Dynamic RAM (DRAM)
Dynamic RAM is a type of RAM chip used for the main memory for modern computers. It is slower and less expensive. It has an access time of 50 to 70 nanoseconds (ns). High density is the biggest benefit of DRAM. By high density it is meant that numerous bits can be packed into a very small chip.
In DRAM, each data bit is stored as an electrical charge within a cell. A DRAM chip has memory cells which are tiny capacitors that retain a charge indicating a bit. Since DRAM is dynamic in nature, this causes a problem. Its content changes with every keystroke or mouse swipe. It is so designed that it must constantly be refreshed else the electric charge in capacitors will drain and result in data loss. Refreshing the memory consumes processor time and takes it away from other tasks.
In DRAM, a pair of transistor and capacitor is used for each bit, making it dense and offering more memory capacity than other types of memory. The transistor is responsible for reading the charge state of the adjacent capacitor. The presence of charge in a capacitor is indicated by 1 and no charge is indicated by 0. The memory must constantly be refreshed because the charge in the capacitors drains out. Any interference with the refresh cycle or in the power supply can cause the cells to lose the charge and subsequently the data. This may lead to blue screen, corrupted files and system crash.
DRAM is popularly used in PC systems because it is economical and offers a lot of capacity while occupying very little space. However, it is much slower than the processor. Therefore, many architectures of DRAM have been developed to improve the performance. These will be discussed as we proceed with the session.