ROM (Read Only Memory)
The first classification of memory is ROM. The data in this memory can only read, no writing is allowed. It is used to store permanent programs. It a nonvolatile type of memory.
The classification of ROM memory is as follows
- Masked ROM: the program or data permanently installed at the time of manufacturing as per requirement. The data cannot be altered. The process of permanent recording expensive but economic for large quantities.
- PROM (Programmable Read Only Memory): The basic function same as that of masked ROM. but in PROM, we have fuse links. Depending upon the bit pattern, the fuse can burn or keep intact. This job performed by PROM programmer. To do this, it uses high current pulse between two lines. Because of high current, the fuse will get burnt; effectively making two lines open. Once a PROM is programmed we cannot change connections, only a facility provided over masked ROM is, the user can load this program into it. The disadvantage a chance of re-growing of the fuse and changes the programmed data because of aging.
EPROM (Erasable Programmable Read Only Memory)
- the EPROM is programmable by the user. It uses MOS circuitry to store data. They store 1’s and 0 ’s in form of charge. The information stored can erase by exposing the memory to ultraviolet light which erases the data stored in all memory locations. For ultraviolet light, a quartz window provided which covered during normal operation. Upon erasing it can be reprogrammed by using EPROM programmer. This type of memory used in a project developed and for experiment use. Also, The advantage is it can programme erased and reprogrammed. The disadvantage all the data get erased even if you want to change single data bit.
- EEPROM: EEPROM stands for electrically erasable programmable read-only memory. This is similar to EPROM except that the erasing is done by electrical signals instead of ultraviolet light. The main advantage is the memory location can be selectively erased and reprogrammed. But the manufacturing process complex and expensive so do not commonly used.
R/W Memory (Read/Write Memory)
The RAM also called read/write memory. The RAM is a volatile type of memory. It allows the programmer to read or write data. If the user wants to check the execution of any program, user feeds the program in RAM memory and executes it. The result of execution then checked by either reading memory location contents or by register contents.
Following is the classification of RAM memory. It is available in two types
SRAM (Static RAM)
SRAM consists of the flip-flop; using either transistor or MOS.
Moreover, for each bit we require one flip-flop. Bit status will remain as it is; unless and until you perform next write operation or power supply switched off.
Advantages of SRAM
- Fast memory (less access time)
- The refreshing circuit not required.
Disadvantages of SRAM
- Low package density
DRAM (Dynamic RAM)
In this Classification Memory a data stored in form of charge in capacitors. When data 1, the capacitor will charge and if data 0, the capacitor will not charge. Because of capacitor leakage currents, the data will not hold by these cells. So the DRAMs require refreshing of memory cells. It a process in which same data read and written after a fixed interval.
Advantages of DRAM
- High package density
- Low cost
Disadvantages of DRAM
- Also, Required refreshing circuit to maintain or refresh charge on the capacitor, every after few milliseconds.
- Magnetic Disk: The Magnetic Disk Flat, a circular platter with a metallic coating that rotated beneath reading/write heads. It is a Random access device; read/write head can move to any location on the platter
- Floppy Disk: These small removable disks that plastic coated with magnetic recording material. Floppy disks are typically 3.5″ in size (diameter) and can hold 1.44 MB of data. Also, This portable storage device a rewritable media and can reuse a number of times. Floppy disks commonly used to move files between different computers. The main disadvantage of floppy disks is that they can damage easily and, therefore, are not very reliable. Moreover, The following figure shows an example of the floppy disk. Figure 3 shows a picture of the floppy disk.
- Hard Disk: Another form of auxiliary storage a hard disk. A hard disk consists of one or more rigid metal plates coated with a metal oxide material that allows data to magnetically recorded on the surface of the platters. The hard disk platters spin at 5 a high rate of speed, typically 5400 to 7200 revolutions per minute ( RPM).Storage capacities of hard disks for personal computers range from 10 GB to 120 GB (one billion bytes are called a gigabyte).
- Optical Disks: Optical Mass Storage Devices Store bit values as variations in light reflection. They have higher area density & longer data life than magnetic storage. They also standardized and relatively inexpensive. Their Uses: read-only storage with low-performance requirements, applications with high capacity requirements & where portability in a standardized format is needed.
Types of Optical Disk: Classification Memory
- CD-ROM (read-only)
- CD-R: (record) to a CD
- CD-RW: can write and erase CD to reuse it (re-writable)
- DVD(Digital Video Disk )