Modern computer processors can perform several billion operations per second, creating and changing incredible amounts of data in a short period of time. To perform at this level, they have to be able to juggle the information they process, to have someplace to store it until it is needed again for modification or reference.
Computers have a memory structure which can be easily (if somewhat sloppily) compared to the human brain. The hard drive provides long-term memory storage similar to our long-term memories, a place where data is put to be permanently stored. RAM (Random Access Memory) provides a pallet that the computer can work from in normal operation, similar to our short-term memory. It holds information that is essential now but may or may not be transferred to long-term memory, depending on need.
Modern processors also include a memory cache, a comparatively small amount of high-speed memory which stores the data that is currently being used most often. This could be compared to our awareness, the memory that connects one moment to the next and keeps us doing what we were doing a second ago.
Random Access Memory (RAM) can be thought of as the short-term memory, in the sense that once the power is turned off, all information stored there is not saved. All modern computers have hard drives which store data permanently as magnetic information, but even with the improved speed of today's hard drive technology. Hard drives are still too slow to keep up with the needs of the processor since it can operate on considerably more information per second than can possibly be transferred to and from the hard drive.
This is where the need for a fast, short-term memory solution comes in, a memory space that provides very fast access for the processor so data can be written and read as needed without slowing down the system appreciably.
RAM fulfills this need, specifically DRAM (Dynamic RAM), the template for all modern memory types.
DRAM consists of semiconductor chips arranged on a small circuit board, each containing a logical arrangement of cells laid out in rows and columns. These cells use a combination of a capacitor and a transistor to achieve one of two states, filled with electrons (1) or empty (0), thus allowing binary (digital) information to be stored.
A capacitor is like a small bucket that is able to store electrons. To store a 1 in the memory cell, the bucket is filled with electrons. To store a 0, it is emptied. The problem with the capacitor's bucket is that it has a leak. In a matter of a few milliseconds a full bucket becomes empty. Therefore, for dynamic memory to work, either the CPU or the memory controller has to come along and recharge all of the capacitors holding a 1 before they discharge. To do this, the memory controller reads the memory and then writes it right back. This refresh operation happens automatically thousands of times per second.
This refresh operation is where dynamic RAM gets its name. Dynamic RAM has to be dynamically refreshed all of the time or it forgets what it is holding. The downside of all of this refreshing is that it takes time and slows down the memory.
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