Main Memory: RAM & ROM

The memory unit that communicates directly within the CPU, Auxiliary memory and Cache memory, is called main memory. The Main Memory size is larger than cache memory and it has very fast speed to access programs and data stored in external memory. It communicates directly with the CPU. The Main Memory is slower compared to CPU registers and cache memory. It is also known as Primary Memory. Main memory is arguably the most used memory. When discussing computer algorithms such as quick sort, balanced binary sorted trees, or fast Fourier transform, one typically assumes that the algorithm operates on data stored in the main memory. The main memory is reasonably fast, with access speed around 100 nanoseconds. It also offers larger capacity at a lower cost. However, the main memory is volatile. Main memory is made up of RAM and ROM with RAM integrated circuit chips holing the major share.

RAM ( Random Access Memory ) :

RAM stands for Random Access Memory. "Random" means that the memory cells can be accessed in any order. RAM is the internal memory of the CPU for storing data, program, and program result. RAM is volatile memory, which means it read/write data in memory until the machine is working. As soon as the machine is switched off, data is erased. RAM is the most common type of memory found in computers and other devices. Basically there are two types of RAM :

DRAM ( Dynamic Random Access Memory ) : DRAM stands for Dynamic Random Access Memory. It makes use of a transistor and a capacitor to form a memory cell standing for a single bit of data.  They are not able to retain information for a longer time even if the power supply is on throughout. Therefore, it needs refreshing dynamically, and therefore is known as the dynamic type of memory. This is the most common type of computer memory.  It provides more memory per chip due to lesser number of components in comparison to SRAM. It is slower and less expensive than SRAM. This type of memory is primarily used for creating a large system RAM space.

SRAM ( Static Random Access memory ) :  It stands for Static Random Access Memory. In SRAM Flip flops are used for retaining memory. Four to six transistors are used by flip flops*  for a memory cell. This memory can story information till the power supply is on. Static RAM does not need to be refreshed, which makes it faster, but it is more expensive than dynamic RAM. In static RAM, a bit of data is stored using the state of a flip-flop. These are mainly used in processor’s cache memory . It does not require refreshing, as it is quiet faster in comparison to some other types.

ROM ( Random Access Memory ) : 

ROM stands for Read Only Memory. This is a non-volatile type of memory which is found in computers and other electronics devices. It is used for storing data permanently. The information is stored permanently in such memories during manufacture. A ROM stores such instructions that are required to start a computer. This operation is referred to as bootstrap. ROM chips are not only used in the computer but also in other electronic devices. Some of the common types of ROM are as follows :

MROM ( Masked ROM ) : It is a static ROM which comes programmed into an integrated circuit by its manufacturer. In this ROM a bit pattern is permanently recorded by a marking and Metalization process, which is an expensive and specialized one. Memory manufacturers are generally equipped to undertake this process. These kind of ROMs are known as masked ROMs, which are inexpensive.

PROM (Programmable ROM ) : PROM is read-only memory that can be modified only once by a user. Unlike RAM, PROM retains its contents when the computer is turned off. The difference between a PROM and a ROM is that a PROM is manufactured as blank memory and programmed later with a special device called PROM programmer or the PROM burner, whereas the ROM is programmed during manufacturing process. the process of programming a PROM is sometimes called burning a PROM. Inside the PROM chip, there are small fuses which are burnt open during programming. It can be programmed only once and is not erasable.

EPROM ( Erasable and Programmable ROM ) : An EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light. Once erased, it can be reprogrammed. An EPROM is similar to a PROM except that it requires ultraviolet radiation to be erased. For erasing this charge, ultra-violet light is passed through a quartz crystal window (lid). This exposure to ultra-violet light dissipates the charge. During normal use, the quartz lid is sealed with a sticker.

EEPROM ( Electrically Erasable and Programmable ROM ) : EEPROM is programmed and erased electrically. It can be erased and reprogrammed about ten thousand times. Both erasing and programming take about 4 to 10 ms (millisecond). In EEPROM, any location can be selectively erased and programmed. EEPROMs can be erased one byte at a time, rather than erasing the entire chip. Hence, the process of reprogramming is flexible but slow.  The EEPROM is used to store computer system’s BIOS. Unlike EPROM, the entire chip does not have to be erased for changing some portion of it. Thus, it even gets rid of some biggest challenges faced by using EPROMs.

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