Memory in Computers

Memory is primarily of three types −

 Cache Memory
 Primary Memory/Main Memory
 Secondary Memory

Cache Memory
Cache memory is a very high-speed semiconductor memory which can speed up the CPU. It
acts as a buffer between the CPU and the main memory. It is used to hold those parts of data
and program which are most frequently used by the CPU. The parts of data and programs are
transferred from the disk to cache memory by the operating system, from where the CPU can
access them.

Advantages

The advantages of cache memory are as follows −

 Cache memory is faster than main memory.

  It consumes less access time as compared to main memory.
 It stores the program that can be executed within a short period of time.
 It stores data for temporary use.

Disadvantages

The disadvantages of cache memory are as follows −

 Cache memory has limited capacity.

 It is very expensive.

Primary Memory (Main Memory)

Primary memory holds only those data and instructions on which the computer is currently
working. It has a limited capacity and data is lost when power is switched off. It is generally
made up of semiconductor device. These memories are not as fast as registers. The data and
instruction required to be processed resides in the main memory. It is divided into two
subcategories RAM and ROM.

DRAM- Dynamic random-access memory (dynamic RAM or DRAM) is a type

of random-access semiconductor memory that stores each bit of data in a memory
cell consisting of a tiny capacitor and a transistor, both typically based on metal-oxide-
semiconductor (MOS) technology. The capacitor can either be charged or discharged; these
two states are taken to represent the two values of a bit, conventionally called 0 and 1.
SRAM- Static random-access memory (static RAM or SRAM) is a type of random-access
memory (RAM) that uses latching circuitry (flip-flop) to store each bit. SRAM is volatile
memory; data is lost when power is removed.
The term static differentiates SRAM from DRAM (dynamic random-access memory) which
must be periodically refreshed. SRAM is faster and more expensive than DRAM; it is typically
used for the cache and internal registers of a CPU while DRAM is used for a computer's main
memory.

Read Only Memory (ROM)

It is an example of nonvolatile memory. ROM full form is Read Only Memory. It is a class
of storage medium used in computers and other electronic devices. Read Only Memory (ROM),
also known as firmware, is an integrated circuit programmed with specific data when it is
manufactured. The instructions for starting the computer are housed on Read only memory
chip.
ROM chips are used not only in computers, but in most other electronic items as well. Because
data is fully incorporated at the ROM chip’s manufacture, data stored can neither be erased nor
replaced. This means permanent and secure data storage. However, if a mistake is made in
manufacture, a ROM chip becomes unusable.
BIOS CHIP is an example of ROM in computer which is present on motherboard.

Difference between RAM and ROM

• ROM can hold data permanently and RAM cannot.

• ROM chip is a non-volatile and RAM chip is volatile in nature.
Types of ROM :

PROM : Short for programmable read-only memory, a memory chip on which data can be
written only once. Once a program has been written onto a PROM, it remains there forever.
Unlike RAM, PROMs retain their contents when the computer is turned off. The difference
between a PROM and a ROM (read-only memory) is that a PROM is manufactured as blank
memory, whereas a ROM is programmed during the manufacturing process. To write data
onto a PROM chip, you need a special device called a PROM programmer or PROM burner.
The process of programming a PROM is sometimes called burning the PROM.

EPROM : Acronym for erasable programmable read-only memory, and pronounced ee-prom,
EPROM is a special type of memory that retains its contents until it is exposed to ultraviolet
light. The ultraviolet light clears its contents, making it possible to reprogram the memory. To
write to and erase an EPROM, you need a special device called a PROM programmer or
PROM burner.

EEPROM : Short form of electrically erasable programmable read-only memory. EEPROM

is a special type of PROM that can be erased by exposing it to an electrical charge. Like other
types of PROM, EEPROM retains its contents even when the power is turned off. Also like
other types of ROM, EEPROM is not as fast as RAM.

Characteristics of Main Memory

 These are semiconductor memories.

 It is known as the main memory.
 Usually volatile memory.
 Data is lost in case power is switched off.
 It is the working memory of the computer.
 Faster than secondary memories.
 A computer cannot run without the primary memory.

Secondary Memory
This type of memory is also known as external memory or non-volatile. It is slower than the
main memory. These are used for storing data/information permanently. CPU directly does
not access these memories, instead they are accessed via input-output routines. The contents
of secondary memories are first transferred to the main memory, and then the CPU can access
it. For example, disk, CD-ROM, DVD, etc.
Characteristics of Secondary Memory

 These are magnetic and optical memories.

 It is known as the backup memory.
 It is a non-volatile memory.
 Data is permanently stored even if power is switched off.
 It is used for storage of data in a computer.
 Computer may run without the secondary memory.
 Slower than primary memories.

HARD DISK
Logical & Physical Structure of a Hard Disk

Physical Structure of a Hard Disk

The main components of hard disk drive are:

 Platters: These are disk like structures present on the hard disk, stacked one
above the other and store the data
 Head: It is a device present on the arm of the hard drive that reads or writes
data on the magnetic platters, mounted on the surface of the drive
 Spindle: It is the spinning shaft on which holds the platters in a fixed position
such that it is feasible for the read/write arms to get the data on the disks
 Actuator: It is a device, consisting of the read-write head that moves over the
hard disk con to save or retrieve information
 Cylinder These are the circular tracks present on the platters of the disk drive at
equal distances from the center

A hard disk contains a stack of platters, circular metal disks that are mounted inside the hard
disk drive and coated with magnetic material, sealed in a metal case or unit. Fixed in a
horizontal or vertical position, the hard disk has electromagnetic read or write heads above and
below the platters. The surface of the disk consists of a number of concentric rings called as
tracks; each of these tracks has smaller partitions called disk blocks. The size of each disk block
is 512 bytes (0.5 KB). The track numbering starts with zero. When the platter rotates, the heads
record data in tracks. A 3.5-inch hard disk can contain about thousand tracks.
The spindle holds the platters in a fixed position such that it is feasible for the read/write arms
to get the data on the disks. These platters rotate at a constant speed while the drive head,
positioned close to the center of the disk, reads the data slowly from the surface of the disk
compared to the outer edges of the disk. To maintain integrity of data, the head is reading at a
particular period of time from any drive head position. The tracks at the outer edges of the disk
have less densely populated sectors compared to the tracks close to the center of the disk.
The disk fills the space based on a standard plan. One side of the first platter contains space,
reserved for hardware track-positioning information which is not available to the operating
system. The disk controller uses the track-positioning information to place the drive heads in
the correct sector position.
The hard disk records the data using the zoned bit recording technique, also known as multiple
zone recording. This method combines the areas on the hard disk together as zones, depending
on the distance from the center of the disk. A zone contains a certain number of sectors per
track.
Calculation of data density of disk drives is done in the following terms:

 Track density: Refers to the number of tracks in a hard disk

 Area density: Area density is the platters’ storage capacity in bits per
square inch
 Bit density: It is bits per unit length of track

Logical Structure of Hard Disk

A hard disk’s logical structure mainly depends on the file systems used and the software that
defines the process of accessing data from the disk. Operating systems use different types of
file systems, and those file systems use various other types of controlling and accessing
mechanisms for data on the hard disk, Operating systems organize the same hard disk in many
different ways.
The logical structure of the hard disk directly influences the consistency, performance,
compatibility, and expandability of the storage subsystems of the hard disk. The logical
structure depends on the type of operating system and file system used, because these factors
organize and control the data access on the hard disk.
The most common computer file systems are:
 FAT
 FAT32
 NTFS
 EXT
 EXT2 and 3
 EFS
 FLOPPY

A floppy disk is a type of storage media that reads data storage information, also known as a
floppy diskette, floppy, or floppy disk that is used to store electronic data, like a computer
file. It was extremely expensive as it was one of the first types of hardware storage created in
1967 by IBM, which could read/write a portable device.

As the technology advanced, a floppy disk that is becoming smaller and came in three sizes, 8
inches, 5.5 inches, and 3.5 inches. As compared to previous models of floppy, the newer
version can store more data than used more cutting-edge technology.

Another version of the floppy disk, 5.25 inch, was capable of holding 360KB of data and was
introduced in 1978. Later, up to 1.2MB could be store by revisions of the 5.25-inch floppy disk.
Early desktop PCs like the Apple IIIc, were used these disk drives. The 3.5-inch HD (high
density) floppy disk was able to store 1.44MB and was introduced in 1987. As compared to
5.25-inch disks, these versions were more portable and also more durable. For backing up
personal data and distributing commercial software titles, the 3.5-inch floppy disk became the
standard for the next decade.

Over time, floppy disks were capable of reading and write data. At that point, it contained four
basic components:

o Magnetic read/write heads

o Containing all of the electronics, it includes a circuit board.
o It comprises a lever with a frame that helps to open and close the device.
o It is placed through a spindle clamping device because it is spinning 300 to 360 rotations
every minute.
The read/write head was used for reading and writing, which could read both sides of a disk.
For erasing data, a wider head was used and it offered surety that data was erased. Floppy discs
began to replace by CD-ROMs in the late 1990s. Later, consumers also started to use recordable
CDs for keeping backup their data. The first mainstream computer, Apple's original iMac, was
introduced in 1998. It did not include a floppy disk drive. Eventually, various PC manufacturers
followed suit because it took several years.

Now many people use recordable CDs or USB flash drives to back up their data, and most
software is distributed on CDs and DVDs. Finally, floppy disks have become a good thing of
the past for storing data. As it was the data storage medium and many people grew up with
that; hence, the floppy disk still has an important role in the veteran computer users. In modern
times, as ZIP drives, CDs and USB are har drives have been introduced.

How were floppy disks used?

In the early time of the computer, floppy disks were only the option to backup your information
and install a new program onto a computer because there were no CD-ROM drives, USB, and
floppy disks included on the computers. The program could be able to install from one floppy
disk if the program size was less than 1.44 MB. Similarly, most programs needed multiple
floppy disks if they have a larger size than 1.44 MB.

Types of Floppy Disk

There are several kinds of floppy disk drives that came on the market and gone with the times
due to not use more.
8-inch Drive
In the early 1970s, 8-inch was the first floppy design that was used as a read-only format then
become able for both read and write. Floppy was its physicality that gave the entire series of
the floppy drive its name.

5 ¼ -Inch Drive
During the 1980s, a 5 ¼-inch floppy disk drive was produced that was widely in use on PCs.
In the early 1990s, 5 ¼-inch floppy disk drives were also included on computers that could
have the ability to store data between 360 kilobytes and 1.2 megabytes. Some 5 ½-floppy disks
were able to write data to both sides of the disk and also allowed to modify data. After that,
manufacturers of floppy disks began to develop double-sided drives.

3 ½ -Inch Drive
Another type of floppy disk, 3 ½-inch floppy drive that is encased in plastic, which can hold
1.44 megabytes on a high-density disk and 730 kilobytes on a double- density disk. In older
times of the computer, multiple disks had to use to install the program, such as Windows 3.0.

CD ROM
Short for Compact Disc Read-Only Memory, a CD-ROM is an optical disc containing audio
or software data whose memory is read-only. A CD-ROM Drive or optical drive is the device
used to read them. CD-ROM drives have speeds ranging from 1x to 72x, meaning it reads the
CD roughly 72 times faster than the 1x version. As you would imagine, these drives are capable
of playing audio CDs and reading data CDs, including CD-R and CD-RW discs.
Note: A CD-ROM drive cannot read a DVD, including movie DVDs and data DVDs. The
DVD format is different than a CD, and a CD-ROM drive is not designed to read the format of
a DVD. A DVD-ROM drive is required to read a DVD.

Disc drive overview

Below is a picture of the front and back of a standard IDE CD-RW disc drive from Memorex.
A CD-ROM drive can be opened by pressing the tray eject button on the front of the drive
(pictured right). To close the CD-ROM drive, press the tray or the eject button again.

CD-ROM and disc drive interfaces

Below are the different interfaces that allow a CD-ROM and other disc drives to
connect to a computer.

 IDE/ATA - One of the most commonly used interfaces for disc drives.
 Panasonic - Older proprietary interface.
 Parallel - Interface used with old external CD-ROM drives.
 PCMCIA (PC Card) - Interface sometimes used to connect external disc drives
to laptop computers.
 SATA - Replacing IDE as the new standard to connect disc drives.
 SCSI - Another common interface used with disk and disc drives.
 USB - Interface most commonly used to connect external disc drives.

STORAGE CAPACITY CHART