4. Cache Memory

Secondary Storage Devices
1. Hard Disk
2. Floppy Disk
3. CD-ROM (Compact Disc Read only Memory)

RAM
RAM is that part of main memory in which data and instructions are held temporarily. RAM provides a working area to the user to enter and process data. In RAM each data element has its own address (location). Any data element can be read easily and quickly by using that address. It is also called as Read-And-Write Memory (RAM) since the computer can store or write data at any selected location (address) and can retrieve or read data when needed. It is a temporary memory of a computer used to store data and an instruction when the computer is on; everything that is stored in RAM is lost when the computer is turned off. For this reason it is also called volatile memory. Large RAM sizes provide larger data that computer can hold and process. Additional RAM chips can be installed in a computer simply by plugging them on motherboard. This increases the storage capacity of RAM of computer.
RAM is a semiconductor memory with no moving part. Data can be accessed from RAM at very high speed very close to the speed of light. A memory chip less than one-fourth size of a postage stamp can store more than 500,000 bytes equivalent to the printed matter on a popular daily newspaper.

ROM
ROM stands for Read Only Memory. The instructions written in ROM can only be read but cannot be changed or deleted. The ROM is, there fore, also called non-volatile memory. It is not possible to write new information or instructions into the ROM. This is the reason why it is called Read Only Memory.
ROM stores data and instructions permanently. When the power is turned off, the instructions stored in ROM are not lost. The instructions are written into the ROM chips at the time of its manufacturing. When the computer is switched on, the instructions in the ROM are automatically loaded into the memory of the computer. These instructions prepare the computer system for use and display the initial operating system screen.
A variation of ROM is programmable read only memory (PROM). PROM is a ROM into which you can load read only programs and data. Some microcomputer software packages, such as electronic spreadsheets are available as PROM units as well as on interchangeable disks. ROM and PROM are used in variety of capabilities within a computer system.

Difference Between RAM and ROM
Everything that is stored in RAM is lost when the computer is turned off. For this reason it is also called volatile memory.
In contrast the Read Only Memory (ROM) is non-volatile memory. Instructions can only be read from ROM. Instructions once written on ROM chip cannot be changed. It is a permanent memory of a computer, i.e. the contents of this type of memory are not lost when the computer is turned off. As soon as the computer is turned on, a program that resides on ROM automatically makes the computer ready for use. ROM is a semiconductor memory generally programmed by the manufacturer.

Difference Between SIMMs and DIMMs
The capacity of RAM in a computer affects computer's power. More RAM can make computer run faster. The computer does not necessarily have to load a complete program into its main memory to run it. However greater is the part of a program it can fit into memory, the faster the program will run.
Many RAM chips are installed on a small circuit board. These RAM chips are wired together to form a single module of large memory called Single In-Line Memory Modules (SIMMs). SIMM has contact terminals on it one side. It can easily be plugged into the motherboard. SIMMs had different capacities such as 1 Mbyte, 4Mbytes, and 16Mbytes with varying speed.
SIMMs were popular by PC users in 1980s but has now been replaced by DIMMs. Like SIMMs, Dual In-Line Memory Modules (DIMMs) have been developed installing many RAM chips. DIMMs provide larger capcity as compared to SIMMs. DIMMs have wider data bus and thus have higher rate of data transfer. DIMMs are available in several configurations having different capacities and speeds. You can expand RAM capacity of your computer by simply plugging in more DIMMs.

Short Notes
Floppy Diskette
It is a small removable, portable storage device that is made of polyester film coated with a metal oxide compound. It looks like a 45 rpm phonograph record except that it is enclosed in a jacket square in shape. It is readable by a computer with a floppy disk drive. The physical size of disks has shrunk from the early 8 inch to 5.25 inch square and about 1 millimeter thick (mini-floppy) to 3.5 inch square and about 2 millimeters thick (microfloppy) while the data capacity has risen.

Hard Disk
A hard drive is a a magnetic disk on which you can store computer data. The term hard is used to distinguish it from a soft, or floppy disk. Hard disk holds more data and is faster than floppy disks. In general, hard disks are less portable than floppies, although it is possible to buy removable hard disks. Today's computers typically come with a hrd disk that contains several billion bytes (giga bytes) of storage.

CD-ROM
CD-ROM (Compact Disc, Read Only Memory) is an adaption of the CD that is designed to store computer data in the form of text and graphics, as well as hi-fi stereo sound. It is a non-volatile optical data storage medium that is physically the same as an audio CD and is capable of storing large amounts of data up to 1 GB, althrough the most common size is 650 MB about 12 million bytes per pound weight. All CD-ROMs confirm to a standard size and format, so you can load any type of CD-ROM into any CD-ROM player. In addition CD-ROM players are also capable of playing audio CDs. A standard CD is 120mm (4.75 inches) in diameter and 1.2 mm (0.05 inches) thick.

Magnetic Tape
A magnetic tape is also a storage device from which you can access data serially like an audio cassette from which you can hear the songs in the order in which they are stored. Serial access is slower than the direct access provided by media such as diskettes. When you want to access some particular data from the tape, the device has to scan through all the data you do not need but that data is stored before the required one. The access speed can be quite slow when the tape is long and what you want is not near the start. So the best use of the magnetic tape is for the data that you do not use very often.

Magnetic Disk
A magnetic disk is a metal or plastic disk coated with magnetic material. Data is recorded into the magnetic material in machine code. Disks have became popular due to their random access. These disk spin in the disk drives Read/Write heads moving on the disk along its radius can read data at any location under the heads.
Different types of disk drives and magnetic disks are in use. The most popular types of magnetic disks are interchangeable disks called the floppy disks and fixed disks called the hard disks.
Generally PCs are configured with at least one hard disk drive and on floppy disk drive. Hard Disk provided a very big storage capacity and are very fast but these are not inter changeable. Floppy disks are smaller than the hard disk, because they always consists only one platter. Floppies are available in size of 3.5" and 5.25" inches. Both of them are available in double density and high-density storage capacities. The storage capacities of these diskettes used for IBM compatible computer.
Floppy Disk, although slower than a hard disk, are inexpensive and can easily moved from computer to computer. They are comparatively fragile and must be handled with care.

Difference in the Capacity Between Hard Disk and Floppy Disk
A hard disk contains several disk platters stacked on a single rotating spindle. Data are stored on all recording surfaces. For a disk with four platters, there are eight recording surfaces on which data can be stored. A floppy disk has only one platter on which data is written, that is why the storage capacity of a hard disk is more as compared to the floppy disk. Hard disks or fixed disks are also called Winchester disk are permanently installed or fixed with in the system unit.