arithmetic operations: addition, subtraction, multiplication, division (often via hardware or microcode).
• Logical operations: bitwise comparisons and Boolean logic (AND, OR, NOT, XOR).
• Bit shifting and rotation: shift data left/right, logical or arithmetic shifts, rotate operations.
• Comparison operations: equality and relational comparisons (=, <, >, ≤, ≥) used in conditional branches.

Registers

Registers are small, very fast storage locations inside the CPU used to hold instructions, addresses, intermediate data and status information during processing. Registers constitute the top of the memory hierarchy and provide the fastest way to manipulate data.

Common register types and purposes:

• Program Counter (PC) - holds the address of the next instruction to fetch.
• Instruction Register (IR) - holds the currently fetched instruction.
• Memory Address Register (MAR) - holds the address of the memory location to read or write.
• Memory Data Register (MDR) - holds data read from or to be written to memory.
• Accumulator (AC) - often used to hold intermediate arithmetic or logic results.
• General Purpose Registers (GPRs) - used by programs for temporary storage.
• Status/Flags Register - contains condition codes (zero, carry, sign, overflow) set by ALU operations.

The number and width (size in bits) of registers differ between processor architectures and strongly affect performance.

Control Unit (CU)

The Control Unit coordinates the operation of the CPU and the rest of the computer. It retrieves instructions from memory, decodes them to determine required actions, and generates the necessary control signals to direct the ALU, registers and I/O devices.

Functions of the control unit:

• Fetch the next instruction from main memory using the PC and place it into the IR.
• Decode the instruction to determine operation type, operands and the devices involved.
• Generate and issue control signals to the ALU, registers, memory and I/O devices to carry out the operation.
• Manage sequencing of micro-operations and handle interrupts and exceptions.

Two common implementation approaches:

• Hardwired control - control signals are produced by fixed electronic circuits. It is fast but less flexible.
• Microprogrammed control - control signals are produced by a microcode stored in control memory; easier to design and modify.

Memory Unit

The memory unit stores programs, data and intermediate results. Memory may be temporary (volatile) or permanent (non-volatile). It is organised in a hierarchy according to speed, cost and capacity.

• Primary memory (main memory) - usually volatile memory such as RAM (Random Access Memory). It stores the program currently in execution and the data being processed.
• Read-Only Memory (ROM) - non-volatile memory that stores firmware or bootstrap code (BIOS/UEFI) that does not change frequently.
• Cache memory - very fast, small memory placed between CPU and main memory to reduce access time for frequently used data and instructions.
• Secondary memory (auxiliary storage) - non-volatile storage used for permanent data and program storage: hard disk drives (HDD), solid state drives (SSD), optical discs and magnetic tape.
• Virtual memory - a technique where part of secondary storage is used to extend apparent main memory capacity, managed by the operating system through paging or segmentation.

Before the CPU processes data, that data and the instructions to be executed are loaded from secondary storage into main memory. The CPU then accesses these items from main memory (and cache) using addresses carried on the address bus.

Microprocessor

A microprocessor is an integrated circuit (chip) that implements the central processing logic of a computer on a single or a small number of semiconductor dies. The microprocessor is mounted on the motherboard (a printed circuit board) and, together with support circuitry, forms the CPU.

• The performance of a computer depends heavily on the microprocessor design, clock frequency, pipeline structure, cache sizes and instruction set efficiency.
• Intel introduced the first commercially available microprocessor Intel 4004 in 1971. The design team included engineers such as Ted Hoff and Federico Faggin (among others). The 4004 was a 4-bit microprocessor and marked the beginning of single-chip CPUs.
• Well-known microprocessor families include Intel Pentium series (Pentium IV), Intel Core (i3, i5, i7, i9), and many multi-core and embedded processors used in modern systems.

Motherboard

The motherboard (also called main board, system board or logic board) is the main printed circuit board that connects the processor, memory, storage interfaces, expansion slots and peripheral connectors. It provides electrical connections and slots for components and controllers.

Components on the Motherboard

• CMOS battery - a small battery that powers the real-time clock and preserves BIOS/UEFI settings when external power is off.
• BIOS/UEFI chip - firmware that initialises hardware at power-up and starts the operating system (bootstrap process).
• Cooling fan / heat sink - provides thermal management for the CPU and other heat-generating components.
• Expansion slots - slots such as PCI Express (PCIe) for graphics cards, network cards and other add-in cards.
• Power connectors / PSU (SMPS) - the switched-mode power supply (SMPS) / PSU supplies regulated power to the motherboard via connectors; the SMPS itself is usually a separate unit, not mounted on the motherboard.
• PCI / PCIe slots - interfaces for peripheral cards.
• Processor socket - the physical socket or slot where the microprocessor is placed.
• Buses and connectors - include data and address buses on the board, USB ports, SATA connectors, M.2 slots and other I/O connectors.

Interconnection of Units

The CPU communicates with memory and I/O devices using a collection of electrical pathways called a bus. A bus is a group of parallel wires or signal traces; each wire carries one bit at a time. Buses provide the channels over which addresses, data and control signals travel between components.

A computer bus is typically divided into three logical categories:

• Address bus - carries the memory or I/O addresses that identify the source or destination of data. It is unidirectional (CPU to memory/I/O). The width of the address bus (number of lines) determines the maximum addressable memory space.
• Data bus - carries the actual data between CPU, memory and I/O devices. It is typically bi-directional. The width (8, 16, 32, 64 bits) affects the amount of data moved per transfer.
• Control bus - carries control and timing signals (read/write, interrupt, clock, mem-ready) used to coordinate operations and signal states between devices.

Internal bus (system bus) connects internal components on the motherboard such as CPU, cache and main memory. 
External bus (expansion bus) connects peripheral devices and expansion cards (examples today: PCIe, USB, SATA). Bus performance, arbitration, width and clock rate are key factors in overall system throughput.

Instruction Cycle

The instruction cycle (or fetch-decode-execute cycle) is the sequence of steps the CPU performs to execute a single instruction from a program.

A simple instruction cycle can be described as follows:

1. Fetch: The CPU places the address of the next instruction (from the PC) on the address bus. The instruction is read from memory into the MDR, then placed into the IR. The PC is incremented to point to the following instruction.
2. Decode: The control unit decodes the instruction in the IR to determine the operation, addressing mode and operands required.
3. Execute: The CPU performs the operation - ALU performs arithmetic/logic operations, branch decisions are made, and data transfers between registers and memory or I/O occur.
4. Store / Write-back: The result of the execution is written back to a register or to main memory as required.

In practice the first two steps are commonly discussed together as the fetch-decode portion, while the last two form the execute portion. Modern processors use pipelining, where multiple instructions are overlapped in different stages of this cycle to improve performance.

Tid-Bits

• UPS (Uninterruptible Power Supply) - a device that provides emergency power to a computer or other equipment when the main power source fails, allowing safe shutdown and protection against voltage spikes.
• Power strip - a device providing multiple power outlets from a single wall socket; often includes surge protection.
• Instruction code - a binary pattern (set of bits) that specifies a particular operation for the CPU to perform; together an architecture's instruction set defines supported instructions and their encodings.