Revision for Computer Science test: Storage

Revision for Computer Science test: Storage

Memory and Storage

Primary Memory

Primary memory, also known as main memory, is the only type of memory that could be directly accessed by the microprocessor. It is transistor-based, therefore is faster to do reading and writing. There are two types of primary memory: ROM and RAM.

ROM (Read Only Memory)

ROM or Read Only Memory, is used for system startups. For example, one data it saves is BIOS, Basic Input / Output System. The contents of the ROM chip can only be read and cannot be modified.

RAM (Random Access Memory)

RAM or Random Access Memory, stores the memory of running applications. It can also be used to store data and files. It can write into and read from and the contents of the memory could be changed. The larger the RAM, the faster the computer runs. When RAM gets nearly full, the computer needs to save some of the files at the hard disk drive. When the file is needed, the computer needs to swap from RAM memory to the hard disk drive, then swap back. Swapping needs a lot of time and the computer needs to do a lot of work. Therefore, the computer will be slower and the performance of it will be affected. Its only drawback is that it is temporary and volatile.

When there is more RAM, the computer will run faster.

read-onlyread and write
instructions for startuptemporary files and data used by running applications
data it stored cannot be modifieddata can be modified
CheaperMore expensive

Secondary Storage

Secondary storage is not directly accessible to the computer system. Different from primary memory, secondary storage is auxiliary and is usually used for permanent data storage (as implied in its name, it is storage instead of memory). There are two types of secondary storage: HDD – Hard-Disk Drive and SSD – Solid-state Drive.

HDD (Hard-disk drive)

Hard-disk drive is the most common way to store data on a computer. Data is stored in a digital format on the magnetic surfaces of a disk, also called platters. These platters can spin at a high speed of 7000 times a second. There are also a number of read-write heads that can move from the edge of a disk to the center of the disk for 50 times a second.

sectors and tracks

Data are stored on the surfaces of sectors and tracks. Each sector in a given track stores a given number of bytes.

Hard-disk drives are slow in data access when compared with RAM in primary memory. Many applications need to constantly access data and files so there will be a large amount of head movement, causing latency.

applications not responding is a result of latency

SSD (Solid-state Drive)

SSD solves the problems that exist in Hard-disk drives. It is at least 15 times faster than a hard-disk drive. It doesn’t rely on magnetism and has no moving parts within it. They store data by controlling the movement of electrons on the NAND chips (that sounds amazing but I don’t really know the details of it).

– has moving parts– doesn’t have moving parts, thus more reliable
– very common for computers but heavier– lighter and is more suitable for laptops
– needs time to start the platters and head accelerating– does not need to get up to speed before working properly
– gets hot when running because of the moving parts in it– run much cooler so again more suitable for laptops
– thicker because of the moving parts– thinner since no moving parts
– slower in accessing, reading and writing data– faster in accessing data and no latency
– Unlimited reads and writes– limited reads and writes and doesn’t last long
– Large storage capacity– Smaller storage capacity

Offline Storage

Offline storage is disconnected from the computer and has to be plugged into the computer to be used. Offline storage is portable, contains data that needs to be shared.


CD and DVD disks

CDs and DVDs use optical storage. They have a thin layer of alloy or light-sensitive organic dye to store data. Their tracks, unlike platters in HDD, are single and spiral from the center of the disk to the edge. Data are stored in the pits and bumps on the track, and a read laser is used to read the data. CDs and DVDs can be designed to be R, read only, or RW, which means that it is able to read and write into it for multiple times.

spiral shaped disk

The difference between DVDs and CDs is that DVDs are dual layer and has plastic reflectors sandwiched between. DVDs can store more data because of this.

DVDs are usually 4GB. Songs are stored in DVDs because DVDs can be stacked and are easier to sell.

Other types of offline storage media

Magnetic storage system

Transistor based

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power.

Optical storage system

  • CD
  • DVD

Binary Systems and Hexadecimal

Binary Systems

Binary is the only language machines understand without a compiler. It is based on number 2. When converting from binary to denary, times each place of the binary number by its corresponding value, from right to left, 1, 2, 4, 8, 16. When converting from denary into binary, see the values 1, 2, 4, 8, 16, and choose the ones that add up into the denary number by writing “1” under them. If you are not choosing a number, write “0” under them. An easier way of doing so is to simply divide the number by 2 and take all the remainders from top to bottom and arrange them from right to left.


Computers do not understand hexadecimals, but it is easier for humans to understand them. These numbers are base 16. To convert from denary to hexadecimals, divide the denary number by 16 and take the remainder from top to bottom, arrange them from right to left to form the hexadecimal number. If the remainder is larger than 10, write them in letter form, for example, A, B, C.

Uses of hexadecimal

Hexadecimal are very common. They are used to express colors, known as “hex colors”. The size of each hex color is 3 bytes and 6 nibbles. Each digit in a hexadecimal is 1 nibble.

Hexadecimal is also used in MAC addresses. MAC address is unique for every device and will never change. With MAC address it is possible to find a device.

A MAC address (not mine)

The format of a MAC address is NN:NN:NN:DD:DD:DD, where N is the identity of the manufacturer and D is the serial number of the device.


Difference between Primary Memory and Secondary Storage

Difference between hard-disk drives and floppy diskettes

That’s the end. Happy revision and test on Tuesday!

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