A B C OF COMPUTING
[[Image:NEC 128mb ram.JPG|right|thumb|One module of 128MB NEC SD-RAM.]] Many computer systems have a memory hierarchy consisting of [[CPU register]]s, on-die [[Static random access memory|SRAM]] caches, external [[cache]]s, [[DRAM]], [[paging]] systems, and [[virtual memory]] or [[swap space]] on a hard drive. This entire pool of memory may be referred to as "RAM" by many developers, even though the various subsystems can have very different [[access time]]s, violating the original concept behind the ''random access'' term in RAM. Even within a hierarchy level such as DRAM, the specific row, column, bank, rank, channel, or [[interleave]] organization of the components make the access time variable, although not to the extent that rotating [[storage media]] or a tape is variable. (Generally, the memory hierarchy follows the access time with the fast CPU registers at the top and the slow hard drive at the bottom.) In most modern personal comp...