For many years, computer main memory has been dominated by dynamic random access memory (DRAM) because of its high density, fast access time, and low cost. However DRAM is volatile and loses its contents when power is removed. DRAM's volatility has required computer users to wait for several minutes every time the computer starts up while the basic input-output system (BIOS) and operating system load information from a slower computer disk and set up the DRAM memory for normal program execution.
Floating-gate non-volatile memory is another memory technology that has low cost and high density. Some computers today use solid-state drives based on floating-gate non-volatile memory to store the operating system and therefore allow very fast startup. However floating-gate non-volatile memory suffers from long write times and cannot replace all the DRAM in the system.
In recent years, new types of non-volatile memory have been developed. These new memory types include phase change memory (PCM), magneto-resistive random access memory (MRAM), ferro-electric random access memory (FeRAM), and resistive random access memory (RRAM). These new memory types have significantly faster access times, and specifically write times, than floating-gate non-volatile memory. Thus they appear to be more promising replacements for DRAM as main computer memory.
However these newer memory technologies wear out after a certain number of write cycles, which can necessitate their replacement in as little as two years or less. Frequent replacement is expensive and disruptive to the user. Moreover certain operating systems and computer programs cause memory locations to be accessed non-uniformly, which could limit the lifetime of PCM and other similar emerging memory technologies to the lifetime of the most frequently used memory locations.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.