The term random access memory (RAM) is commonly used as a synonym for the main memory (also called primary memory or just memory) of a computer. This is because such memory is generally composed entirely of RAM chips.
The main memory is used to hold portions of the operating system, application programs and data that are currently being used or which are frequently used. This includes the kernel (i.e., the core of the operating system), which is the first part of the operating system to load into memory during booting and which remains there for the entire duration of a computer session. Booting is the process by which a computer starts and automatically loads the operating system into memory.
Physically, RAM consists of a number of DRAM (dynamic RAM) chips, which are combined into modules that are plugged into slots on the motherboard (i.e., the main circuit board on a computer). DRAM is a type of RAM that features relatively low cost and small space consumption. Additional DRAM modules can be added (by even a minimally skilled user) to improve computer performance as long as slots are available on the motherboard.
Characteristics of RAM
RAM derives its name from the fact that its contents can be accessed (i.e., read from and written to) in any order. (Perhaps it should have more descriptively been named non-sequential memory, as it is not accessed in a random manner.) This is in contrast to sequential storage devices and media, such as hard disk drives (HDDs), floppy disks, CDROMs and magnetic tape, for which the data must be accessed to some extent in a fixed order. Thus, the times required to access any locations in RAM are virtually identical, in contrast to disks and tapes, which have delay times that vary according to the location on the media.
RAM features much higher access speeds than storage devices. This is due to the fact that access is entirely electronic and thus there are no moving parts (e.g., magnetic heads and spindle motors) to slow things down. RAM access time is expressed in nanoseconds (millionths of a second), whereas HDD access time is expressed in milliseconds (thousandths of a second).
Another characteristic of RAM is that it is volatile, in contrast to ROM (read-only memory) and storage devices and media. This means that any data stored in RAM is retained only as long as the RAM chips are connected to a power supply. In the case of DRAM chips, moreover, not only must they be connected to a power supply, but also they must be refreshed at frequent intervals (i.e., multiple times per second) by an electric current, hence the term dynamic in their name. The reason is that each bit of data is stored as a charge in a microscopic capacitor, and such charges quickly dissipate without refreshment. Thus, when the power supply is interrupted (e.g., the computer is turned off), the memory contents are lost. When a computer is turned on, the operating system and other files are once again loaded into RAM, usually from the HDD.
SRAM (static RAM) is another type of RAM that is faster and more reliable than DRAM. The term static is derived from the fact that it does not need to be refreshed as does DRAM, although it is still volatile (i.e., it needs to be connected to a power supply in order to retain its contents). SRAM has the disadvantages that it holds less data (and thus consumes more space) than DRAM and that it is considerably more expensive. SRAM should not be confused with SDRAM (synchronous DRAM), which is a frequently used type of DRAM that is synchronized with the clock speed of the CPU (central processing unit) in order to boost processing speed.
DRAM is the least expensive type of RAM, and it also has a relatively small space consumption. Because of the large amount of RAM that is required for modern computers and the desire for low cost, DRAM is generally the only type of RAM that is used for the main memory.
The main memory of the first electronic computers (e.g., ENIAC) consisted of a special type of vacuum tube (named the Williams tube after one of its inventors). However, these tubes were soon replaced by the much more reliable and considerably less bulky ferrite core memories, which were composed of small, ring-shaped pieces of an electromagnetic material that were connected by wires in a grid arrangement.
The first single-transistor DRAM cell (which could hold a single bit of data) was developed in 1966 by Dr. Robert H. Dennard at IBM. In 1970, the newly formed Intel Corporation released the world's first commercially available DRAM chip, the 1103, which had a capacity of about a thousand bits. This best-selling product featured a much lower cost and smaller size than comparable core memories and immediately began replacing them.
RAM technology subsequently improved at a swift pace as a result of the ability to produce ever finer circuit line widths and the consequent ability to cram more and more circuit elements into a single chip. These improvements include faster speeds, smaller chip sizes and lower power consumption. At the same time, RAM prices have continued to come down at a dramatic rate.
As of mid-2006, the largest DRAM chips being mass produced contained more than one billion transistors and had a capacity of one gigabit (i.e., one billion bits). This vast capacity has been made possible through the development of techniques to reduce the average circuit line width to less than a tenth of a micron (a micron is one millionth of a meter or one thousandth of a millimeter). Moreover, 4G (four gigabit) DRAMs have been developed and large-scale production is expected to begin in the near future.
More is Better
Having more RAM in a computer can speed up its operation. This is because it reduces the number of times that the CPU has to access the HDD (i.e., read data from it and write data to it), an operation that takes much longer than reading data from or writing data to RAM.
Although the amount of RAM on computers has continued to increase, so have the requirements for RAM from memory-hungry application programs. As of mid-2006, most personal computers being sold contained between 512MB (megabytes) and 2GB (gigabytes) of RAM. While this is huge in comparison to what was common just a few years ago, it is still sometimes insufficient to simultaneously operate all of the programs that users attempt to run. The reason is that the size of many programs has continued to increase along with the growth in memory sizes and hard disk capacities, largely in order to add more features (including fancier graphics).
When all of the RAM is being used (e.g., if there are many programs open simultaneously or if one very large program is in use), a computer can employ virtual memory to effectively increase the system's total memory. Virtual memory is the utilization of space on a HDD to free up space in the RAM and thus simulate additional main memory. The operating system accomplishes this by transferring data that is not immediately needed from the RAM to swap space on the HDD, and when that data is required again, by copying it back into memory.
Created July 9, 2004. Updated June 10, 2006.