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Classification of ROM

ROM (Read Only Memory) The data is permanently stored in the memory and these devices are mask programmed during manufacturing. ROMs cannot be reprogrammed and are of nonvolatile type. These devices are cheaper than programmable memory devices. The applications of ROM are fixed programmed instructions, look-up tables, conversions, and some specific operations.
 

Classification of ROM
 
PROM (Programmable Read Only Memory) The data can be electrically stored. It can be programmed by blowing built-in fuses. PROM can be reprogrammed and is of nonvolatile type. These memory devices are of very low memory density and occupy more space.
 
EPROM (Erasable Programmable Read Only Memory) These are strictly a MOS device and programmed by storing charge on insulated gates. These devices are erasable with ultraviolet rays and reprogrammable after erasing. These memory devices are of nonvolatile type.
 
EEPROM (Electrically Erasable Programmable Read Only Memory) These memory devices are electrically programmable by the programmer and the stored data can be erased by ultraviolet light. This is of nonvolatile type. This is also called Electrically Alterable Programmable Read Only Memory (EAPROM).

Classification of RAM

Table : Comparison of memories
 
Types of Memory Category Erasing property Writing mechanism Volatility
Read only Memory (ROM) Read only Memory Not possible Masks Nonvolatile
Programmable ROM (PROM)   Not possible Electrically Nonvolatile
Erasable PROM (EPROM)   Ultra-violet light and chip level Electrically Nonvolatile
Electrically Erasable PROM   Electrically and byte level Electrically Nonvolatile
(EPROM)         
Flash memory   Electrically and block level Electrically Nonvolatile
Random access memory (RAM) Read and write memory Electrically and byte level Electrically Volatile
 

Static RAMs  These RAMs are built with static or dynamic cells. Five or six transistors are used to store a single bit. Data can be written and read in nanoseconds. Usually TTL, ECL, NMOS and CMOS technology are used to manufacture static RAMs. When the power is shut off, data stored in cells can be lost.
 
Dynamic RAMs In a dynamic memory, data can be stored on capacitors and to retain data every cell has to be refreshed periodically. One transistor is used to build a memory cell and requires less space. These memories consume less power compared to static RAMs. The comparison between SRAM and DRAM is given below:
 
Table: Comparison of Static RAM and Dynamic RAM
 
Static RAM Dynamic RAM
Stored data will not be changed with time. Stored data will be changed with time.
 
Consume more power. Consume less power than static RAM.
SRAM is expensive. SRAM is less expensive.
SRAM has less packing density. DRAM has high packing density.
These memories are not easy to construct. These memories construction are simple.
No refreshing is required. As refreshing is required, additional circuit is incorporated with memory.
No maintenance is required. Maintenance is required.
 
Classification of RAM

Comparison of  static RAM and Dynamic RAM Memories

Table:- Comparison of Static and Dynamic RAM
 
Static RAM Dynamic RAM
Stored data is retained as long as power is ON. Stored data will be erased and repeated refreshing is required to store data.
Stored data will not be changed with time. Stored data will be changed with time.
Consume more power. Consume less power than static RAM.
SRAM is expensive. SRAM is less expensive.
SRAM has less packing density. DRAM has high packing density.
These memories are not easy to construct. These memories construction are simple.
No refreshing is required. As refreshing is required, additional circuit is incorporated with memory.
No maintenance is required. Maintenance is required.
 




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