1 3 Computing System Fundamentals 3.2 Computer Architecture

3.2.6 Secondary Storage

3 Storage It is necessary to keep data which is not needed in primary memory all of the time, and which may indeed be too large to fit into the primary memory. Both programs and data are held in secondary memory (or storage).

4 Types of storage media The medium (pl. media) is the physical material the data are kept on. Types include: ‣ magnetic tape e.g. DAT ‣ magnetic disk e.g. floppy disk ‣ optical discs e.g. compact disc (CD-ROM, CD-R, CD-RW), DVD, Blu-ray, ‣ solid-state e.g. flash drives (memory sticks)

5 Magnetic tape Often used for backing up large volumes of data. Used to be reel-to-reel 1" (25 mm) wide. Modern versions are narrower e.g. Digital Linear Tape (DLT) can store about 100GB.

6 Tape cartridges About twice the size of a standard tape cassette Tapes are slow to access but can be removed and stored somewhere physically secure. They use sequential access

7 Disk or disc? Magnetic disks (floppy, hard drive) were invented in America and so generally take the American spelling. Optical discs (compact disc, digital versatile disc) were invented in Europe and take the British spelling. Compare this with computer program vs. television programme.

8 Magnetic disks Faster to access (they use direct access) A spinning disk coated in a layer that can be magnetised by a read/write head. The different directions of magnetism represent the 0's and 1's.

9 Floppy disks Slowest, as they spin (about 6 rps) only when being accessed. Can contain 1.44MB (equivalent to a novel). Removable and cheap.

10 Hard disks Permanently enclosed in their drive mechanism (the read/write heads are only a fraction of a mm away from the disk). They spin constantly (about 250 rps +) and comprise a stack of metal or ceramic platters that have magnetic coating on both sides.

11 Hard disks Capacity is typically 500GB - 2TB for a microcomputer, usually several TB for a server. Faster to access than floppy disks but not usually portable.

12 Optical discs Do not use a magnetic surface but encode the data onto a reflective surface which is then read by a laser. Types: ‣ CD-ROMs (up to 720MB), ‣ DVD ROMs (up to 17GB) and ‣ Blu-ray (up to 50GB).

13 Optical discs CD/DVD-R and -RW can be written to (R once, RW multiple times). Example uses are audio, video and file backup. Resistant to damage. Cheap and portable but comparatively slow to access compared to other types of memory. Use random access.

14 Storage methods Not to be confused with media, this refers to how the data are organised. Three main ways: ‣ serial access, ‣ sequential access and ‣ direct (or random) access.

15 Serial access Data are written and accessed one record at a time, one after the other with no particular regard to order. Magnetic tape can be a serial access medium: to find a given record, you must first pass by all of the preceding ones checking each in turn (very slow). Example: backup or a payroll program.

16 Sequential access The data are kept in a specified order according to a certain key field e.g. alphabetical order by surname. The computer will still have to go through the previous records but access is quicker as the computer can estimate where a record is and jump there.

17 Direct (random) access Going directly to the location of the data required, giving fast storage and access. Possible with discs and flash memory. Records in a random access file can be stored anywhere, so an addressing system needed (e.g. an index table or a hashing calculation). Example: supermarket database (needs to serve the checkouts quickly).