Different Types Of RAM

Static Ram (SRAM) and  Dynamic Ram (DRAM):

 The major difference between them is the lifetime of the data they store. SRAM retains its contents as long as electrical power is applied to the chip. If the power is turned off or lost temporarily, its contents will be lost forever.

DRAM is dynamic in that, unlike static RAM (SRAM), it needs to have its storage cells refreshed or given a new electronic charge every few milliseconds. Static RAM does not need refreshing because it operates on the principle of moving current that is switched in one of two directions rather than a storage cell that holds a charge in place. Static RAM is generally used for cache memory, which can be accessed more quickly than DRAM. 

DRAM stores each bit in a storage cell consisting of a capacitor and a transistor. Capacitors tend to lose their charge rather quickly; thus, the need for recharging.

DRAM Types :

SDRAM (synchronous DRAM) is a generic name for various kinds of dynamic random access memory (DRAM) that are synchronized with the clock speed that the microprocessor is optimized for. This tends to increase the number of instructions that the processor can perform in a given time. The speed of SDRAM is rated in MHz rather than in nanoseconds (ns). This makes it easier to compare the bus speed and the RAM chip speed. You can convert the RAM clock speed to nanoseconds by dividing the chip speed into 1 billion ns (which is one second). For example, an 83 MHz RAM would be equivalent to 12 ns.

RDRAM -- Rambus DRAM

RDRAM make sure that all memory sockets of a channel are filled with either a memory chip or a continuity module. 

RDRAM often has to be installed in pairs of the same type of memory chips.

RDRAM devices may be configured into single-, dual- or quad-channel RIMM modules. For dual-channel or quad-channel (4-channel) RDRAM chipsets and motherboards, memory module upgrades should be in matched pairs. For instance, to add 512MByte of memory into a dual or 4-channel system, two matched 256MByte modules should be inserted. 32bit RIMM modules, such as RIMM 4200, 4800, and 6400 modules, can be upgraded singly on dual channel systems.

DDR SDRAM -- Double Data Rate SDRAM

DDR SDRAM is synchronous dynamic RAM (SDRAM) that can theoretically improve memory clock speed to at least 200 MHz*. It activates output on both the rising and falling edge of the system clock rather than on just the rising edge, potentially doubling output. It's expected that a number of Socket 7 chipset makers will support this form of SDRAM. When released DDR SDRAM memory was about twice as expensive as conventional SDRAM memory.

Cache

Cache Memory is fast memory that serves as a buffer between the processor and main memory. The cache holds data that was recently used by the processor and saves a trip all the way back to slower main memory. The memory structure of PCs is often thought of as just main memory, but it's really a five or six level structure:

The first two levels of memory are contained in the processor itself, consisting of the processor's small internal memory, or registers, and L1 cache, which is the first level of cache, usually contained in the processor.

The third level of memory is the L2 cache, usually contained on the motherboard. However, the Celeron chip from Intel actually contains 128K of L2 cache within the form factor of the chip. More and more chip makers are planning to put this cache on board the processor itself. The benefit is that it will then run at the same speed as the processor, and cost less to put on the chip than to set up a bus and logic externally from the processor.

The fourth level, is being referred to as L3 cache. This cache used to be the L2 cache on the motherboard, but now that some processors include L1 and L2 cache on the chip, it becomes L3 cache. Usually, it runs slower than the processor, but faster than main memory.

The fifth level of memory is the main memory itself.

The sixth level is a piece of the hard disk used by the Operating System, usually called virtual memory. Most operating systems use this when they run out of main memory, but some use it in other ways as well.