Types of Computer Memory
- Primary or main or internal memory
- Secondary or auxiliary or backup or external memory
1. Primary Memory
Primary memory is directly related to the processor of the computer system.
It stores:
- All the data and program instructions that can be directly retrieved .
- Result or the output prior to the transfer to the output or the storage device.
In modern machines, memory is provided by the semiconductor, which are very expensive.
Primary memory is further divided into following parts:
Technically, the speed of the main memory of the computer is much less than the speed of the processor. In order to give an excellent performance, the processor cannot spend much of its time in waiting to access instructions and data from the main memory.
Hence, the best way to utilize the processor ins to make it work as fast as possible. An efficient solution to this problem is to use a fast cache memory, which makes the main memory appear to the processor to be faster than it really it.
Cache Memory
The Cache Memory is a temporary storage area, placed between the processor and the main memory. It maps the contents of the main memory for quick access by the processor. It works by storing the contents of frequently RAM locations and addresses that store frequently used data. When the processor looks for a memory address, the cache checks whether it holds that address or not. If it does, then the data is transferred to the processor at a quick speed. That is why the cache memory is always considered faster than the Random Access Memory (RAM).
Secondary Memory
Secondary memory auxiliary or external storage memory is located within the computer cabinet, though it is external to the main circuit board. While some device of this memory type are fixed inside the computer cabinet (like hard disk), others are removable (like optical discs such as CDs, DVDs and external hard disk).
Secondary memory devices are non-volatile in nature, that is, the programs stored in them are not lost when the power is switched off. They store data as charges on a disk coated with a fine layer of metallic particles. The most common secondary storage devices are hard disks, compact discs, digital video discs and Blu-ray discs.
Secondary storage devices also act as the input devices. When the computer is switched on, it transfers the operating system on the hard disk. All the additional programs are also transferred from the disk to the computer's memory. When the data stored on the computer's disk drive is accessed, it is also transferred to memory. In all these ways, storage devices act like input devices.
Storage devices can "play" recorded data as well as "record" new data. In computing, the "play" function is called reading, and the "recording" function is called writing. Not all storage devices can do both. A CD-ROM drive, for example, is called read-only storage device it can read the data encoded on CD-ROM discs. However, a variant of CD-ROM drive, called CD-RW, can write to record-able CD-ROM discs as well as read any CD-ROM disc.
Storage devices that can perform both functions are called read/write device.
Terms related to disks:
Disk
Disk, in computer science, is a round, flat piece of flexible material that can be electrically, magnetically, optically or mechanically influenced to hold information recorded in digital (Binary) form. A disk, in most computers, is the primary means of storing data on a permanent or semi-permanent basis. Because disks are random access devices, they are reasonably speedy. Also, disks enable the computer to write and read data.Terms related to disks:
- Tracks: Data on disks is recorded in concentric circular bands called tracks.
- Sectors: Each track is divided into parts called sectors.
- Cluster: Two or more sectors combine to form a cluster.
Disk Drive
Disk drive, in computer science, is an electromechanical device that reads from and writes to disks. The main components of a disk drive include:
- A spindle on which the disk is mounted.
- A drive motor that spins the disk when the drive is in operation.
- One or more read/write heads that perform the actual reading and writing.
- A controller circuitry that synchronizes read/write activities and transfers information to and from the computer.
Common disk drives used are hard disk drives, CD-ROM drives, DVD, Blue-ray drives and Pen drives.
Hard Disk and Drive
Hard Disk is the fixed disk that cannot be removed from its drive cabinet. This drive contains the electron to read and write data on to the disk surface. Hard disks presently used with personal computers can store hundreds of gigabytes (1 gigabytes is a billion bytes) of data. A hard disk stores the computer's operating system and applications. It also provides space for the user's data, instructions and information. Most personal computers have only one hard disk, although some have as many as four. In such cases, the main disk is called master, and the other disks are called slaves.
Hard Disk Assembly and Working
A hard disk drive consists of a platters, spindle, actuator, frame, motor, read/write heads, air filter and electronics. The frame contains all the mechanical parts of the drive and is sealed with a cover. This sealed part of the drive called Hard Disk Assembly or HDA. The electronics parts of the drive consists of one of more printed circuit boards. these are mounted on the bottom of the HDA.
A hard disk has one of more platters and each platter usually has a head on each of its sides. The platters are made form glass or ceramic and a layer of magnetic material is coated on the surface. The platters are mounted on the spindle which is turned by the drive motor.
The read/write head doesn't actually contact the disk's surface. Instead, it "floats" just above the disk, at a distance almost three hundred times less than the width of a human hair,. If the read/write head were to encounter an obstacle, such as dust or smoke the head would bounce on the disk surface, causing serious damage.
Information is stored on the platters by sending pulses of current form the drive electronics to the head. The direction of the current and thus the direction of the diverging magnetic field across the gap in the head determine binary one (1) or zero (0).
Hard disk storage capacities are measured in gigabytes or terabytes. A normal hard disk can store from 320 GB to 4 TB of data.
Compact Disk Drive
Compact Disk (CD) is an optical disk that uses laser beam to burn tiny holes on the surface of the disc to represent digital data. An optical drive reads this data by focusing a laser-generated beam of light on the disk's surface. The drive detects the light fluctuations caused by the holes and translates these into data that the computer can use. Optical media like CDs can store extremely large amounts of data. Other optical discs are DVD, Ferro Magnetic Disc (FMD), Giant Magneto-Resistive (GMR) and Magneto Optical (MO).
A Standard CD has a diameter of 4.7 inches and can store up to 700 MB of data. The mini CD has various diameters ranging from 2.4 to 3.1 inches. They are usually used to store and deliver device drivers.
Lection of CDs are:
- CD-ROM is a means of providing read only access to a large amount of data for use on computer systems. The data on CD-ROM is encoded in the form of a spiral of minute pits impressed into one surface of the disk at the time of manufacture. As the name indicates, these disks come prerecorded and cannot be altered; CD-ROM is, in other words, a read-only storage medium. CD-ROM provides an excellent way to distribute large amounts of data at low cost.
- CD-R is a write-once technology; after one saves data to the disc, one can't erase it or write over it. An advantage of CD-R discs is that they aren't expensive.
- CD-RW (Compact Disc Re-Writable) provides full read/write capabilities using erasable CD-RW discs, which are more expensive than CD-R discs.
Digital Video Disc and Drive
Digital Video Disc (DVD) is also an optical disc. This type of disc is capable of storing data up to 17 GB. Unlike compact discs, digital video discs can store an entire digitized movie. Digital video discs are designed to work with a video player and television. For computer data, a different format is used, called DVD-ROM (Digital Video Disc-ROM). DVD-ROMs are designed to work with a spacial drive (a DVD-ROM drive) fit inside a computer. DVD-ROM drives can play DVD videos and can also read existing CD-ROM discs.
Read/write versions of DVD, called DVD-RW and DVD-RAM are also available in the market.
Pen Drive/USB Flash Drive
A pen drive aka USB flash drive is a removable and rewriteable data storage device. It consists of flash memory with a built-in Universal Serial Bus (USB) interface. Most pen drives weigh as less as 30 grams. However, small size doesn't mean small capability. They can store up to 256 gigabytes (GB) of data. Pen drives as large as 2 terabyte (TB) are planned for the near future.
A pen drive consists of a small Printed Circuit Board (PCB) which also carries the circuit elements and a USB connector,. These are insulated electrically and protected inside a case, which could be made of plastic, metal or rubber. A pen drive can easily be carried in a pocket or on a key chain. Most pen drives use a standard USB connection that allows plugging into a port on a computer system form where they draw power. However, other interfaces also exist.
There are typically three parts to a pen drive:
Blu-ray Disc Drive
Blu-ray disc is optical disc storage medium. It is made up of plastic and has a diameter of 120 mm with a thickness of 1.2 mm, the same size as a DVD or CD. A blue laser beam is used to read the disc, hence the name Blu-ray. This blue laser beam allows information to be stored at a greater density as opposed to the red laser beams used for DVDs.
Blu-ray discs have a storage capacity of up to 25 GB per layer, with dual layer discs having 50 GB Memory space. Triple layer discs (100 GB) and quadruple layer discs (128 GB) are also available. The first Blu-ray disc prototypes were unveiled in October 2000, and the first prototype player was released in April 2003 in Japan. Afterwards, it continued to be developed until its official release in June 2006.
Blue-ray disc drive works in similar manner as the Digital Video Disc drive. Also, this drive can run DVDs and CDs too.
Memory Cards and Sticks
Memory cards and sticks are storage device that are used to store digital content. These are mainly used in digital camera and various types of computers for storing/transferring multimedia content.
Memory sticks, a patent of Sony, is a removable flash memory device. All Sony products of Cameras, Camcorders, Computers, PSPs, etc have memory stick slots. There are many variations of memory sticks such as Memory Stick PRO, Memory Stick-Duo, Memory Stick Micro, etc.
Memory cards, also known as Secure Digital (SD) cards, are also removable flash memory devices and follow the same format is memory sticks. These are used in GPS navigation devices, digital cameras, mobile phones, and various type of computers.
Memory cards, also known as Secure Digital (SD) cards, are also removable flash memory devices and follow the same format is memory sticks. These are used in GPS navigation devices, digital cameras, mobile phones, and various type of computers.
