Storage Unit:
A storage unit is that part of the computer system which is used to store the data and instructions to be proccessed. There are two types:
1-Primary Storage
2-Secondary Storage.
–:Characteristics Of Storage Unit:–
*Speed of operation of the computer.
*Efficiency
*Cost and amount of data which computer cam store.
PRIMARY STORAGE DEVICES
Alternatively referred to as internal memory, main memory, and primary memory, a primary storage device is a medium that holds memory for short periods of time while a computer is running. Although it has a much lower access time and faster performance, it is also about two orders of magnitude more costly than secondary storage.The Primary storage is volatile.
Normally,primary memory size ranges from a few kilobytes on small computer to several thousnand kilo bytes and megabytes on larger machines.
Characteristics of Main Memory :-
These are semiconductor memories.
It is known as main memory.
Usually volatile memory.
Data is lost in case power is switched off.
It is working memory of the computer.
Faster than secondary memories.
A computer cannot run without primary memory.
Types of primary storage:-
1.RAM
RAM(Random Access Memory) is the internal memory of the CPU for storing data, program and program result. It is read/write memory which stores data until the machine is working. As soon as the machine is switched off, data is erased.
Access time in RAM is independent of the address that is, each storage location inside the memory is as easy to reach as other locations and takes the same amount of time. Data in the RAM can be accessed randomly but it is very expensive.
RAM is volatile, i.e. data stored in it is lost when we switch off the computer or if there is a power failure. Hence a backup uninterruptible power system(UPS) is often used with computers. RAM is small, both in terms of its physical size and in the amount of data it can hold.
Functions:-
1-Stores data and insturctions that are currently being uesd by the CPU
2-Loses its content whenever it loses electricity(volatility)
3-Stores data that is waiting to be processed
4-Stores information that is waiting to be output and backed up.
2.ROM
ROM stands for Read Only Memory. The memory from which we can only read but cannot write on it. This type of memory is non-volatile. The information is stored permanently in such memories during manufacture. A ROM, stores such instructions that are required to start a computer. This operation is referred to as bootstrap. ROM chips are not only used in the computer but also in other electronic items like washing machine and microwave oven.
Functions:-
1-Stores content that cannot be edited(can only be read,hence its name)
2-The data on a ROM chip is added at the time of manufacturing,
hence cannot be changed
3-Stores instructions that tell that computer what to do,for eg.how to
load the operating system into RAM
4-It is really on;y suitable foe storing data which is not expected to
need modification for the life of the device
PROM(Programmable Read only Memory):-
PROM is read-only memory that can be modified only once by a user. The user buys a blank PROM and enters the desired contents using a PROM program. Inside the PROM chip there are small fuses which are burnt open during programming. It can be programmed only once and is not erasable
EPROM(Erasable and Programmable Read Only Memory):-
The EPROM can be erased by exposing it to ultra-violet light for a duration of up to 40 minutes. Usually, an EPROM eraser achieves this function. During programming, an electrical charge is trapped in an insulated gate region. The charge is retained for more than ten years because the charge has no leakage path. For erasing this charge, ultra-violet light is passed through a quartz crystal window(lid). This exposure to ultra-violet light dissipates the charge. During normal use the quartz lid is sealed with a sticker.
EEPROM (Electrically Erasable and Programmable Read Only Memory) :-
The EEPROM is programmed and erased electrically. It can be erased and reprogrammed about ten thousand times. Both erasing and programming take about 4 to 10 ms (milli second). In EEPROM, any location can be selectively erased and programmed. EEPROMs can be erased one byte at a time, rather than erasing the entire chip. Hence, the process of re-programming is flexible but slow.
3.Cache Memory
Cache Memory : The Cache Memory is the Memory which is very nearest to the CPU , all the Recent Instructions are Stored into the Cache Memory. The Cache Memory is attached for storing the input which is given by the user and which is necessary for the CPU to Perform a Task. But the Capacity of the Cache Memory is too low in compare to Memory and Hard Disk.
SECONDARY STORAGE DEVICES
Secondary Memory :-
This type of memory is also known as external memory or non-volatile. It is slower than 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. Contents of secondary memories are first transferred to main memory, and then CPU can access it. For example: disk, CD-ROM, DVD etc.
Characteristic of Secondary Memory :-
These are magnetic and optical memories.
It is known as backup memory.
It is 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 secondary memory.
Slower than primary memories.
Information stored on secondary devices can be accessed in two ways:
1-Sequential Access
2-Direct Access
1.Sequential Access
Sequential Access Memory (SAM) is a class of data storage devices that read their data in sequence. This is in contrast to random access memory (RAM) where data can be accessed in any order. Sequential access devices are usually a form of magnetic memory.these types of devices are useful in application like pay slip printing where the data is to be accessed one after the other.
Types Sequential access devices:-
a.Punch Paper Tape
Early method of data storage used with early computers. Punch cards also known as Hollerith cards and IBM cards are paper cards containing several punched holes that were punched by hand or machine to represent data. These cards allowed companies to store and access information by entering the card into the computer.The punched paper tapes are a low cost storange medium and their storage capacity unlimited.Nowdays,punched paper tapes are rarely being used.
b.Magnetic Tape
A magnetic tape is a ribbon of mylar which is coated with a thin layer of iron oxide material on one side.This tape is stored on a cassette or cartridge,or reel.The iron material can be magnetized and the data is recorded on the tape in the form of magnetized and non-magnetized spots.A magnetic tape drive is used to read data from the tape or write information to the tape.The tape drive has a read/write head to access or storage information respectively.
Physically,the tape medium is a plastic ribbon,which is usually 1/2 inch or 1/4 inch wide and 50 to 2400 feet long.
Advantages Of Magnetic Tape:
-High data density and virtually unlimited storage.
-low in cost.
-Easy to handle and portable from one computer to another.
Disadvantages Of Magnetic Tape:
-Support only Sequential access.
-Tapes are sensitive to dust,humidity and temperature,Hence require proper storage facilities.
2-Direct Access
In Direct Access the data at any location on the storage unit can be accessed directly without having to follow the sequence in which it has been stored.Typical devices that support direct access are the magnetic disk and magnetic drum.
a.Magnetic Disk
The most common physical device for storing files is the magnetic disk.Actually, a disk typically contains several rotating disks, or platters. The surfaces of the platters are covered in metal oxide, and read/written by electromagnetic recording heads, rather like those on an audio cassette recorder. There is one head for each surface, and all the heads move together. The disk rotates at around 3600 rpm (or approx 90mph), with the heads floating microscopic distances above the surfaces. Modern disks for workstations typically hold 500MB – 9GB, and cost of the order of £200 – 3000; prices are currently dropping rapidly.
The surface of a platter is organised as a number of concentric tracks. Each track is divided into sectors. The information held in one sector, a block, is the unit of transfer between the disk and primary memory (typically 4K bytes or so). The operating system determines where the blocks for each file are placed.
The time taken to access a particular block consists of:
- Time to move the heads to the right track – the seek time.
- Time waiting for the sector to come round to the head – the latency.
- Time to actually transfer the data – the block transfer time
b.Floppy Disks
Floppy disks are made up off flexible mylar coated with iron oxide.The disk is enclosed in a square plastic jacket to protect the surface of the disk from dust.A floppy disk is to be inserted in a floppy disk drive of the computer system to read or write information.The read/write head of the drive makes a direct contact with the floppy disk.
while accessing or storing data ,floppy disk comes in various sizes
-8 inch floppy disks
-5 1/4 inch floppy disks
-3 1/2 inch floppy disks.
c.winchester Disks
In a winchester,the disks are permanently enclosed in a sealed container.The disks are coated with a special lubricant to reduce friction with the read/write head.This technology allows for an increase in the number of tracks on the disks and higher storage density. winchester disks are fast and highly reliable.
d.Magnetic Drum
A magnetic drum, also referred to as drum, is a metal cylinder coated with magnetic iron-oxide material on which data and programs can be stored. Magnetic drums were once used as a primary storage device but have since been implemented as auxiliary storage devices.
Storage Devices 