Our computers rely on a motley mix of technologies to meet their data-storage needs. Document files and other occasionally accessed data typically live on magnetic hard disks or flash drives, which are slow but can store information reliably for decades. Data that must be retrieved more frequently, including processor instructions for calculations, are stored in dynamic random-access memory (DRAM), whose capacitor-based bits are short-lived but manipulable on nanosecond time scales. And for the most demanding calculations, known as cache operations, there is static random-access memory (SRAM). Composed of six-transistor circuits, SRAM bits take up copious chip real estate but can be written or erased in a fraction of a nanosecond.
Shuttling data between the different types of memories—all housed at different physical locations—makes for sluggish processing, especially in applications that handle massive amounts of data. About a decade ago, computer scientists began dreaming of a memory that would do it all—one with the requisite combination of stability, scalability, and speed to serve as long-term storage, cache memory, and all things in between.1
The dream of a universal memory was inspired in large part by the emergence of phase-change memory, in which bits are stored in tiny alloy fragments that can be switched between crystalline and amorphous-solid states by electric pulses. The bits could easily preserve their state for a decade, and they could be written or erased in tens of nanoseconds—almost as fast as DRAM.
But after years of failed attempts to substantially improve on those speeds, the dream faded. “It is generally agreed that the vision of a ‘universal memory’ is not realistic,” Stanford University’s Philip Wong and University of California, Berkeley’s Sayeef Salahuddin wrote in a 2015 commentary.2
Now researchers at Xi’an Jiaotong University and the Shanghai Institute of Microsystem and Information Technology (SIMIT), both in China, have demonstrated a phase-change memory cell3 that team member Wei Zhang says “should bring the universal memory concept back into consideration.” The group used a rational-design approach to concoct a quick-freezing alloy that can switch phases in just 700 ps—on par with SRAM write times.
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