Types of computer memory 

Computer memory is the storage space in the computer, where dat is to be processed and instructions required for processing are stored. The memory is divided into large number of small parts called cells. Each location or cell has an unique address which varies from zero to memory size minus one. There are different types of computer memory. Some of them are discussed below: 

a. Primary memory 

Primary memory is the main memory or internal memory and most important types of computer memory. . It is a temporary storage area built into the computer hardware. This memory consists of some Integrated Circuit (IC) chips either on the motherboard or on a small circuit board attached to the motherboard of a computer system. This memory allows CPU to store and retrieve data very quickly. The rate of fetching data from this memory is of the order of 50 nanoseconds/byte. Hence the rate of data fetching from main memory about 100 times faster than that from a high speed secondary memory (storage) like disk. The primary memory includes RAM and ROM. 

i. RAM (Random Access Memory) 

RAM is a commonly used volatile memory. It is also called temporary memory. They are RAM needs regular power supply to hold its content. RAM chips are of two types. Static RAM (SRAM) and Dynamic RAM (DRAM). 

Static Random Access Memory (SRAM) 

SRAM is a fast memory technology which is used for high speed registers and system caches. SRAM takes six transistors to store 1 bit. It retrieve data bits in its memory as long as power is supplied. This memory does not need periodic refreshing. 

Dynamic Random Access Memory (DRAM) 

Dynamic RAM uses an external circuitry to periodically 'regenerate' or refresh the storage charge to retain the stored data. It won't reliably retain information for very long time. Fortunately this capacitor-based system can remember long enough to be useful- a few or few dozen milliseconds- before the disappearing charges make the memory unreliable. During the refresh period, the memory is not available for normal operation. 

Difference between SRAM and DRAM

SRAM	DRAM
i. SRAM stands for Static Random Access Memory.	i. DRAM stands for Dynamic Random Access Memory
ii. SRAM does not need periodic refreshing.	ii, DRAM needs periodic refreshing.
iii. It uses transistor to store single bit of data.	iii. It uses a separate capacitor to store each bit of data.
iv. It stores bit as a voltage.	iv. It stores bit as charge.

 

ii. Read Only Memory (ROM) 

An acronym for Read Only Memory, ROM is computer memory on which data are pre-recorded. The programming code and/or data on a ROM chip is written to the chi8p at the factory. It can be read, but it cannot be erased or removed. SO it is called non-volatile or permanent memory. ROM retains its data or content even when the computer is turned off, unlike a computer's main memory RAM, which needs a constant charge of electricity to keep its information. ROM chips are used in all kinds of electronic devices from calculators to video games. The following are sub types of ROM. 

PROM (Programmable Read Only Memory) 

PROM is a non-volatile programmable ROM but only once. Once the program is written in it, it cannot be erased but can only be read. PROMs are programmed to record information using a special device known as PROM - Programmer. Once the chip has been programmed, the PROM becomes a ROM. That is, the information recorded in it can only be read but cannot be changed. 

EPROM (Erasable Programmable Read Only Memory) 

An EPROM is a special kind of PROM chip that can be reprogrammed to store the new information. Its information is stored as electrical charges deposited on the chip (1s and 0s). An EPROM can be erased by removing it from the circuit. lt can then be reprogrammed using an EPROM burner. EPROMs are still considered non-volatile; they won't lose their information when the computer is turned off. EPROM chips are of two types- The EEPROM and UVEPROM. 

The EEPROM (Electrically EPROM) is a kind of EPROM chip which doesn't have to be removed and sent back to the manufacturer to be reprogrammed. It can be electronically reprogrammed while in circuit using a software program designed specifically for that purpose. On the other hand, the UVEPROM (Ultra Violet EPROM) is a kind of EPROM in which the stored information is erased by exposing the chip for some time to ultraviolet light. 

Differences between primary memory and secondary memory 

ROM	RAM
i. ROM stands for Read Only Memory.	i. RAM stands for Random Access Memory
ii. ROM is non volatile (Permanent) reading memory.	ii. RAM is volatile (temporary) reading and writing memory.
iii. The contents of ROM are not erased as soon as electricity is not supplied.	iii. The contents of RAM are erased as soon as electricity is not supplied.
iv. The types of ROM are PROM, EPROM and EEPROM.	iv. The types of RAM are SRAM, and DRAM.

 

b. Secondary Memory (Storage) 

The primary memory is of limited capacity and they are volatile memory. To overcome these limitations, types of computer memory, i.e. secondary storage devices are used. Secondary memory are non volatile and have lower cost per bit stored but they generally have an operating speed far slower than that of primary memory. It is used to store the large volume of data on permanent basic that can be partially transferred to primary storage, whenever required for processing. Magnetic disk, magnetic tape, optical disk are the examples of secondary storage devices. Secondary storage devices are: Sequential Access Device, Direct Access Device, Magnetic Disk, Optical Disks, CD-ROM, DVD, Hard -Disk, Floppy Disk. 

Differences between primary memory and secondary memory 

Primary memory	Secondary memory
i. Primary memory is the main memory of computer which is used to store data for present purpose.	i. Secondary memory is an auxiliary memory of computer which is used to store data, software etc for future purpose.
ii. Primary memory is directly accessed by CPU.	ii. Secondary memory is accessed by CPU via an interface.
iii. Primary memory is more expensive than secondary memory.	iii. Secondary memory is cheaper than primary memory.
iv. Example of primary memory: ROM, RAM, etc.	iv. Example of secondary memory: Hard disk, CD ROM, etc .

 

The secondary storage devices are broadly categorized into the following types. They are:

1. Sequential storage device
2. Magnetic storage devices 
3. Optical storage devices 

i. Sequential Storage device 

A sequential access storage device is the one in which information can be retrieved in the same sequence only in which it is stored. In sequential access storage devices, the arrival at desired location may be preceded by sequencing through other locations so that access time varies according to the locations. For example, if any 10 items are stored in a sequential access storage devices, to access the eight item we must first access the first seventh items coming before the eight item. Magnetic tape is an example of sequential access storage device. 

Magnetic Tapes 

Magnetic tapes are mass sequential storage or secondary storage devices capable of storing and retaining large volumes of data. Magnetic tapes are similar in operation and function to tape recorder and VCR tapes. Magnetic tapes are designed to store large quantities of data. They are typically 0.5 inches wide and covered with magnetized material, over which data can be recorded. The storage part of the tape is commonly a ribbon that is coated or impregnated with a magnetic material. 

Advantages of magnetic tape 

• Magnetic tapes are cheaper than other types of sequential access devices. 
• Data stored on magnetic tapes can be erased and they can be reused multiple times. 
• Magnetic tapes have high storage capacity and can backups substantial amounts of information. 

Disadvantages of magnetic tape 

• Magnetic tapes tend to depreciate over time. 
• Data access is slow and a tape drive has to read the entire contents of a magnetic tape before finding and accessing data. 

ii. Magnetic Storage Devices 

The surfaces of Magnetic storage devices are coated with magnetic tapes such as iron oxide or ferrous oxide, which can be magnetized. That is  which reacts to a magnetic field. Each of these particles can act as a magnet. The write/read heads of disk drives or tape drives contain electromagnets that generate magnetic fields in the iron on the storage medium as the head passes over the disk or tape. The presence of a magnetic field represents a '1' bit and its absence represents a '0' bit. Some commonly used magnetic storage devices are described below. 

Zip disk 

Zip disk is a removable storage disk specifically designed to be used with Iomega's zip drives. The Zip disk was one of the more popular floppy disks of era because of its storage capacity. The original Zip disks could hold 100 megabytes of data. Iomega later created 250 megabyte and 7500 megabyte zip disks. 

Super-Disk 

Super-Disk is a secondary storage device developed in the mid 1990s by Imation Corporation, It was released as competition for Iomega's zip disks. It is capable of storing about 120 MB of data. Super-Disk was an example of what has since been called Elliptical (secret) technology. Elliptical technology combined traditional magnetic tape storage with laser heads. 

Hard Disk 

Hard disk is the most commonly used secondary storage device in the personal computer. It is also called the magnetic disk, HDD, hard drive, fixed drive, or fixed disk. It is the main and the largest data storage device in the computer. It can store a large amount of data and information permanently. Most application programs and operating systems require hard disk for installation and operation. It contains one or more metal platters mounted on a central spindle. A hard disk is actually, a stack of platters. Each platter is coated on both sides with magnetic material. Both surfaces of each disk or platter are used to store information except for the top and bottom platters. The hard disk and drive is a single unit. It includes the hard disk, the motor that rotates the platters. Each platter has two read/write heads, one for each side. The hard disk has also arms that move the read/write heads to the proper locations on the platter to read or write data. The entire unit is enclosed in an air tight sealed case. 

Advantages of hard disk 

• Hard disk has large storage and retrieval capacity of data. 
• It is usually fixed inside the computer so cannot get mislaid. 
• It is more durable and reliable than other storage media. 
• It is cheaper on a cost per megabyte compared to other storage media. 
• Hard disks can be replaced and upgraded as necessary. 

Disadvantages of hard disk 

• Hard disks eventually fail which stops the computer from working. 
• Regular 'head' crashes can damage the surface of the disk, leading to the loss of data in that sector. 
• The disk is fixed inside the computer and cannot easily be transferred to another computer. 

Flash Memory

 

Flash memory is an advanced type of secondary memory which is available as USB memory sticks. It is user friendly, portable and cheaper. Example: Pen drive 

Pen drive 

Pen drive is the most popular portable magnetic secondary storage device. It is also called flash drive, or USB (Universal Serial Bus) drive or thumb drive. Pen drive is small in size and can be carried in a pocket like a pen so it is named as pen drive. It is a convenient way of transferring data from one computer to another. It can be considered as a small external hard disk. Its storage capacity ranges from 128 Mb to a few Gigabytes. 

iii. Optical Storage Devices 

An optical storage media consists of a flat, round, portable metal disc, which is coated with a thin metal, plastic or other material that is highly reflective. The information on the optical disk is stored in the form of pits and lands. The pits are the tiny reflective bumps that are related with laser beam. The lands are flat areas separating the pits. A land reflects the laser light, which is read as binary digit 1. A pit absorbs or scatters light, which is read as binary digit 0. The high-powered laser beam creates the pits. A lower-powered laser light reads data from the disc. Like tracks on a magnetic disk, the tracks of an optical disk are divided into sectors but shape of these sectors is different than sectors of magnetic disk. Some commonly used optical storage devices are described below: 

CD-ROM (Compact Disk Read Only Memory) 

CD-ROM stands for Compact Disc Read Only Memory. It is the most popular and widely used secondary storage device. CD-ROM is a type of optical disc that uses laser technology to store and read data to and from the disc. A large amount of data can be stored on a single disk. Once the information is stored on the CD-ROM, it becomes permanent and cannot be changed (altered). Therefore, it is called Compact Disc Read Only Memory. Nowadays, CDs are commonly used in personal computers. 

Advantages of CD ROM 

• CD-ROM is portable and able to store large amount of text, images, audio and videos. 
• It is quite cheaper in terms of cost per character stored. 
• It has a longer life span than a magnetic media. (About 20 years) 
• It is comparatively reliable and easy to use. 

CD RW (Compact Disk Read Writable) 

Compact Disk Read Writable (CD-RW) refers to an optical secondary storage device that may be written and rewritten multiple times. CD-RW allows for data erasing during each rewritable section. However, data cannot be changed during CD-RW sessions. It is a compact disc (CD) format that allows repeated recording on a disc. The CD-RW format was introduced by Hewlett Packard, Mitsubishi, Philips, Ricoh, and Sony, in a 1997 supplement to Philips and Sony's Orange Book. 

Advantages of CD RW 

• CD RW can record and erase data for several times. 
• The CD RW is cheaper for frequent changing files and data. 
• CDRW are better for the environment by the fact they are reusable. 
• The space available on the CDRW is usually lower than one time use. 

Disadvantages of CD RW 

• The burn and read speed of CD RW is slower than a non-rewriteable disc. 
• Compact Disk Read Writable disks are not very durable. 

DVD-ROM: 

DVD stands for Digital Video Disc or Digital Versatile Disc. A DVD is also an optical disk like CD but it has very high storage capacity than CD. It is an extremely high capacity optical disc with storage capacity from 4.7 GB to 17 GB. A DVD-ROM looks like a CD-ROM. Although the size and shape is similar to CD-ROM but DVD-ROM stores information in different manner. It is developed for a Compact Digital Video format. DVD disc is specially used to store movie films. It is also used to store huge databases, music, large software, etc. 

Blue Ray: 

The Blue Ray is the new technology for replacing DVDs in the near future. A blue laser is used to record more data onto a disc which is the same physical size as a DVD. It can store large amounts of data compared to DVDs, up to 50 GB for dual layered discs. But it is currently expensive compared to DVDs and most computers don't have Blue ray players/ recorders as standard. 

Topics covered: What is computer memory, what is memory, computer memory, types of computer memory, Types of computer memory, classification of computer memory, computer memory and it's types