Main Memory

The main memory is the central unit of the computer system. It is relatively large and fast memory to store programs and data during there execution. These memories are designed with semiconductor integrated circuits. The basic element of the semiconductor memory is the memory cell. 

The memory cell has three functional terminals which carries the electrical signal.                        

• The select terminal: It selects the cell.                                                                                               
• The data in terminal: It is used to input data as 0 or 1 and data out or sense terminal is used for the  output of the cell's state.                                                                                                              
• The control terminal: It controls the function i.e. it indicates read and write. 

Most of the main memory in a general purpose computer is made up of RAM integrated circuits chips, but a portion of the memory may be constructed with ROM chips.

RAM– Random Access memory: 

• Memory cells can be accessed for information transfer from any desired random location. 
• The process of locating a word in memory is the same and requires of locating a word in memory is the same and requires an equal amount of time no matter where the cells are located physically in memory thus named 'Random access'.  
• Integrated RAM are available in two possible operating modes, Static and Dynamic. 

SRAM versus DRAM 

• Both volatile, Power needed to preserve data 
• Static RAM 

Uses flip flop to store information 

Needs more space o Faster, digital device 

Expensive, big in size 

Don't require refreshing circuit 

Used in cache memory

• Dynamic RAM 

Uses capacitor to store information 

More dense i.e. more cells can be accommodated per unit area 

Slower, analog device o Less expensive, small in size  

Needs refreshing circuit 

Used in main memory, larger memory units 

ROM– Read Only memory: 

Read only memory (ROM) contains a permanent pattern of data that cannot be changed. 

A ROM is non-volatile that is no power source is required to maintain the bit values in memory. 

While it is possible to read a ROM, it is not possible to write new data into it. 

The data or program is permanently presented in main memory and never be loaded from a secondary storage device with the advantage of ROM. 

A ROM is created like any other integrated circuit chip, with the data actually wired into the chip as part of the fabrication process. 

It presents two problems:

• The data insertion step includes a relatively large fixed cost, whether one or thousands of copies of a particular ROM are fabricated. 
• There is no room for error. If one bit is wrong, the whole batch of ROM must be thrown out. 

Types of ROM 

Programmable ROM (PROM) 

• It is non-volatile and may be written into only once. The writing process is performed electrically and may be performed by a supplier or customer at a time later than the original chip fabrication. 

Erasable Programmable ROM (EPROM) 

• It is read and written electrically. However, before a write operation, all the storage cells must be erased to the same initial state by exposure of the packaged chip to ultraviolet radiation (UV ray). 
• Erasure is performed by shining an intense ultraviolet light through a window that is designed into the memory chip. 
• EPROM is optically managed and more expensive than PROM, but it has the advantage of the multiple update capability. 

Electrically Erasable programmable ROM (EEPROM) 

• This is a read mostly memory that can be written into at any time without erasing prior contents, only the byte or byte addresses are updated. 
• The write operation takes considerably longer than the read operation, on the order of several hundred microseconds per byte. 
• The EEPROM combines the advantage of non-volatility with the flexibility of being updatable in place, using ordinary bus control, addresses and data lines. 
• EEPROM is more expensive than EPROM and also is less dense, supporting fewer bits per chip. 

Flash Memory 

• Flash memory is also the semiconductor memory and because of the speed with which it can be reprogrammed, it is termed as flash. It is interpreted between EPROM and EEPROM in both cost and functionality. 
• Like EEPROM, flash memory uses an electrical erasing technology. 
• An entire flash memory can be erased in one or a few seconds, which is much faster than EPROM. 
• In addition, it is possible to erase just blocks of memory rather than an entire chip. However, flash memory doesn't provide byte level erasure, a section of memory cells are erased in an action or 'flash'.