Introduction to Memory: Physical devices used to store programs (sequences of instructions) or data on a temporary or permanent basis for use in a computer. Memory-Data stored in the form of chips. Storage-Data stored in tapes or disks.

Memory hierarchy:

The memory is characterized on the basis of two key factors— capacity and access time. A ' memory hierarchy' in computer storage distinguishes each level in the ' hierarchy' by response time, complexity, and capacity.

Primary Storage Devices

Random Access Memory consists of integrated circuit(IC) either on the motherboard or small circuit board attached to motherboard. Memory enhanced easily by adding memory chips. It is Volatile memory that loses its contents when the computer or hardware device loses power.

Volatile memory is memory that loses its contents when the computer or hardware device loses power. Computer RAM is a good example of a volatile memory. A computer's main memory, which is made up of dynamic RAM or static RAM chips

Non-volatile memory, sometimes abbreviated as NVRAM. Memory that keeps its contents even if the power is lost. CMOS is a good example of a non-volatile memory. Example of non volatile memory is the ROM (read only memory), and the flash memory (PEN DRIVE).

Read Only Memory. Non-volatile memory chip in which data stored permanently and cannot be altered. Burning in the data-Data are stored permanently using fuse links (Once burnt it is permanent). Non-Volatile memory cannot lose its contents when the computer or hardware device loses power. Synonyms- Field stores, Permanent stores or Dead stores.

Computer Science 2210 Memory Notes

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Programmable Read-Only Memory. That can be modified once by a user. A form of digital memory where programming is applied after the device is constructed. They are manufactured blank and, depending on the technology, can be programmed in system. This machine supplies an electrical current to specific cells in the ROM this process is known as burning the PROM. Ex: Video games, mobile phones, radio-frequency identification.

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Erasable Programmable Read Only Memory. Enable to remove information or data from EPROM Chips and also reprogrammed to store new information. They are often used by Experimenters to test efficiency of computers through small micro programs.

Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. Flash memory is a flexible type of storage. Flash memory technology is a mix of EPROM and EEPROM technologies. Ex: Personal computers, digital cameras, mobile phone, Pen Drives etc.

Primary storage (or main memory or internal memory) is the only one directly accessible to the CPU. Physical memory which is internal to the computer. Every file and program that is accessed or executed is stored or copied in the main memory. The CPU continuously reads instructions stored there and executes them as required. RAM used for primary storage is also volatile.

Secondary storage (also known as external memory or auxiliary storage) not directly accessible by the CPU. Larger capacity but whose access time is slower than that of the main memory. This memory is external to the CPU the computer usually uses its input/output channels to access secondary storage and transfers the desired data.

Built into a CPU or located next to it on a separate chip. The CPU uses cache memory to store instructions that are repeatedly required to run programs, improving overall system speed. Cache memory is random access memory (RAM) which can be accessed more quickly by microprocessor. The advantage of cache memory is that the CPU does not have to use the motherboard’s system bus for data transfer, CPU direct communicate with cache. A collection of data duplicating original values stored elsewhere on a computer.

Secondary Storage Devices:

Secondary Storage Devices: Magnetic Tape, Magnetic Disk, Floppy Disk, Hard Disk, CD DVD, Magneto-optical

Magnetic tape data storage uses digital recording on to magnetic tape to store digital information. A sequential storage medium used for data collection, backup and archiving. Like videotape. Modern magnetic tape is most commonly packaged in cartridges and cassettes. Plastic ribbon usually ½ inch or ¼ inch wide and 50 to 2400 feet long. It consists of a thin film of magnetic oxide material bonded to a polyester-based strip. Magnetic tape was first used to record computer data in 1951 UNIVAC I. The major drawback of tape is its sequential format. Locating a specific record requires reading every record.

Types of Magnetic tape: ½ inch tape reel, ½ inch tape cartridge, ¼ inch streamer tape, 4 mm digital audio tape

Magnetic tape Access method: Magnetic storage media can be classified as either sequential access memory or random or serial access. The access time can be defined as the average time needed to gain access to stored records. In the case of magnetic wire, the read/write head only covers a very small part of the recording surface at any given time they have many parallel tracks across the width of the media and the read/write heads take time to switch between tracks and to scan within tracks. Different spots on the storage media take different amounts of time to access. They take as much as 100 s.

Magnetic Disk: Bits of data (0’s and 1’s) are stored on circular magnetic platters called disks. A disk rotates rapidly (& never stops). A disk head reads and writes bits of data as they pass under the head. Often, several platters are organized into a disk pack (or disk drive). Disk contains concentric tracks. Tracks are divided into sectors A sector is the smallest addressable unit in a disk.

Storage has major implications: READ: transfer data from disk to main memory (RAM). WRITE: transfer data from RAM to disk.

Floppy Disk: It is a flat, circular piece of flexible plastic coated with magnetic oxide and also known as diskettes. Drive- Floppy disk drive used to read/write data from/to floppy disk. Enables easy loading/unloading of floppy disks by inserting the disk inside or pressing the eject button.

3 ½ inch Floppy Disk: 3 ½ inch diameter disk enclosed in a square high jacket plastic cover. Jacket cover having cut for read/write head to make contact with disk. They are double sided which record data on both disk surfaces. Capacity: Double Density, High Density and Very High Density.

Hard Disk: Device for storing and retrieving digital information, primarily computer data. It consists of one or more hard rapidly rotating discs (platters) coated with magnetic material, and with magnetic heads arranged to write data to the surfaces and read it from them. A typical hard disk drive has two electric motors; a disk motor that spins the disks and an actuator (motor) that positions the read/write head assembly across the spinning disks. A typical HDD design consists of a spindle that holds flat circular disks, also called platters, which hold the recorded data. The platters are made from a non-magnetic material, usually glass, or ceramic, and are coated with a shallow layer of magnetic material.

CD: Compact disc that contains data accessible to, but not writable by a computer for data storage and music playback. Discs are made from a 1.2 mm thick disc of polycarbonate plastic, with a thin layer of aluminum to make a reflective surface. The most common size of CD-ROM disc is 120 mm in diameter, though the smaller Mini CD standard with an 80 mm diameter. A laser is shone onto the reflective surface of the disc to read the pattern of pits and lands ("pits", with the gaps between them referred to as "lands"). A CD-ROM sector contains 2,352 bytes, divided into 98 24-byte frames. Several formats are used for data stored on compact discs, known as the Rainbow Books. Recordable (CD-R) and rewritable (CD-RW) discs are manufactured by a different method, whereby the data are recorded on them by a laser changing the properties of a dye material in a process that is often referred to as "burning".

DVD: DVD is an optical disc storage format, invented and developed by Philips, Sony, Toshiba and Panasonic in 1997. DVDs offer higher storage capacity than Compact Discs while having the same dimensions. DVD is an optical disc technology with a 4.7 gigabyte storage capacity on a single-sided, which is enough for a 133-minute movie. DVDs can be single- or double-sided (two-layered DVD) will hold up to 17 gigabytes of video, audio, or other information. Blank recordable DVD discs (DVD-R) can be recorded once using a DVD recorder and then function as a DVD-ROM. Rewritable DVDs (DVD-RW) can be recorded and erased multiple times.

CD/DVD Access time: Optical drives like CD and DVD are relatively slow and can demand up to several hundred milliseconds to access information. In fact, because of the massive amount of data and the greater density, DVDs are actually slower than ordinary CD-ROMs. For example, access time for a normal CD is 180ms, while currently available DVD kits sport access times that range between 200 and 470ms.

DVD/CD Access Method: DVD/CD disc is divided into tracks and data are stored in a spiral trail of tiny pits on a reflective metal surface embedded in plastic that correspond to the zeros and ones of the digital information contained within the tracks. Laser beam follows the spiral, reads the information from the pits, and then plays back that information.

Magneto-Optical: A magneto-optical drive is a kind of optical disc drive capable of writing and rewriting data upon a magneto-optical disc. The magneto-optical (MO) drive is a popular way to back up files on a personal computer. MO device employs both magnetic and optical technologies to obtain ultra-high data density. The technology was introduced commercially in 1985. Ex: Fujitsu Dynamo, PowerPC Apple PowerBook, a note book computer etc.

Magneto-Optical Access Method: An MO system achieves its high data density by using a laser and a magnetic read/write head in combination. Both the laser and the magnet are used to write data onto the disk. The laser heats up the disk surface so it can be easily magnetized, and also to allow the region of magnetization to be precisely located and confined. A less intense laser is used to read data from the disk. Data can be erased and/or overwritten an unlimited number of times.