RAM AND ROM
- Firstly, there are many memory arrays to store the data and those are divided into many ways:
RAM: Random Access Memories are volatile in nature. When the power is supplied then only we can retrieve the data and when power gets interrupts data is lost. RAM is also called PERMANENT storage memory. For volatile memory, there must refresh the memory data periodically. RAM is divided into two ways:
Static RAM (SRAM):
- Memory behaves like latches or flip-flops
- SRAM cell stores only one bit with a six-transistor circuit.
- Retains esteem uncertainly, as long as it is kept controlled.
- Relatively harsh to unsettling influences, for example, electrical commotion.
- Faster and more costly than DRAM.
- SRAM cell is utilized for reserve memory.
- It involves more space than the DRAM cell.
Dynamic RAM (DRAM):
- The memory lasts only for a few milliseconds.
- DRAM cells must “refresh” locations by reading or writing.
- DRAM cell stores only one bit with a capacitor and a transistor.
- The esteem must be revived each 10-100 ms.
- Sensitive to unsettling influences.
- Slower and less expensive than SRAM.
- A DRAM cell is utilized for the Main memory.
- It involves less space than the SRAM cell.
ROM: READ ONLY MEMORY are non-volatile in nature. When there is no power also it can hold the data if, there is any power interruption also data will not be lost. ROM is also called as TEMPORARY storage memory. As this is non-volatile memory, it does not require the memory data to be refresh periodically. ROM is divided into five types:
- Mask ROM
- Flash ROM
- Masked ROMs are non-volatile memories in which the information is permanently stored through the use of custom masks during fabrication.
- The terminology mask comes from integrated circuit fabrication, where regions of the chip are masked off during the process of photolithography.
PROM: The PROMs were originally manufactured in bipolar technology, but are now available in MOS technology as well, including the radiation hardened CMOS devices.
- Bipolar PROMs
- CMOS PROMs
- It is a ROM that can be electrically programmed in a system by using a special 12V programming pin.
- It can be erased by removing from the system and exposing the ceramic package with a quartz window to ultraviolet light about 20 min.
- EEPROM takes advantage of an effect known as Fowler-Nordheim tunneling.
- In the best-known EEPROM technology, known as floating- gate tunneling oxide (FLOTOX), the PROGRAM and ERASE operations are carried out by electrons tunneling through the thin oxide.
- 8-bit data is erased at a time, hence slower.
- Flash memories are the devices for which the contents of all memory array cells can be erased simultaneously through the use of an electrical erase signal.
- They are based on their EPROM or EEPROM technology, and their selection for a particular application requires making tradeoffs between the higher densities of the EPROM technology versus the in-circuit programming flexibility of the EEPROM technology.
- Programmed using high electrical voltage. Erases data in blocks hence faster.