Speed, Size and Cost of Memory

The choice of a RAM chip for a given application depends on several factors. Fore most among these factors are the speed, power dissipation, and size of the chip. Static RAMs are generally used only when very fast operation is the primary requirement. Their cost and size are adversely affected by the complexity of the circuit that realizes the basic cell. Dynamic RAMs are the predominant choice for implementing computer main memories. The high densities achievable in these chips make large memories economically feasible.

An ideal main memory would be fast, large and inexpensive. A static RAM are a very fast memories. But these chips are expensive because their basic cells have six transistors, which precludes packing a very large number of cells on to a single chip. Thus, for cost reasons, it is impractical to build a large memory using static RAM chips. The only alternative is to use a DRAM chips, which have much simpler basic cells and thus are much less expensive. But such memories are significantly slower.

Although Dynamic RAMs allow main memories in the range of tens of megabytes to be implemented at a reasonable cost, the affordable size is still small compared to the demands of large programs with voluminous data. A solution is provided by using secondary storage, mainly magnetic disks, to implement large memory spaces. Very large disks are available at a reasonable price and they are used extensively in computer systems. However, they are much slower than the main memory unit. A huge amount of cost-effective storage can be provided by magnetic disks. A large, yet affordable, main memory can be built with Dynamic RAM technology.

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