From LQWiki
RAM - Random Access Memory.
The term comes from the way data is accessed in this memory, and symbolizes the possibility to access any part of the memory at any given time. The contrary would be sequential memory, which mostly has a meaning in tape backup situations. Early computers also used drum memory, which was sequential (kind of).
All computer memory modules used in computers today are of the RAM type. This type of memory can be subdivided into two main groups:
The difference between the two is that as long as power is maintained on the memory modules the DRAM will hold it's information. The content of DRAM memory disappears from the memory within milliseconds, so in order to maintain it's data it has to be refreshed periodically. This makes the DRAM memory much slower than the SRAM.
The computer memory you usually come in contact with are a form of DRAM, like SDRAM, RDRAM, DDR-SDRAM. Because SRAM doesn't have to be refreshed it is much faster than DRAM and also much more expensive.
The speed advantage of SRAM means that it is used in the computers cache memory, which is a small amount of high speed SRAM memory mounted close to or on the processor itself.
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Pedantic Quibble
All semiconductor memory e.g. ROM can be accessed or addressed non-sequentially but that is not random (if data were written randomly how would it be found again?). If the mists of time were cleared the origins of RAM would probably be found to be an alliteration of ROM to both associate and distinguish read/write memory from read only memory.
However, drum memory is (kind of) sequential, in that you would have to wait for your data to come under the read/write heads. The "random" part of RAM might have arisen during the switch-over to fully random-accessible core memory. (Any oldtimers out there want to weigh in?)
Earlier Forms of Memory
One of the earliest forms of memory was drum memory, which was similar to today's hard drives, except that the media was cylindrical instead of disc-shaped. This was the kind of memory used by the computer in Story of Mel. Drum memory was replaced by core memory, which consisted of ferrite "cores" that were hand-strung with conductive wires. Core memory lives on in computer terms such as core dump.
RAM under Linux
If your system does not find all the installed RAM, you may need to reconfigure your kernel. Enable himem in your kernel configuration: Processor type and features -> 'High memory support'. Setting it to '4GB' should solve the problem.
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