What is Computer memory?

The computer memory is a temporary storage area. It holds the data and instructions that the Central Processing Unit (CPU) needs. Before a program can be run, the program is loaded from some storage medium into the memory. This allows the CPU direct access to the program.


Random Access Memory (RAM): Memory is a need for any computer and this memory is called Primary memory/storage or Random Access Memory (RAM). Random access memory (RAM) is used as the working memory of a computer system. It stores input data, intermediate results, programs, and other information temporarily. It can be read and/or written. It is usually volatile, that is all data will be lost when the power is turned off.

Read only memory (ROM): is the permanent memory which is used to store important control programs and systems software to perform a variety of functions, such as booting up or starting up programs. ROM is non-volatile. That means the contents are not lost when the power is switched off.


PROM: There is another type of primary memory in computer, which is called Programmable Read Only Memory (PROM). You know that it is not possible to modify or erase programs stored in ROM, but it is possible for you to store your program in PROM chip. Once the programmers’ are written it cannot be changed and remain intact even if power is switched off. Therefore programs or instructions written in PROM or ROM cannot be erased or changed.

EPROM: This stands for Erasable Programmable Read Only Memory, which overcome the problem of PROM & ROM. EPROM chip can be programmed time and again by erasing the information stored earlier in it. Information stored in EPROM exposing the chip for some time ultraviolet light and it erases chip is reprogrammed using a special programming facility.

How data store?

The computer’s memory consists of many millions of bytes. To make it easier, the unit K (for kilobytes) can be used to express memory capacity. One K equals 1,024. For example, 64K bytes of memory is the same as 65,536 (1,024 × 64 = 65,536) bytes.

bits and bytes charts

An 8-bit byte can accommodate up to 28, or 256, characters. ASCII is the American Standard Code for Information Interchange, an American National Standard computer character code. ASCII is an 8-bit code with 256 characters:

ASCII table to bit conversion


Floppy Disk

A Floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, readable by a floppy disk drive (FDD), and sealed in a rectangular plastic carrier lined with fabric which serves the purpose of keeping the data storage disk free of foreign particles such as dust.

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360KB 5,25″ Floppy: The oldest floppy disk format is the 360 KB 5.25″ floppy disk. This is the type of disk that was used in the very first IBM PCs, which in fact didn’t use a hard disk at all. The 360 KB floppy is the only format that uses 40 tracks per side to record data; the others all use 80 tracks.

1.2MB 5.25″ Floppy: The high density 1.2 MB floppy disk debuted in the IBM AT in 1984, as a standard feature (the 360 KB floppy was optional). The increase in capacity of this disk, over 200% compared with the 360 KB version, all but obsoleted the smaller format rather quickly. The 1.2 MB floppy disk can still read and write 360 KB floppies, but problems can occasionally result. Since the floppy uses a higher bit density, the 1.2 MB floppy requires a floppy disk controller capable of 500 Kbits/s data transfer.

720KB 3.5″ Floppy: The original version of the 3.5″ floppy disk held 720 KB of data and was introduced in 1986. This version of the 3.5″ never became very popular both because it offered 40% less capacity than the 1.2 MB 5.25″ drive, and because it was so quickly replaced by the high density 3.5″ disks.

1.44MB 3.5″ Floppy: The only floppy disk format still in wide use, the 1.44 MB 3.5″ drive was introduced by IBM in 1987 as part of its PS/2 line. Since that time they have grown immensely popular, and this format is the standard for floppy disks today. Virtually every PC made since 1987 uses one of these drives, and there are many non-PC computers that will read them as well. They have become very cheap due to the aging of the technology and the fact that they are produced in such high volume. Their universality is what has allowed the floppy to continue to be a default part of every PC despite their rather tiny storage capacity compared to today’s hard disks.

2.88MB 3.5″ Floppy: The highest-capacity format for floppy disks is the 2.88 MB 3.5″ disk that was developed by Toshiba in the late 80s., but there are no popularity received to this as compared to 1.44MB. The reason could be the introduction of Hard Disk.

Hard Disk

Hard disk drives were introduced in 1956 as data storage for an IBM real-time transaction processing computer and were developed for use with general-purpose mainframe and minicomputer.

A disk drive is a randomly addressable and rewritable storage device. The term can be broadly interpreted to include optical drives and in earlier times, floppy drives. However, in popular usage, it has come to relate mainly to hard disk drives (HDDs).

Hard disk

Platter: Hard drives are normally composed of multiple disks called platters. These platters are stacked on top of each other. It is the platter that actually stores the data. It consists of a substrate coated with magnetic media. The substrate is there to act as a rigid support for the magnetic media. The magnetic layer is protected by a thin layer of carbon and a lubrication layer to prevent damage in case the head comes in contact with the platter surface. Typically, both sides of the platter have magnetic media on which to store data. Tens of thousands of tracks per inch can be laid down on these platters.

Spindle/Motor: The platters are attached at the center to a rod or pin called a spindle that is directly attached to the shaft of the motor that controls the speed of rotation.

Head-Actuator Assembly: This assembly consists of an actuator, the arms, the sliders and the read/write heads. The actuator is the device that moves the arms containing read/write heads across the platter surface in order to store and retrieve information. The head arms move between the platters to access and store data.

Head Slider: At the end of each arm is a head slider, which consists of a block of material that holds the head and acts as an airfoil to keep it positioned at the precise height above the surface. The read/write heads convert the electronic 0s and 1s in the magnetic fields on the disks.

Logic Board: Logic boards consisting of chips, memory and other components control the disk speed and direct the actuator in all its movements. It also performs the process of transferring data from the computer to the magnetic fields on the disk.

The disks features:

Tracks: A track is a concentric ring on the disk where data is stored.

Cylinders: On drives that contain multiple platters, all the tracks on all the platters that are at the same distance from the center are referred to as a cylinder. The data from all the tracks in the cylinder can be read by simply switching between the different heads, which is much faster than physically moving the head between the different tracks on a single disk.

Sectors: Tracks are further broken down into sectors, which are the smallest units of storage on a disk, typically 512 bytes. A larger number of sectors are recorded on the outer tracks, and progressively fewer toward the center. Data can also be read faster from the outer tracks and sectors than the inner ones.

Clusters: Sectors are grouped together into clusters. As far as the operating system is concerned, all the sectors in one cluster are a single unit.

Extents: A set of contiguous clusters storing a single file or part of a file is called an extent. It is best to keep the number of extents for any file to a minimum, as each extent requires a separate input/output (I/O) operation.

History of Storage

  • Hard disk – 1956
  • Floppy Disk – Late 60’s
  • Optical Disk (CD ROM) – 1978 – 1982
  • Flash Memory – 1980
  • Compact Flash Disk – 1994 – 1995

Hard Disk and their time line can be found here. Currently High speed drive 7,200RPM to 10,000RPM drives are available. The latest technology in the market is varies from 2.5“ and 3.5” hard disks size. Introduction of new SSD drives (Solid State Drives) is way ahead of bringing the new technology to peek. Read SSD vs flash drive comparison



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