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secondary storage
not directly accessible by processor, must be copied into RAM first, nonvolatile, slower access speed
magnetic data storage
electromagnets create fields and set tiny magnetized dots to N or S on the magnetic surface of the disc which represent binary 1s and 0s
HDD
hard disk drives. circular platters coated in magnetic material, stored in concentric tracks and sectors making data blocks. several read write heads move in and out of platters to find required data block
magnetic tape drives
used for backups and archiving, data stored in series along a tape which loops around two wheels in a casette.
solid state storage
NAND based integrated circuits use millions of floating gate transistors to stores data as 1s and 0s. (cheaper and most common) NOR used in EEPROM(expensive)
SSD
solid state drive. a grid of electrical cells are used to quickly send and recieve data. the grids are seperated into pages which are clumped together into blocks. it stores data by flashing it onto the chips.
hhd cost
cheaper per unit of data stored
hhd latency
slower than ssd, as read write head has to search for correct block
hdd robustness
as it has moving parts it is fragile. whereas SSD is shock resistant and has no moving parts
hdd longevity
greater longevity over time for read write functions. ssd have limited number
hdd power
uses more power and is much hotter
hdd weight
hdd is heavier and bigger, ssd is lighter and portable and small
where is hdd often used
web servers
virtual memory
when computers are doing heavy processing
they run out of space in RAM
so the operating system sections of some secondary storage and temporarily uses it to store data
The data it moves to virtual memory is data that was sitting in RAM not being used
When it is needed again it is copied back into RAM
offline memory
non volatile, not fixed within the computer, it is portable and can be removed easily
optical media
uses polycarbonate,plastic disks coated with aluminium. data is burned by lasers into pits and lands on the tack representing 1s and 0s. to read data the lasers are shone onto the pits and lands, where land bounces back differently, the optical device can read the binary from reflected light
how to store more data on optical disk
shorter laser wavelength so smaller pits and lands so more pits and lands, smaller distance between spiral tracks on disk
pros of optical disks
good data transfer rate, cheap
cons of optical disks
reflective surface easily scratched and damaged, optical media require a separate drive, data has to be burnt/ finalized before being used on another device
CD
compact disk, slowest data transfer rate, red 780nm lase, single layer disk, 80mins of music 6 to 700MB
DVD
digital versatile disk, medium data transfer rate, red laser 650nm, dual layer disk (birefringence giving read errors), 8 to 9GB 1.5 hours of video + sound
Blu Ray
fastest data transfer rate, blue laser 450nm, more layer disks, 25GB, 2.5 video+sound at very high quality, uses built in encryption
DVD RAM
data stored in concentric tracks and sectors so direct access to data. read and write operations can be done at the same time, so users can watch a recorded programme at same time as recording another - tv box
flash memory
sd cards/usb sticks, uses solid state technology, connect through ports, cheap and portable, power efficient and little heat but easily lost and broken