Memory in a computer system is required for storage on subsequent retrieval of the instructions and data. Memory measured in bytes.
8 bits - 1 Byte
(BIT stands for Binary Digit. That means 0 and 1)
1024 Bytes - 1 KB (Kilo Byte)
1024 K Bytes - 1 MB (Mega Byte)
1024 M Bytes - 1 GB (Giga Byte)
1024 G Bytes - 1 TB (Tera Byte)
In computers there are two types of memories.
1). PRIMARY MEMORY (OR) MAIN MEMORY:
Primary memory consists of semiconductor memory chips and is used to store the data and programs currently in used. Each storage element of memory is directly (Randomly) accessible and can be examined and modified without effecting other cells and hence primary memory is also called RAM (Random Access Memory). Main memory stores a variety of critical information required for processing the CPU. The memory unit stores all the information in memory cells also called memory locations in binary digits. Each memory location has a UNIQUE address. The RAM is a volatile memory.
2). SECONDARY MEMORY (OR) AUXILIARY MEMORY:
Secondary memory is used to store the information permanently. It is non-volatile memory. This is also called auxiliary or permanent memory. The devices which are used for permanent memory are called as secondary storage medias. Which is external to a computer. The following are some of the commonly available categories of secondary
storage devices.
MAGNETIC TAPES: There are used as external storage devices apart from being used as high speed input and output devices. These devices use a tape which is about 12.5mm to 25mm wide and about 500 meters to 1200 meters long. This tape is usually made of a plastic known as mylar. This tape is quite similar to the audio cassette tape. The tape is coated with very thin layer of iron oxide, a magnetic material.
MAGNETIC DISKS: Magnetic disks are very popular means of secondary storage. A magnetic disk is usually made up of plastic like material called mylar coated with ferromagnetic materials. These disks come in various sizes. These magnetic disks can be divided into two major categories. They are:
a). floppy disks (b). fixed or hard disks
All these disk surfaces are composed on concentric circles called tracks. Data is stored on these tracks. The tracks are further divided into sectors.
OPTICAL DISKS: Optical disks are alternate mass storage devices with huge capacity. In optical storage devices the information is written using laser beam. These devices which are memories can store large amount of data.Ex: CD (Compact Disc), DVD (Digital Versatile Disc)
FLOPPY DISKS:
A floppy disk is small, flexible, faster and cheap alternative to storage using magnetic film. This was developed in early 1970's, is one of the popular media for data storage. This disk is made up of flexible plastic material, coated with magnetic materials like ferrous oxide. The floppy disk is packed in protective paper or plastic envelope. All the disks have two sides. Floppy diskette comes in two
basic sizes at present as follows:
a). 5 1/4 inch floppy disk (mini floppy)
b). 3 1/2 inch floppy disk (micro floppy)
The floppy's surface has to be divided into tracks and sectors.
a). MINI FLOPPY: These floppy's are again divided into two tydependends upon their storage capacity.
1. DSDD (Double Sided Double Density)
2. DSHD (Double Sided High Density)
DSDD DSHD
The maximum capacity is 360 KB The maximum capacity is 1.2 MB
40 tracks per side (TPS) 80 tracks per side (TPS)
9 sectors per track (SPT) 15 sectors per track (SPT)
Each sector can store 512 Bytes Each sector can store 512 Bytes
b). MICRO FLOPPY: Micro floppy's are again divided into 2 types.
The maximum capacity is 720 KB The maximum capacity is 1.44 MB
80 tracks per side (TPS) 80 tracks per side (TPS)
9 sectors per track (SPT) 18 sectors per track (SPT)
Each sector can store 512 Bytes Each sector can store 512 Bytes
HARD DISK
Hard disk also called as fixed disk. These fixed disk is usually made from aluminum or other materials instead of mylar or other plastic like material. These are non-removable disks,which typically holds 10 MB to 160 GB of data. These are provide substantially fasterdata access compared to floppy disk and provide very large data storage for on-line retrieval. These disks have multiple platters, which are stacked vertically, normally at a distance of an inch. This is known as a disk pack. A disk pack can contain multiple heads mounted with the same arm. This entire pack is called as Hard Disk Drive.
COMPACT DISC
The compact disc is creating revolution in the secondary storage media. In CD's the information is stored evenly across the disk in segments of the same size. As discussed earlier, the data is stored sequentially along a spiral track. In this disc random access becomes more difficult because locating a desired address involves first moving the head to the specific area then adjusting the rotation speed and then reading the address, and then to find and access the specific sector.