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RAM is considered volatile storage because as soon as the power supply is stopped, the contents of RAM are lost. Hence, the data in RAM stays as long as the computer is running. RAM is much faster to read from and write to, when compared to other storage devices like hard disk, CDROM, Floppy disk etc. RAM can be both read and written; therefore, it is sometimes also called read-write memory.
There are two fundamental types of RAM:
(a) Dynamic RAM (DRAM) (b) Static RAM (SRAM)
(a) Dynamic RAM (DRAM): Dynamic random access memory is the most common kind of RAM, which stores each bit of a data in a separate capacitor and a transistor. Capacitors tend to leak electrons, thus the need for recharging arises. Hence, DRAM, unlike SRAM needs to have its storage cells refreshed or given an electric charge every few milliseconds. This refresh requirement operation is from where dynamic RAM gets its name. DRAM is a volatile memory because it loses data when the power supply is stopped.
DRAM’s structural simplicity gives it an advantage over SRAM. It consists of only one transistor and a capacitor per bit, due to which it has very high density. SRAM, on the other hand, requires six transistors.
There are three basic types of DRAM:
(i) Synchronous DRAM (SDRAM): While all the other types of DRAM work asynchronously to the system clock, Synchronous dynamic random access memory is a type of DRAM that synchronizes itself with the microprocessor clock speed. It is a type of RAM that is twice as fast as EDO DRAM. The speed of SDRAM is rated in MHz than in nanoseconds. SDRAM chips are mounted on DIMM memory modules. These memory modules are classified according to the CPU speed that they are designed to support.
(ii) RAMBus DRAM ( RDRAM): Rambus dynamic random access memory is a type of synchronous dynamic RAM, developed by Rambus Corporation. It is a high speed memory technology, which transfers data up to 800 MHz. Since, RDRAM narrows the memory bus to 16-bit, systems get more speed by running channels in parallel. RDRAM has heatsinks to dissipate heat, which is a major issue with RDRAM. Manufacturing of RDRAM is expensive as compared to other RAMs
RDRAM memory modules (RIMM) can only be used on the motherboards specifically designed for them. A 16-bit and 32-bit RDRAM RIMMS have 184 and 232 pins respectively.
(iii) DDR-SDRAM: stands for double-data-rate synchronous dynamic random access memory. It is a type of Memory integrated circuits that are used in computers. DDR transfers data over the bus over both (rising and falling) edges of the clock cycle, doubling the data throughput over that of standard SDRAM. DDR memory runs at bus speeds up to 400 MHz. DDR memory chips are mounted on 184-pin DIMM modules. DDR SDRAM consumes less power, hence is best suited for laptops.
(b) Static RAM (SRAM): Static random access memory is the kind of RAM that holds its data without external refresh, as long the power remains applied, unlike DRAM that needs to be periodically refreshed. It is a type of memory that is faster and more reliable than DRAM (Dynamic RAM). Since, Static RAM operates on the principle of moving current, it is a volatile memory.
There are three basic types of Static RAM (SRAM):
• Async SRAM: Short for asynchronous Static Random Access Memory. It is an older type of SRAM, which works independently of the system clock, that is, asynchronous.
• Sync SRAM: Short for synchronous Static Random Access Memory. This type of memory is synchronous with the system clock. This SRAM is more expensive.
• Pipeline Burst SRAM: This is the commonly used memory. Pipeline means larger packets of data are re sent to the memory at once and is acted on very quickly. This type of SRAM can operate at bus speed higher than 66 MHz.