Data Organization and Formatting.

External Memory

The goal of the lecture : analyze and study types of external memory, data organization and formatting, RAID (Six levels of RAID), Optical memory.

Contents

1. Types of external memory. Data organization and formatting.

2. RAID (Six levels of RAID).

3. Optical memory.

Literature.

1. Stallings W. Computer Organization and Architecture. Designing and performance, 5^th ed. ? Upper Saddle River, NJ : Prentice Hall, 2002.

2. V. Carl Hamacher, Zvonko G. Vranesic, Safwat G. Zaky. Computer organization,4^th ed. ? McGRAW-HILL INTERNATIONAL EDITIONS, 1996.

Tanenbaum, A.S. Structured Computer Organization, 4^th ed. - Upper Saddle River, NJ : Prentice Hall, 2002

Types of External Memory

à Magnetic Disk

? RAID

? Removable

à Optical

? CD-ROM

? CD-Writable (WORM)

? CD-R/W

? DVD

à Magnetic Tape

Magnetic Disk

à Metal or plastic disk coated with magnetizable material (iron oxide ? rust)

à Range of packaging

? Floppy

? Winchester hard disk

? Removable hard disk

Data Organization and Formatting.

à Concentric rings or tracks (between 500 and 2000 tracks on one side)

à Tracks divided into sectors (data are read by blocks = sectors; there can be between 10 and 100 sectors)

à Adjacent tracks are separated by gaps. This prevents errors due to misalignment (???????????) of the head (conducting coil).

à

Gap

ID Field

Gap

Data Field

Gap

Gap

ID Field

Gap

Data Field

Gap

Gap

ID Field

Gap

Data Field

Gap

		600 Bytes/Sector

Synch Byte

Track #

Head #

Sector #

CRC

Data are transferred to and from the disk in blocks, accordingly data are stored in block-size regions known as sectors. To avoid imposing unreasonable precision requirements the adjacent sectors are separated by intra?track(record) gaps.

In order to identify positions within a track there are must be starting points on the tracks and ways for identification the start and the end of each sector. These requirements are handled by means of control data recorded on the disk. Thus, the disk is formatted with some extra data used only by the disk driver, and they are not accessible to the user.

In Fig. Winchester Disk Track Format each track contains 30 fixed-length sectors of 600 bytes each. Every sector holds 512 bytes data, plus control information useful to the disk controller.

The ID field is a unique identifier or address used to locate a particular sector.

The SYNCH byte is special bit pattern that determines the beginning of the field.

The track number identifies a head, since this disk has multiple surfaces.

The ID and data fields contain an error-detecting code (CRC).

Date: 2016-06-13; view: 9