Data Storage Technology By Maika

Introduction A storage device consists of a read/write mechanism and a storage medium. The storage medium is the device or substance that actually holds data. The read/write mechanism is the device used to read or write data to and from the storage medium.

A typical computer system has many storage devices Primary storage: Static RAM (SRAM) Dynamic RAM (DRAM) Flash RAM ROM Secondary storage: Magnetic tape Magnetic disk Optical disk Multiple devices are required because no one device or technology can cost-effectively meet all storage needs for a single computer system or user.

Primary Characteristics Speed Volatility Access method Portability Cost and capacity

Speed Is the most important characteristic that differentiate primary and secondary storage With current technology, primary storage speed is typically greater than secondary storage speed by a factor of 105 or more.

Storage device speed is called access time. Is the time required to execute on read or write operation completely. Access time of primary storage devices generally are expressed in nanoseconds. Access time of secondary storage devices typically are expressed in milliseconds.

A complete measure of data access speed consists of access time and the unit of data transfer to/from the storage device. Data transfer units vary from one storage device to another: The data transfer unit for primary storage devices is usually a word. The data transfer unit for secondary storage devices is block. The date transfer unit for magnetic and optical disk drives is sector.

A storage device’s data transfer rate is computed by dividing 1 by the access time and multiplying the result by the unit of data transfer. Eg. The data transfer rate for a primary storage device with 15 nanosecond access time and a 32-bit word data transfer unit can be computed as: (1second / 15 nanoseconds) * 32 bits = 266,666,667 (bytes/second)

Volatility A storage device or medium is non-volatile if it holds data without loss over long periods of time. A storage device or medium is volatile if it cannot reliably hold data for long periods. Primary storage devices are generally volatile. Secondary storage devices are generally non-volatile.

Access Method The physical structure of a storage device’s read/write mechanism and storage medium determines the way(s) in which data can be accessed. Three types of access methods: Serial access Radom access Parallel access

Serial Access A serial access storage device stores and retrieves data items in a linear or sequential order. Once written, data can be read back only in that same order. Serial access time depends on the current position of the read/write mechanism and on the position of the desired date item within the storage medium. Serial access devices are not used for frequently accessed data but for holding backup copies of data stored on other storage devices.

Random Access A random access device is not restricted to any specific order when accessing data but directly access any desired data item stored on the storage medium. All primary storage devices and disk storage devices are random access devices. Access time may or may not be a constant. It is a constant for most primary storage devices. It is not a constant for disk storage.

Parallel Access A parallel access device is capable of simultaneously accessing multiple storage locations. If one considers the unit of data access to be a bit, then access is parallel. Parallel access also can be implemented by subdividing data items and storing the component pieces on multiple storage devices.

Portability Data can be made portable by storing it on a removable storage medium or device. External disk drives are portable if they can be added or removed easily from a computer system and transported without damage to the device or its data content. Portable devices typically sacrifice access speed to compensate for the lack of environmental control.

Cost and Capacity Each of the storage device attributes is directly related to device cost Characteristics Cost Speed Volatility Access method Portability Capacity Cost increases as speed increases For devices of similar type, cost decreases as volatility increases Serial is the least expensive, Random is more expensive than serial, Parallel is more expensive than non-parallel For devices of similar type, portability increases cost Cost usually increases in direct proportion to capacity

Primary Storage Devices Storing Electrical Signals Data are represented within the CPU as digital electrical signals – the basis of data transmission among all devices attached to the system bus Electrical power can be stored directly by various devices including batteries and capacitors An electrical signal can be stored indirectly by using its energy to alter the state of a device such as a mechanical switch

Random Access Memory (RAM) RAM is a generic term describing primary storage devices with the following characteristics: Microchip implementation using semiconductors Ability to read and write with equal speed Random access to stored bytes, words, or larger data units There are two basic RAM types Static RAM (SRAM) Dynamic RAM (DRAM)

Static RAM Is implemented entirely with transistors The basic unit of storage is a flip-flop circuit A flip-flop circuit which contains two transistors for bit storage is an electrical switch that remembers its last position SRAM is volatile unless a continuous supply of power can be guaranteed.

Dynamic RAM Uses transistors and capacitors. The capacitors are the dynamic element DRAM chips include circuitry that automatically performs refresh operations Each refresh operation is called a refresh cycle

SRAM is more expensive to fabricate than DRAM due to its more complex circuitry. DRAM is slower than SRAM due to its required refresh cycles. Neither RAM type can match current microprocessor clock rates which range from 500MHz to 1.5GHz. The fastest DRAM is at least 25 times slower than modern microprocessors. SRAM is at least 2.5 times slower.

Read-Only Memory (ROM) Is a random access memory device that can store data permanently or semipermanently. Is used to stored programs such as computer system boot subroutines like the system BIOS. Two ROM technologies are currently in use are: Electrically Erasable Programmable Read-Only Memory (EEPROM) and Flash Memory.

EEPROM can be programmed, erased, and reprogrammed by signals sent from an external control source, such as a CPU. The primary drawback of EEPROM is that read and erase operations require relatively high voltages and repeated cycles to cause permanent change. Flash Memory can be erased and rewritten much more quickly and is used for programs and data that aren’t frequently updated. It tends to wear out after 100,000 or more write operations, making it unsuitable for primary storage.

Memory Packaging Memory circuits are embedded within microchips and groups of chips are packed on a small circuit board that can be installed or removed easily from a computer system. Early RAM and ROM circuits were packaged in dual in-line packaged (DIPs) Later, single in-line memory module (SIMM) is adopted which incorporated multiple DIPs on a tiny printed circuit board. The double in-line memory module (DIMM) is a newer packaging standard. It is a double-sided SIMM with memory DIPs and electrical contacts on both sides of the module.

CPU Memory Access The physical organization of memory, The organization of programs and data within memory, And the method(s) of referencing specific memory locations are critical design issues for both primary and secondary storage devices and processors.

Physical Memory Organization The main memory can be regarded as a sequence of contiguous or adjacent memory cells. Cell 0 Cell 1 Cell 2 Cell 3 Cell 4 In a byte sequence, the leftmost byte is called the most significant byte and the rightmost byte is called the least significant byte.

Big endian describes architectures that store the most significant byte at the lowest memory address. Little endian describes architecture that store the least significant byte at the lowest memory address. The addressable memory of a CPU is the highest numbered storage byte that can be represented. Addressable memory is determined by the number of bits used to represent an address. Physical memory is the actual number of memory bytes that physically are installed in the machine. Physical memory is usually less than addressable memory.

Memory Allocation and Addressing Memory allocation describes the assignment of specific memory address to system software, application programs, and data. Unused Program data Program code Operating system FFFFH High memory 3000H Program offset Low memory 0000H

Absolute addressing describes memory address operands that refer to actual physical memory locations. Usually, the operating system calculates and stores the program offset in a register when the program first is loaded into memory. During program execution, the CPU automatically adds the program offset to all memory address operands before accessing memory. The method of automatically computing physical memory addresses is called indirect addressing. The register that holds the offset value is called offset register.

Magnetic Storage A magnetic storage device converts electrical signals into magnetic charges, captures the magnetic charge on a storage medium, and later uses the stored magnetic charge to generate an electrical current. Magnetic storage devices must control or compensate for some undesirable characteristics of magnetism and magnetic storage media

Magnetic Decay and Leakage Is the tendency of magnetically charged particles to lose their charge over time. Is constant over time and proportional to the power of the charge. Will cause the stored charge power to fall below the threshold required for a successful read operation  the data content of the storage medium effectively is lost. Magnetic leakage: The strength of individual bit charges also can decrease due to magnetic leakage from adjacent bits.

Storage Density Coercivity is the ability of a substance or magnetic storage medium to accept and hold magnetic charge. It varies widely among elements and compounds. The amount of surface area allocated to a bit is referred to as the recording density, which is expressed in bits, bytes, or tracks per inch. TIP: Designers and purchasers of magnetic media and devices must find a suitable balance between high recording density and the reliability of the media, especially over relatively long periods of time.

Media Integrity The integrity of magnetic storage medium depends on the nature of its construction and the environmental factors to which it is subjected. Loss of coercible coating represent a loss of strength in stored magnetic charges  data becomes unreadable when the remaining charge falls below the threshold of readability. TIP: To extent the life of magnetic media, they must be protected from physical abuse and temperature and humidity extremes

Magnetic Tape Is a ribbon of plastic with a coercible surface coating. Tapes are mounted in a tape drive for reading and writing. Tape drive contains motors that wind and unwind the tape. Tapes primarily are used to make back up copies of data stored on faster secondary storage devices and to physically transport large data sets.

There are two basic approaches to record data onto a tape surface: Linear recording places bits along parallel tracks that run along the entire length of the tape. Helical scanning reads and writes data to or from a tape by rotating the read/write head at an angle to the tape and moving from tape edge to tape edge.

Magnetic disk Magnetic disk media are flat, circular platters with metallic coating that are rotated beneath read/write heads. Data normally are recorded on both sides of a platter. A track is one concentric circle of a platter. A sector is a fractional portion of a track. A read/write head is placed for each side of each platter. Read/write heads are mounted on the end of an access arm. Access arms are attached to a positioning servo. The servo moves the access arms so that the read/write heads can be positioned anywhere between the outermost track and the innermost track.

Hard disk: Is a magnetic disk media with a rigid metal base. Typical platter size is between three and five inches in diameter. High speeds is up to 10,000 revolutions per minute. Drive capacity depends on the number of platters, platter size, and recording density. Multiple hard drives often are enclosed in a single storage cabinet and referred to as a drive array.

Floppy disk (diskette): Uses a base of flexible or rigid plastic material. Rang from 2.5 to 3.5 inches in diameter. Contains a single flexible plastic platter coated with an iron or other metallic compound on both sides. The platter is mounted permanently in a plastic case that can be removed from a diskette drive. The case has an access door that is closed when the diskette is removed from the drive and opened automatically when the diskette is inserted into the derive.

Disk access time depends on several factors including: Time required to switch among read/write heads Time required to position the read/write heads Rotational delay – the time that the disk controller must wait for the proper sector to rotate beneath the heads

Optical Mass Storage Devices Came of age in the 1990s Primary advantages: higher recording density and longer data life Can retain data for decades Not subject to problems of magnetic decay and leakage Popular due to standardized and relatively inexpensive storage media. Store bit values as variations in light reflection

Optical storage devices are available currently in a wide variety of storage formats and write technologies including: CD-ROM WORM (CD-R) Magneto-optical CD-RW DVD