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Computing Academy GCSE Computer Science

Computers use several different types of memory to ensure that they don’t forget all of this data. Permanent Storage / Secondary Storage Hard Disk Drives (HDDs) and Solid State Drives (SSDs) are used in modern computers as permanent, secondary or persistent storage. When you save a piece of work or copy photos from your digital camera to your computer they will be stored on a HDD or SSD. HDD A HDD is a stacked set of magnetic platters or plates. These platters spin up to 10,000 revolutions per minute (rpm), and mechanical arms seek backward and forwards across the spinning platter in order to read or write data to a particular sector of the disk. Data is stored to a HDD by manipulating and adjusting the magnetic charge into one of two directions. Each direction represents 0 (off) or 1 (on). HDDs are a type of secondary or permanent storage. They are persistent which means that the data will remain intact even when the power is turned off. In 1956 when IBM invented the first HDD it cost around $65,000 and stored about 5MB. Today, £100 will buy you a 5TB HDD (which is 5,242,880MB). 27

As well as making it very affordable to store large amount of data, HDDs are fairly reliable. Often, when HDDs fail it is either because the disk has been corrupted due to interference from other magnetic sources (such as speakers), or because the mechanical parts have failed. Advantages Disadvantages Large Capacity Relies on moving parts Faster than optical disks e.g. DVDs Disk surface can become damaged Persistent storage Heavy power consumption Easily replaced / upgraded Noisy Slower read / write speed than RAM SSD A SSD can be used as an alternative to a HDD. Rather than using spinning platters and mechanical arms there are no moving parts in SSDs. SSDs use microchips to store data using electricity rather than magnetism. Special transistors are used which can retain their 1 (on) or 0 (off) state even when there is no power. SSDs have very fast read / write speeds as there is no need to wait for a mechanical disk to spin and an arm to move into position. As there are no moving parts SSDs use much less power than HDDs so they are often used in laptop computers in order to extend battery life. As of 2013, SSDs are still a relatively new technology and as with all new technologies, when they first emerge the price is extremely high. Due to the high costs involved, the size of a SSD in a laptop is usually only around 300GB, compared with laptops which use HDDs which can be over 2TB. 28

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