-- Summary: Data retention in semiconductor memory --

Contrary to conventional wisdom, "volatile" semiconductor memory does not entirely lose its contents when power is removed. Both static (SRAM) and dynamic (DRAM) memory retain some information on the data stored in it while power was still applied. SRAM is particularly susceptible to this problem, as storing the same data in it over a long period of time has the effect of altering the preferred power-up state to the state which was stored when power was removed. Older SRAM chips could often "remember" the previously held state for several days. In fact, it is possible to manufacture SRAM's which always have a certain state on power-up, but which can be overwritten later on - a kind of "writeable ROM".

Is this a new discovery? When I used to work with DOD classified data, not so long ago, disk drives had to be declassified using an approved program, such as Norton Utilities' "WIPEINFO". (That was approved up through the SECRET/SAR level, anyway. I don't know about TS/SCI/SI.) But those same regulations said that RAM was considered declassified within a certain time (30 seconds, I think) after power was removed. (That time figure was UNclassified, BTW.) I think it was just to allow time for the voltage to bleed off of the power supply's filter capacitors, and not related to the relative volatility of DRAM.