Storage Devices

Storage Storage refers collectively to all the various media on which a computer system can store software and data. Storage is also called mass storage and auxiliary storage.

Storage cont. The difference between a storage device and memory is: Storage devices are like file drawers. Memory is a temporary workspace, like a desk.

Why is storage necessary? Storage retains data when the current is switched off. The computer’s random access memory (RAM) is volatile (when you switch off the computer, all the information in RAM is irretrievably lost). Storage devices are nonvolatile (they don’t lose data when the power is off). Storage is cheaper than memory. RAM is designed to operate very quickly so it can keep up with the computer’s CPU.

How is Storage Capacity Measured? Data stored as a stream of bits 1 byte = 8 bits (usually one ASCII character) 1 kilobyte (KB) = 1024 bytes 1 megabyte (MB) = 1024 kilobytes 1 gigabyte (GB) = 1024 megabytes 1 terabyte (TB) = 1024 gigabytes

Types of storage devices Read/write media enables the computer to perform writing (output) operations as well as reading (input) operations. CD-ROM drives are read-only. Read-only can not perform writing operations. CD-R drives are read/write media.

Types of storage devices Cont. Sequential storage devices have to go through a fixed sequence of stored items to get to the one that’s needed, like a cassette tape. Sequential storage devices are slow but inexpensive. Random-access storage devices can go directly to the requested data without having to go through a sequence. This device is faster but more expensive.

Types of storage devices Cont. Magnetic storage media uses disks or tapes that are coated with tiny, magnetically sensitive materials. In all magnetic storage devices, an electromagnet called a read/write head, records information by transforming electrical impulses into a varying magnetic field. Optical storage media use tightly focused laser beams to read microscopic patterns of data encoded on the surface of plastic disks.

Floppy Disk Media type: magnetic on flexible mylar plastic Capacity: 1.44MB Speed: slow Rewritable Random access Removable disks Floppy disks are designed to work with a floppy disk drive. As computer programs and user’s data files have grown significantly in size, floppies are becoming less useful. Their capacity is so small they cannot accommodate some of the larger files that people create.

Hard Disk Media type: magnetic on hard platters made of glass or aluminum Capacity: typically up to 120GB Speed: fastest: 10,000 RPMs, 10msec access, 100MB/sec Reliability: very reliable as compared to floppies, not typically used for long-term storage, device failure most frequent cause of loss Rewritable Random access

Magnetic Tape Media type: magnetic on rolls of tape Capacity: highest, up to 200GB per tape! Speed: slowest: sequential access, 3MB/sec throughput Reliability: most often used for long-term storage and backups (tape archives). Rewritable Sequential access

CD-ROM Media type: optical on a CD The most popular and least expensive type of optical disc standard is CD-ROM, short for compact disc-read only memory. CD-ROM discs are capable of storing up to 650 MB of data, the equivalent of over 400 floppy disks. Random access Removable disks

CD-R vs. CD-RW CD-R is a “write-once” technology. After you’ve saved data on the disc, you can’t erase it or write over it. CD-RW drives provide full read/write capabilities using erasable CD-RW discs, which are more expensive than CD-R discs.

DVD Media type: optical on DVD Capacity: up to 18GB, double-sided/double-layered NOT rewritable Random access Removable disks