Computer

• A computer is a machine that takes in, stores, transforms and displays data.

• Data is normally in binary form and is processed according to program instructions

• Computer Systems consist of hardware and software components that follows the concept of input, process, output and storage model.

Input, store, process, output

A computer

• • Accepts data or instructions as input from an input device

  • Data or instructions are stored and processed by the computer system

    • • It may be the case that other data or instructions, apart from the input, are necessary during processing.

  • The computer outputs the processed data into information that we can see and use using an output device

1. Arithmetic logic unit (ALU)

• • part of the processor that performs the arithmetic and logic operations (e.g. comparisons and arithmetic operations.)

  • specific operations:

    • Arithmetic operations: including bit addition and negation of binary numbers.

    • Logical operations: including AND, OR, NOT, XOR, NOR, NAND, etc.

    • Bit-shifting operations:  shifting the positions of the bits by a certain number of places to the right or left.

2. Control Unit (CU)

• • part of processor which coordinates all activities in processor and all other parts within a computer;

  • obtains the data/instructions from the memory; 

  • interprets/decodes them into commands/steps/signals; 

  • controls transfer of data and instructions among other units of a CPU (for example, command to ALU for execution); 

  • manages/coordinates all the units of the computer; 

     Control Unit (more details)

• • • Contains various registers;

    • A register in general is a small storage location that can hold data;

    • Usually a multiple of 8 bits;

           Example

• • • • A 64 bit computer has 64 bit registers:

      • The size of the register in bytes is known its as a word;

      • A 64 bit register holds 64 bits = 8 * 8 bits, and since 1 byte is equal to 8 bits

      • 64 bits = 8 bytes

      • A 64 bit computer system has a word size of 8 bytes

      • So each memory location will hold 64 bits or 8 bytes

Types of CPU control

Hardwired control

• • • The CU is made up of flip-flops, logic gates, digital circuits and encoder and decoder circuits that are wired in a specific and fixed way.

    • Thus changes cannot be made to the instruction set.

    • This type of control only has a small number of instructions with each being executed on a single clock cycle.

    • Are widely used in mobile devices because they produce less heat.

    • Reduced Instruction Set Computing (RISC) architecture.

Microprogram control

• • • Are stored in a special control memory on the CPU.

    • Each instruction executes a microprogram.

    • This takes several clock cycles but allows for a larger set of more complex instructions.

    • Complex Instruction Set Computing (CISC) architecture.

Registers

• • • are fast stand-alone storage locations that hold data temporarily.

    • a small set of data memory that is a part of a CPU itself.

    • provides temporary storage for operands and results.

    • can be accessed quickly by the CPU when performing operations, without having to access main memory (a much slower operation).

    • may hold a computer instruction, a storage address, or any kind of data (a bit sequence or individual characters).

3. The Accumulator (ACC): 

A register in the processor unit in which partial calculations are stored by the ALU. Without an ACC, it would be necessary to write the result of each calculation to main memory.

4. Instruction Register 

Stores instructions to be executed

5. Program counter 

Keeps track of the instruction currently being executed. After execution of the instruction, the counter is incremented to point to the address of the next instruction in memory. It contains the memory of the next instruction to be executed.

Functions of the Control Unit (CU) with the registers

• • Controls sequential instruction execution      (through the PC - Program Counter);

  • Interprets instructions       (through the IR - Instruction Register);

  • Guides data flow through different computer areas   (through the MAR and MDR);

  • Regulates and controls processor timing;

  • Sends and receives control signals from other computing devices;

  • Handles multiple tasks, such as fetching, decoding, execution handling and storing results.

Clock Speed

• • Every computing device contains an internal clock that regulates the rate at which instructions are executed.

  • Also synchronizes the actions of all the various computer components.

  • Clock speed is the number of pulses per second generated by an oscillator that sets the tempo for the processor.

    • Indicates how fast the CPU can run.

    • Is measured in megahertz (mHz) or gigahertz (gHz) and corresponds with how many instruction cycles the CPU can deal with in a second.

  • A faster CPU uses more energy and creates more heat.

Motherboard and buses

• • The motherboard is a circuit board that connects the CPU to the memory and all the other hardware.

  • The CPU sits on the motherboard, also called the logic board.

  • Buses are circuits on the motherboard that connect the CPU to other components.

  • There are many buses on the motherboard.

  • A bus moves instructions and data around the system.

  • The speed of a bus is measured in megahertz (mHz).

  • The faster the bus, the faster data is communicated.

  • The speed of the mother bus is defined by the bus speed.

  • Buses are limited by their width in bits. They are usually 8, 16, 32 or 64-bits wide.

  • The bus that connects the CPU to the main memory is called the Front-Side Bus (FSB). The FSB contains two types of buses: