RAM is an acronym for random-access memory. It is used to temporarily store the programs currently in use by the user and by the main processing unit of the computer, the central-processing unit (CPU). When you first turn your computer on, the RAM is empty. There will be no data and no information in it until you open or load a program. These programs are originally permanently stored in the external storage devices, like a hard disk, a floppy disk, or a CD-ROM. When you open up the program, it is copied from the external device it is located on to the RAM, which is directly connected to the CPU. This is because if the CPU was to directly access this program from the external storage device, it would be extremely slow! However, programs cannot be permanently stored on the RAM because the RAM, unlike hard drives, is not big enough to store all the programs a computer system contains. RAMs are also expensive, so if you did make a computer with a large RAM, only millionaires like Bill Gates would be able to afford it!
The RAM is useful because it can feed information to the central processor at extremely high speeds. However, it does not permanently store information. The RAM is volatile, which means that when you exit out of a program or when the computer is turned off, all the information stored in it will be erased. This is why you need to save all of your work before you close a program. Once the program is closed, it is purged from the RAM, and if the information was not saved to a permanent location like the hard drive or a floppy disk, which are non-volatile (meaning that the information stored on them will not be deleted when the computer is shut off), then it will be lost forever!
The RAM is a read-write memory. As stated above, the information in a read-write memory can be read, and the new information can be written into the memory.
The most common form of RAM is the DRAM, or dynamic random-access memory. DRAMs have the spreadsheet layout mentioned above, and these rows and columns are etched into a tiny silicon chip. Each cell in a DRAM contains one bit of data (it contains either a 1 or a 0). The two major components of a DRAM are transistors and capacitors. The capacitor holds the bit of data, while the transistor acts like a switch that allows the information held by the capacitor to be read or for the state of the capacitor to be changed (i.e. for the bit to be changed from a 1 to a 0 or from a 0 to a 1).
The capacitor can be described as a bucket that has a leak in it. If the "bucket" is full of electrons, then the bit of information it stores will be a 1. If the bucket is empty, then a 0 will be stored. However, because this bucket has a leak, then in a few milliseconds, a full bucket can become empty! The memory has to be constantly recharged. This refresh function is automatically performed 1000s of times every second by electrical pulses. If the DRAM is not constantly refreshed, then it will lose all of the data it contains.
To write information into the DRAM, you must make sure all of the cells have the right state. The capacitor in the cell must either be full of electrons to represent a 1, or empty to represent a 0. When reading the information, a capacitor with a level of charge of more than 50% would be read as a 1, while less than 50% is read as a 0.
The memory cells in a DRAM are organized
so that the CPU can access the information in one cell in the same amount
of time as it can access information in another cell.
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