CY1001 Introduction to Computer Systems Storage Devices Lecture objectives: Introduce types of memory and data storage systems.

The (very) basic architecture Input Output Auxiliary (secondary) Storage Arithmetic Unit Main (primary) Memory

Overview

Data storage Analogue signals are continuous wave forms, vary in frequency and amplitude to represent information from sound and data. Digital signals are described using two states: on (1) or off (0). Computers are digital and require digital data.

Memory - overview To be able to run a program, a computer must first have the program in its memory. Main memory consists of memory chips, which are mounted within the computer itself. Secondary storage is used for long-term storage of computer programs and data.

Memory - Cache Cache memory is used to facilitate faster transfer of instructions and data to processor. Cache increases the rate at which work can be performed by a computer. It is a high speed holding area for program instructions and data holds instructions and data most likely to be needed next by processor to reduce access to slower RAM.

Main Memory – Primary storage RAM (Random Access Memory) is a read-and-write memory. It is electronic circuitry and has no moving parts, so access is at electronically fast speed. 128 – 512 MB  1 GB

Main Memory – Primary storage Common RAM technologies are: Dynamic RAM (DRAM), slow, must refresh Static RAM (SRAM), faster, no refresh needed, but more expensive, needs more space and power Enhanced Data Output (EDO) RAM, 50% faster than DRAM Synchronous dynamic RAM (SDRAM) enables data transfers at more than twice the speed of previous RAM technologies (standard memory on most new PCs).  

Buses Buses are used to transport electrical signals (bits) from one place to another. There are two types of buses: address bus data bus Source and destination addresses are sent over address bus to access memory locations data and instructions are transferred using data bus.

Secondary storage and files Information is stored permanently in Secondary Storage and retrieved in milliseconds. Within a computer system, programs and information (text, image, audio, video) are retrieved from secondary storage. stored temporarily in RAM (primary storage) for processing.

Secondary storage and files

Secondary storage and files A file is simply a recording of information. Everything we do on a computer involves a file and therefore secondary storage. The operating system manages where a file is stored in secondary memory.

Data access – type of secondary storage Two methods for storing and accessing data which are sequential and random. Sequential Processing the computer searches the storage medium from the beginning to find the desired programs, data, or files. Magnetic tapes can be used for sequential access only. Random Processing the desired programs, data, and files are accessed directly from the storage medium. Magnetic disks have random or direct-access capabilities Also allow sequential access

Magnetic disks Two types of magnetic disks which are: Interchangeable Magnetic Disks Can be stored offline and loaded to the magnetic disk drives as they are needed Fixed Magnetic Disks (hard disks) which are permanently installed, or fixed. These are rigid and usually made of aluminum with a surface coating of easily magnetized elements, e.g. iron, cobalt, nickel.

Interchangeable disks Virtually all PCs today are configured with at least one hard disk drive and one interchangeable disk drive. There are three types of interchangeable disk drives are commonly used on PCs

Interchangeable disks 3 1/2-inch Floppy Diskettes This is a thin disk, permanently enclosed in a rigid plastic jacket and comes in two capacities - 720 KB or 1.44 MB. SuperDisk It combines hard and floppy disk technology and can store 120 MB. It has the same size as floppy disks but different disk densities. Zip Drive & Zip Disk the Zip drive also combines floppy and hard disk technology to read and write up to 100-MB Zip disks. Its storage capacity equivalent to 70 floppy diskettes.

Hard disks Hard Disk manufacturers are working to put more information in less disk space and to enable a more rapid transfer of information to/from RAM. There are two types of hard disks: Permanently installed Interchangeable hard disks

Magnetic disks

Hard disks Permanently installed Storage capacities of 2 to 120 GB (~85,000 floppy disks!). It contains several disk platters stacked on a single rotating spindle. Data stored on all recording surfaces. Disks spin continuously at a high speed within a sealed enclosure which keeps the disk-face surfaces free from contaminants, Allows greater density than interchangeable diskettes.

Hard disks Interchangeable hard disks Can store up to 1 GB of information. It is inserted and removed as easily as the 3 1/2-inch floppy its performance is almost as good as that of a permanently installed hard disk

Magnetic tapes In the 1950s and 60s, the foundation of many information systems was sequential processing using magnetic tape master files. Today, magnetic tape storage is no longer used for routine processing. Once loaded, the magnetic tape is online, i.e. the data on the tape is accessible to the computer system.

Optical laser disks High density storage technology which may make magnetic disk and tape storage obsolete. One laser beam writes to the recording surface by scoring microscopic pits in the disk another laser reads the data from the light-sensitive recording surface. A light beam is easily reflected to the desired place on the optical disk.

Optical laser disks Currently there are three main categories of optical laser disks: CD-ROM and DVD WORM disks Re-writable optical disks

CD-ROM CD-ROM was introduced in 1980. The extraordinarily successful CD, or compact disc, is an optical laser disk designed to enhance the reproduction of recorded music CD-ROM is a spin-off of audio CD technology and stands for Compact Disk-Read-Only Memory. Once inserted into the CD-ROM drive, the text, video images, etc can be read into RAM for processing or display. Data on the disk are fixed, i.e., cannot be altered with up to 680 MB of data can be stored on a CD.

CD-ROM

DVD DVDs (digital videodisks) are poised to replace CD-ROMs. The DVD looks like CD-ROM, but can store from 7 to 14 times as much information (up to 10 GB). It can store the video for a full-length movie. DVD drives are backwards compatible, i.e. they can also play CDs and CD-ROMs

WORM Disks Write Once, read many (WORM) optical laser disks are used by end user companies to store their own proprietary information. In this case, data can only be read, not updated or changed. WORM disk cartridges can store greater volumes of information than a CD-ROM. WORM applications involve image processing or archival storage (e.g. electronic catalogue) and a mainframe-based WORM disk has capacity of 200 GB.

Re-writable Optical Disks Use several technologies, including magneto-optical (MO) technology, to integrate optical and magnetic disk technology to enable read-and-write storage. A 5 1/4-inch re-writable disk cartridge can store up to 5 GB. Usually used for applications using large volumes of storage with little update activity

Summary Memory Cash memory RAM ROM Secondary memory Secondary storage and files Magnetic disks Optical laser disks Cash memory CD-ROM DVD WORM disks Rewritable Optical Disks