Introduction to Memory Devices
Computers and many electronic gadgets usually rely on stored information which is mainly data which can be used to direct circuit actions. The digital information is stored in memory devices. Memories can be divided into 2 categories based on what memory cells can be accessed at a given instant.
SAM (Sequentially Access Memory) is accessed by stepping through each memory location until the desired location is reached. Magnetic tape is an example of SAM.
The second category of memory devices is called RAM (Random Access Memory) where the memory can be randomly accessed at any instant, without having to step through each memory location. It is generally faster to access a RAM compared to SAM. Most of the electronics gadgets memory are of RAM type.
Random Access Memory (RAM) Memory Device
RAM memory is "volatile" which means that the information stored in the RAM will be lost once the power to it is removed. Two common types of RAM are DRAM (Dynamic RAM) and SRAM (Static RAM). DRAM stored a bit as the presence or absence of charge on MOSFET gate substrate capacitance.
As the capacitance has leakage, it must be refreshed every few miliseconds. SRAM is an array of flip flops of which the bit stored in the flip flop will remain until power is removed or another bit replaces it. SRAM does not need to be refreshed. DRAM is usually 1.5 to 4 times as dense as SRAM and hence cheaper. However, SRAM has faster access times than DRAM.
Read Only Memory (ROM) Memory Device
ROM is non volatile in that its contents are not lost when power to it is removed. All ROMs can be programmed at least once. Mask ROMs are programmed by having "1"s and "0"s etched into their semiconductors at the time of manufacturing.
Programmable ROM (PROM) can be written after manufacturing by electrically burning specific transistors or diodes in the memory array. EPROM can be erased and reprogrammed by using ultraviolet light.
EEPROM (electronically erasable PROM) data can be erased electronically but it takes longer time compared to RAM. Read and write time for RAM is almost the same but PROM has slower write times. PROM must be erased before they can be reprogrammed and it often needs a higher programming voltage than its operating voltage.
ROM is usually used to store data or programs that do not change frequently and must still be there when power supply cuts off.
Non Volatile RAM (NVRAM) Memory Device
A NVRAM chip consists of both RAM and ROM. During power on reset, the contents of the ROM are copied to RAM. Before the power turns off, the system will copy the entire contents of the RAM into ROM for non volatile storage. The RAM in an NVRAM is called shadow RAM. NVRAM fills the gap between easily written memory and non volatile memory.
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