Types of Memory

Types of Memory

The computer memories can be divided into following categories:

·        Primary Memory

·        Secondary memory

·        Cache Memory

1.    Primary Memory: Primary memory or main memory is used for storing program and data during the execution of the program. It is directly accessible to CPU. The main memory is divided into many storage locations, each of which can store data or instructions. Each memory location is of the same size and has an address. With the help of the address, the computer can find any data easily without having to search the entire memory. Broadly primary memory can be of two types – RAM (Random Access Memory) and ROM (Read only memory).

a.     Random Access Memory (RAM)

RAM is also known as volatile memory is a computer memory that requires power to maintain the stored information. The data is primarily stored on RAM. This is also known as Read-Write memory as both the operation can take place on it. It is volatile in nature because as soon as the power is off its contents are also removed. It can be of two types: -

1. Static RAM or SRAM.
2. Dynamic RAM or DRAM

SRAM retains its contents as long as the power is connected and is easy to interface to but uses six transistors per bit.

DRAM is more complicated to interface to and control and needs regular refresh cycles to prevent its contents being lost. However, DRAM uses only one transistor and a capacitor per bit, allowing it to reach much higher densities and, with more bits on a memory chip, be much cheaper per bit. SRAM is not worthwhile for desktop system memory, where DRAM dominates, but is used for their cache memories.

b.    Read Only Memory (ROM)

ROM is also known as Non-volatile memory is a computer memory that can retain the stored information even when not powered. As the name suggests we can perform only read operation on ROM. It is permanent in nature. In ROM booting instructions for computer in the form of firmware are stored. Other examples of non-volatile memory are flash memory and PROM/EPROM/EEPROM.

·        Programmable Read Only Memory (PROM): This type of ROM can be programmed even after its manufacture using a PROM programmer circuit. But once a PROM is programmed, it becomes just like ROM i.e. it cannot be changed.

·        Erasable Programmable Read Only Memory (EPROM): In this type of ROM, the contents can be erased and the memory can be reprogrammed. To erase the data, an EPROM is exposed to ultraviolet light and then it can be reprogrammed using a PROM programmer circuit. When the EPROM is in use, then it behaves like a ROM that means the information can only be read.

·        Electrically Erasable Programmable Read Only Memory (EEPROM): The contents of this type of ROM can be erased and then reprogrammed using electric signals. This makes it an excellent back up for RAM whose contents are lost when the power is switched off. When the power is returned, the contents of EEPROM are copied back into the RAM and the computer continues working without any data loss. Nowadays RAMs and EEPROMs are integrated in a single chip.

2.    Secondary Memory

The major limitation of primary memory is that it has limited storage capacity and is volatile. To overcome this limitation, we have secondary memory storage devices. This type of memory is also called external memory. It refers to the various storage media on which a computer can store data and programs. It is an additional storage, not part of the main computer.

The Secondary storage media can be fixed or removable. Fixed Storage media is an internal storage medium like hard disk that is fixed inside the computer. Removable storage media is a data storage medium that is portable and can be taken outside the computer.

Why do we need Secondary Memory?

Secondary memory is needed because of the following reasons:

·        For permanence: As the RAM is volatile, i.e. it loses all information when the electricity is turned off, and something is needed for permanence. Secondary storage devices serve this purpose. They do not lose data when electricity is turned off.

·        For portability: Secondary storage, like the CDs, flash drives can be used to transport data from one computer to another.

Secondary Storage Media

There are the following main types of storage media.

             I.            Magnetic

          II.            Optical

    III.            Solid State

I.            Magnetic storage media: Examples of magnetic storage media are hard disks, floppy disks and magnetic tapes. Magnetic media is coated with a magnetic sensitive layer and this layer is magnetized in clockwise or anticlockwise directions, which then are interpreted as binary 1s and 0s at reading.

a.     Floppy Disk: A floppy disk is a flexible disk made up of Mylar with a magnetic coating on it. It is packaged inside a protective plastic envelope. These were one of the oldest type of portable storage devices that could store up to 1.44 MB of data but now they are no longer in use. Floppy disks comes in two sizes: 5.25 inches with 1.2 MB capacity and 3.5 inches with 1.44 MB capacity.

b.     Hard Disk: Another form of auxiliary storage is a hard disk. A hard disk consists of one or more rigid metal plates coated with a metal oxide material that allows data to be magnetically recorded on the surface of the platters. The hard disk platters spin at 5 a high rate of speed, typically 5400 to 7200 revolutions per minute (RPM). Storage capacities of hard disks for personal computers range from 10 GB to 2TB.

II.            Optical Storage Media: On an optical storage media information is stored and read using a laser beam. The data is stored as a spiral pattern of pits and ridges denoting binary 0 and binary 1. Examples of optical media are CDs, DVDs etc.

a.     CD- ROM: It stands for Compact Disk - Read Only Memory and data is written on these disks at the time of manufacture. Thereafter this data cannot be changed but can only be read by a laser beam in the form of a continuous spiral. CD- ROMs are used for text, audio and video distribution like games, encyclopaedias and application software.

b.     CD-R: It stands for Compact Disk- Recordable. Data can be recorded on these disks but only once. So, we can write data on these disks through a read/write CD drive but after that the disk cannot be erased/modified.

c.      CD-RW: It stands for Compact Disk-Rewritable. It can be read or written multiple times. But a CD-RW drive needs to be installed on your computer.

d.     DVD: It stands for Digital Versatile Disk or Digital Video Disk. It looks just like a CD and use a similar technology as that of the CDs discussed above but employ a shorter wavelength red laser that permits a narrower beam. This allows tracks to be spaced closely enough to store data that is more than six times the CD's 700MB capacity. CD-like discs glued back-to-back. This protects the delicate reflective coating as it is on the inside of the disc. Also it makes possible to have double sided DVDs—where data can be stored on each half disc. A DVD holds 4.7 GB to 17 GB of data. That means a complete movie can be stored on one side of a DVD. Like CDs DVDs also come in three varieties –

·        DVD- ROM

·        DVD- R

·        DVD-RW

e.     Blue Ray Disk: This is the latest optical storage media to store high definition audio and video. It looks like a CD or DVD but can store up to 27 GB of data on a single layer disk and up to 54 GB of data on a dual layer disk. Where CDs or DVDs use red laser beam, the blue ray disk uses a blue laser to read/write data on a disk. Blue-ray Disc (BD) was developed to enable recording, rewriting and playback of high-definition video (HD), as well as storing large amounts of data.

III.            Solid State Memories: The term ‘solid-state’ essentially means ‘no moving parts’. Hence Solid-state storage devices are based on electronic circuits with no moving parts (no reels of tape, no spinning discs, no laser beams, etc.) Solid-state storage devices store data using a special type of memory called flash memory. SSD, Solid-state drive (or flash memory) is used mainly in digital cameras, pen drives or USB flash drives.

a.     Pen Drives: Pen Drives or Thumb drives or Flash drives are the recently emerged portable storage media. It is an EEPROM based flash memory which can be repeatedly erased and written using electric signals. This memory is coupled with a USB connector through which it can be plugged into the computer. They have a capacity smaller than a hard disk but greater than a CD.

3.    Cache Memory

Cache memory is a special high-speed memory made up of high speed static RAMs. It is used to hold frequently accessed data and instructions. We know that the processing speed of CPU is much more than the main memory access time of the computer. This means the CPU has to wait for a substantial amount of time. Alternatively, we have the cache memory which is a small, expensive but fast memory that is placed between the

CPU and the main memory. Whenever some data is required, the CPU first looks into cache. If data is found, we call it a cache hit and the information is transferred to the

CPU. In case of a miss, the main memory is accessed. Memory caching proves to be efficient because most programs repeatedly access the same data and instructions, so access of frequently used data becomes very fast with cache memory. There are two types of cache memory:

·        L1 cache: It is small and is built inside the CPU. It is fast as compared to L2 cache.

·        L2 cache: It is large but slower and is mounted on the motherboard