Data Storage

• Memory is the space where data and programs are stored. Data in memory is accessed through instructions from the CPU.
• Memory in a computer system is classified into primary and secondary memory.

Types of Memory

Primary memory

• Primary memory is a type of memory that is accessed directly by the CPU.
• For example: RAM, ROM and cache.

Secondary memory

• Secondary memory refers to external storage devices such as hard drives, CDs, flash drives, etc.

Latency

• Latency is the time taken by components to respond to a request.
• To avoid short delays between the CPU’s request for data and finding the data in the memory, some instructions and data are copied to the cache.

RAM

• RAM is a temporary memory which stores data, files and parts of the operating system that is currently in use.
• When a program is loaded from the hard drive, all its contents, such as its instructions and data, are loaded to the RAM and the CPU accesses this information.
• It is also called the main memory.
• RAM is used by the operating system, applications and any data that are currently used.
• The access time for a CPU to access any data from the RAM is less when compared to accessing data from a hard drive.
• The larger the size of RAM, the faster the computer operates.
• Buffers also use RAM.
• When the power is lost, the contents of RAM are lost.
• The contents of RAM can be read from, written to, and changed.
• Each memory location in RAM has a unique address. (0x00000)
  

DRAM and SRAM

Dynamic RAM (DRAM)

• DRAM consists of transistors that act as switches and capacitors that hold binary data.
• DRAM has to be constantly refreshed (every 15 microseconds) to maintain the charge in the capacitor.
• DRAM is used for main RAM memory.
  

Static RAM (SRAM)

• SRAM uses ‘flip-flops’ which hold each bit of memory.
• SRAM need not be refreshed constantly.
• SRAM is faster than DRAM.
• SRAM is used in cache.
• SRAM is more complex to build than DRAM and is therefore more expensive.
• Due to the complex design and more transistors and additional wiring required in SRAM, it offers smaller capacity compared to DRAM.

ROM

• Read Only Memory (ROM) is a permanent memory that is used to store the instructions that are executed once the computer is switched ON.
• This set of instructions is called a boot process. This is responsible for initialising the hardware and operating system soon after the power is switched ON.
• The contents of ROM are not erased, even when the power is switched OFF.
• The contents of ROM can only be read and cannot be changed.
• ROM is made by interconnecting several transistors.
• It is an example for non-volatile memory.
  

Flash Memory

• Flash memory is a type of ROM. It is programmed by applying a slightly larger electric current that forces an electron through a barrier.
• Once the electrons cross this barrier, they get stored in a layer. The electrons can then be detected without affecting its position.
• As a flash of current is used to store data, it is called flash memory.
• Flash memory is also rewritable.

• Users need storage devices to store media and other files that require large amounts of space.
• This problem is solved by storage devices that work based on magnetic, optical and solid-state principles.

Hard disk drives (HDDs)

• Hard disk drives, also called magnetic disk drives, are used in computers and laptops.
• It provides high storage capacity and is cost-effective.
• Large storage facilities also use this technology.
• In a computer, it stores the operating system, installed programs and user’s data.
• External hard disks are available to store data that are not frequently used or to back-up important information.

Structure of an HDD

• The disk is made of a magnetic surface, which is known as a platter.
• Digital data is stored in these magnetic platters.
• This disk can spin at about 7000 revolutions a second.
• Data can be accessed by a number of read-write heads on the surface.
• The read-write heads move from the centre of the disk to the edge of the disk (and back again) 50 times a second.
• The data is read/written using magnetic properties.
• The disk is divided into various sectors and tracks.
• Each sector in a track can store a specific number of bytes.

Latency of an HDD

• The access time of a hard disk drive is high compared to RAM.
• This is due to a large number of head movements.
• Latency is defined as the time taken for a specific block of data to rotate around to the read-write head.
• The effect of latency becomes significant when a user receives messages such as ‘not responding’ and ‘please wait’.

Solid State Drives (SDDs)

• The latency is reduced in SSD compared to HDD as there is no read/write head that needs to be moved.
• Data is stored and retrieved using the electronic properties in NAND chips.
• This type of memory is used in USB devices to transfer information from one device to another.
• This is a type of solid-state device.
• Solid-state drives are a larger version of flash memory.
• Digital data is stored in millions of transistors within the chip.
• An SSD is a non-volatile rewritable memory.
• These are used in portable devices such as tablets and mobile phones.
  

HDD vs SSD

• A computer system has various applications with different requirements.
• Some applications need faster access time.
• A SSD can improve the access time but it is expensive compared to HDD.
• Therefore, a combination of both is provided in a system.
• Applications that require high performance are loaded into an SSD and those that are not frequently used are stored in an HDD.

Basics

• CD, DVD and Blu-ray discs use optics to store data. The surface of CDs and DVDs are made of light-sensitive organic dyes or metal alloys.
• Data is read and written using laser light.
• DVDs (4.7GB) can store large amounts of data compared to CDs (800 MB)
• Blu-ray discs use blue laser light and can hold up to 50 GB. These optical storage systems are used to store music, movies and games.

How it works:

• The clear plastic layer on the surface allows the laser light to pass through it.
• The colour changes in the dye layer when light falls on it.
• This change in colour is reflected and is detected by the computer while the data is read.

Types of Optical Device

Read-only:

• The data can be written only once at the manufacturing stage.
• For example: programs, movies, and song distributed by its producers.

Read/write:

• Read/write optical devices are used as external storage devices to transfer data from one device to another.