Computer Organization

Computer Organization

Computer Organization •Each computer has a visible structure, which is referred to as its architecture. •The concept of architecture and making these hardware components work in a harmonized manner in order to achieve a common objective in an environment is known as computer organization. CENTRAL PROCESSING UNIT (CPU) •CPU is referred to as the brain of a computer system, which converts data (input) into meaningful information (output). • A CPU controls all internal and external devices, performs arithmetic and logic operations, and operates only on binary data (1s and 0s).

Arithmetic/Logic Unit (ALU) ALU contains the electronic circuitry that executes all arithmetic and logical operations on the data made available to it. Arithmetic unit: It contains the circuitry that is responsible for performing the actual computing and carrying out the arithmetic calculations, such as addition, subtraction, multiplication, and division. Logic unit: It enables the CPU to make logical operations based on the instructions provided to it. The unit can compare numbers, letters, or special characters and can then take action based on the result of the comparison.

Registers •Registers are special purpose, high-speed temporary memory units. •These are temporary storage areas for holding various types of information such as data, instructions, addresses, and the intermediate results of calculations. Control Unit (CU) •CU contains circuitry that uses electrical signals to direct the entire computer system to carry out, or execute, stored program instructions. •The control unit controls the I/O devices and transfer of data to and from the primary storage.

System Bus •Bus is a set of connections between two or more components/ devices, which are designed to transfer several/all bits of a word from a specific source to destination. •It consists of multiple paths, which are also termed as lines; each line is capable of transferring one bit at a time. Main Memory Unit •Memory is that part of the computer that holds data and instructions for processing. •Memory stores program instructions or data for only as long as the program they pertain to is in operation. •This memory itself is implemented by two types of memory technologies. Random Access Memory (RAM): Random access memory directly provides the required information to the processor.

Read Only Memory (ROM): •Read only memory stores the initial start-up instructions and routines in BIOS, which can only be read by the CPU. •The contents of ROM are not lost even in case of a sudden power failure, making it non-volatile in nature. Cache Memory •Cache is a very high speed, expensive piece of memory, which is used to speed up the memory retrieval process. •Cache memory can be categorized into three levels: L1 cache, L2 cache, and L3 cache. COMMUNICATION AMONG VARIOUS UNITS •All the units in a computer system work in conjunction with each other to formulate a functional computer system. • There are two types of communication followed in the computer architecture, these are: •Processor to Memory Communication •Processor to I/O Devices Communication

Computer Memory and Storage •Memory refers to the electronic holding place for instructions and data where the computer’s microprocessor can reach quickly. •Memory can classified into two broad categories: • primary memory •secondary memory. Memory Representation The basic unit of memory is bit. Digital computers work on only two states, ON (1) and OFF (0). The various units, used to measure computer memory, are: bit, byte kilobyte, megabyte, gigabyte, and terabyte.

Unit Symbol Bytes Byte Kilobyte Megabyte Gigabyte Terabyte

B KB MB GB TB

20 = 1 byte 210 = 1024 bytes 220 = 1024 KB 230 = 1024 MB 240 = 1024 GB

MEMORY HIERARCHY •The processor is the “brain” of the computer where all the essential computing takes place. •The memory in a computer system is of three types: •Internal Processor Memory: Includes cache memory and special registers. •Primary Memory: Includes Random Access Memory (RAM) and Read Only Memory (ROM), also known as main memory. •Secondary Memory: Includes hard disk, magnetic disk, and magnetic tapes, also known as auxiliary memory.

RANDOM ACCESS MEMORY (RAM) •RAM allows the computer to store data for immediate manipulation and to keep track of what is currently being processed. •It is the place in a computer where the operating system, application programs, and data in current use are kept so that they can be accessed quickly by the computer's processor. Types of RAM Static RAM: •This RAM retains its contents as long as power remains applied to the memory chips. •SRAM does not need to be 'refreshed' (pulse of current through all the memory cells) periodically.

Dynamic RAM: •This RAM must be continually 'refreshed' in order to maintain the data. •This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second. READ ONLY MEMORY (ROM) •A computer system needs special instructions (special boot programs) during a login operation. •These instructions are stored in the special chips, which enables the computer system to perform start operations and transfer the control to the operating system. •This special chip is called ROM (Read Only Memory). ROM chips are also used in electronic items like washing machine and microwave oven.

Types of ROM Masked ROM: •The very first ROM was a hard-wired device that contained a pre-programmed set of data/instructions. •This kind of ROM is known as masked ROM. Programmable ROM (PROM): •Creating a ROM chip from scratch is time-consuming and expensive. •Thus, developers created a type of ROM known as Programmable Read-only Memory (PROM), which can be programmed. Erasable Programmable ROM (EPROM): •An EPROM can be erased by simply exposing the device to a strong source of ultraviolet light for a certain amount of time and programmed repeatedly.

Electrically Erasable Programmable ROM (EEPROM): •This type of ROM can be erased by an electrical charge and then written to by using slightly higher-thannormal voltage. •EEPROM can be erased one byte at a time, rather than erasing the entire chip. Flash ROM: • It is a type of constantly powered non-volatile memory that can be erased and re-programmed in blocks. •It is a variation of EEPROM, which, unlike flash memory, is erased and rewritten at the byte level. SECONDARY STORAGE DEVICES •Secondary storage devices facilitates storing of information and programs permanently. •There are two methods of accessing data from the secondary storage devices:

Sequential Access: •Means the computer system must search the storage device from the beginning until it finds the desired information. •The most common sequential access storage device is magnetic tape where data is stored and processed sequentially. Direct Access: •Also known as random access, means that the computer can go directly to the information that the user wants. •The most common direct access storage are magnetic and optical disks.

MAGNETIC TAPE Magnetic tape looks similar to the tape used in music cassettes. The data is stored in the form of tiny segment of magnetised and de-magnetised portion on the surface of the material. The tape is divided into vertical column (frames) and horizontal rows (tracks).

MAGNETIC DISK Magnetic disks are the widely used popular storage medium for direct access secondary storage. The surface of a disk is divided into imaginary tracks and sectors. Tracks are concentric circles where the data is stored, and are numbered from the outermost to the innermost ring, starting with zero. Data is represented as magnetised spots on a disk. A magnetised spot represents a 1 (bit) and the absence of a magnetised spot represents a 0 (bit).

Types of Magnetic Disk Floppy Disk: It is a removable, round, flat piece of mylar plastic, coated with ferric oxide and encased in a protective plastic cover. Hard Disk: It is the primary storage unit of the computer. It consists of a stack of disk platters that are made up of aluminium alloy or glass substrate coated with a magnetic material and protective layers. Zip Disk: It is a removal storage device whose disk is made up of plastic material on which magnetic oxide particles are coated.

OPTICAL DISK •An optical disk is a flat, circular, plastic disk coated with material on which bits may be stored in the form of highly reflective areas and significantly less reflective areas, from which the stored data may be read when illuminated with a narrow-beam source, such as a laser diode. •An optical disk consists of single long track in the form of spiral shape. •This track starts from the outer edge and spirals inward to the centre of the disk. •Data can be written, but cannot be erased from an optical disk.

Input Output Devices •An input device is an electromechanical device that allows the user to feed information into the computer for analysis, storage, and to give commands to the computer. •After processing the input data, the computer provides the results with the help of output devices. •An output device converts machine-readable information into human-readable form.

Keyboard A keyboard is the most common data entry device that allows the user to enter data into a computer by simply pressing keys.

Pointing Devices A pointing device is used to communicate with the computer by pointing to locations on the monitor screen. Such devices do not require keying of characters, instead the user can move a cursor on the screen and perform move, click or drag operations. Some of the commonly used pointing devices are: Mouse: Mouse is a small hand-held pointing device, which is rectangular-shaped with a rubber ball embedded at its lower side and buttons on the top. Usually a mouse contains two or three buttons, which can be used to input commands or information.

Trackball: Trackball resembles a ball nestled in a square cradle and serves as an alternative to a mouse. In general, a trackball is as if a mouse is turned upside down. It has a ball, which can be rotated by fingers in any direction, the cursor moves accordingly.

Joystick: Joystick is a device that moves in all directions and controls the movement of the cursor. The joystick offers three types of control: digital, glide, and direct.

Light Pen: Light pen (sometimes called a mouse pen) is a handheld electro-optical pointing device which when touched to or aimed closely at a connected computer monitor, will allow the computer to determine where on that screen the pen is aimed.

Touch Screen: Touch Screen is a special kind of display screen device, which is placed on the computer monitor in order to allow the direct selection or activation of the computer when somebody touches the screen.