Basic Computer Model & Different Units of Computer - Computer Architecture & Organisation

Different Units of Computer

The basic high-level model of a computer can be described by four principal units that work together to perform computing tasks. These units are the foundation of computer organisation and are present, in one form or another, in all digital computers.

• Central Processing Unit (CPU)
• Input Unit
• Output Unit
• Memory Unit

Central Processing Unit (CPU)

The Central Processing Unit (CPU) is the primary component that executes instructions and controls the operation of the computer. It interprets program instructions, performs arithmetic and logic operations, and coordinates the activities of all other units by generating control signals.

Major blocks inside the CPU

• Control Unit - Generates control signals to sequence and synchronise operations. It fetches instructions from memory, decodes them and issues the appropriate control signals for execution. The control unit contains registers and control circuitry that help manage the instruction cycle.
• Execution Unit (Data Processing Unit) - Performs arithmetic and logical operations. It contains the Arithmetic and Logic Unit (ALU) and a set of registers used for temporary storage of data during execution.

Registers and their roles

• Program Counter (PC) - Holds the address of the next instruction to be fetched.
• Instruction Register (IR) - Holds the currently fetched instruction for decoding and execution.
• Accumulator (AC) - Often used to store intermediate arithmetic and logic results (architecture dependent).
• General Purpose Registers - Used by programs and the CPU to store operands and intermediate results.
• Status / Flag Register - Stores condition bits (zero, carry, sign, overflow) set by the ALU.

Operation: The Instruction Cycle

The basic instruction cycle carried out by the CPU is commonly described as fetch → decode → execute. A typical sequence is:

• Fetch the instruction from memory using the address in the Program Counter.
• Increment or update the Program Counter to point to the next instruction.
• Decode the instruction in the Instruction Register to determine the operation and operands.
• Fetch any required operands from registers or memory.
• Execute the operation in the ALU or other execution circuitry.
• Store the result in the designated register or memory location and update status flags.

Control unit implementation styles

• Hardwired control - Control signals are produced by fixed combinational and sequential logic. It tends to be faster but less flexible.
• Microprogrammed control - Control signals are generated by a microcode stored in a control memory. It is more flexible and easier to modify or extend.

Input Unit

The Input Unit is responsible for accepting data and programmes from external sources (users or other systems) and converting these inputs into a binary form that the computer can process. Input devices connect to the computer through input controllers or device interfaces which handle signalling and data transfer under the control of the CPU.

Examples of input devices include:

• Keyboard
• Mouse
• Scanner
• Microphone
• Hard disk (as a source of data/programs)
• Floppy disk
• CD-ROM drive

Typical methods used to manage input transfers are programmed I/O (polling), interrupt-driven I/O and Direct Memory Access (DMA).

Output Unit

The Output Unit converts processed binary information from the computer into human-readable form or into a form usable by other devices, and sends it to output devices or secondary storage for permanent retention. Output operations are performed under control of the CPU using output instructions or via DMA and controllers.

Examples of output devices include:

• Monitor (display)
• Printer
• Plotter
• Speakers
• Hard disk (for persistent output storage)
• Floppy disk

Memory Unit

The Memory Unit stores data and instructions that the CPU uses. Memory is organised in a hierarchy according to speed, cost and volatility. The main distinction is between primary (main) memory and secondary (auxiliary) memory.

Primary memory (Main memory)

Main memory is typically semiconductor-based and directly accessible by the CPU. It holds the programs and data currently in use. Primary memory is usually volatile, meaning it loses its contents when power is removed.

• Volatile memory - RAM (Random Access Memory). RAM allows both read and write operations. Two common types are SRAM (faster, used for caches) and DRAM (higher density, used for main memory).
• Non-volatile memory stored in semiconductor form - ROM (Read-Only Memory) and its variants:

• PROM (Programmable ROM) - Can be programmed once after manufacture.
• EPROM (Erasable Programmable ROM) - Can be erased (by ultraviolet light) and reprogrammed.
• EEPROM (Electrically Erasable Programmable ROM) - Can be electrically erased and reprogrammed without removing the chip from the system.

Secondary memory (Auxiliary storage)

Secondary memories are non-volatile devices used for permanent or long-term storage of programs and data. They provide larger capacity at a lower cost per bit but are slower than primary memory.

Examples of secondary storage devices:

• Magnetic devices: Hard disk, Floppy disk, Magnetic tape
• Optical devices: CD-ROM
• Semiconductor devices: Thumb drive (pen drive), SSDs (solid-state drives)

Memory hierarchy and trade-offs

• Higher levels of the memory hierarchy (registers, caches) are faster and more expensive per bit and typically volatile.
• Lower levels (main memory, secondary storage) offer larger capacities at lower cost but higher access times.
• Cache memory is placed between CPU and main memory to reduce average access time by keeping frequently used data closer to the CPU.

How the units interact

The units interact through a system of buses and control signals. The system bus typically contains an address bus (to select memory or I/O locations), a data bus (to transfer data), and a control bus (to carry control and timing signals). The CPU issues memory read/write requests and I/O commands; data moves between memory, CPU registers and I/O devices according to these requests and the control signals generated by the control unit.

Summary

A practical understanding of the basic computer model emphasises the roles and coordination of the CPU, input, output and memory units. The CPU controls and executes instructions using registers and the ALU; the input and output units provide interfaces to the outside world; and the memory unit stores programs and data in a hierarchy that balances speed, capacity and cost. Together these units form the organised structure required to run programmes and solve computational problems.