It occurred to me last year when the iPad was introduced that RAM may soon become a thing of the past, replaced by larger on-chip caches and super fast flash SSDs (solid-state disks.) Of course, something like this has been theorized for decades (consider Tom Baker’s endorsement of bubble memory in the 1981 Dr Who episode Logopolis.) But now it’s starting to look inevitable.
In the old days, chip speeds were so slow that RAM chips could be read almost as fast as processors could use the data. But as chips got faster, it became necessary to add more and more caches of memory between the processor and RAM. Today’s processor clocks are measured in gigahertz—meaning that a clock tick propagating at the speed of light can barely make it across the chip in time for the next tick. Traditional RAM chips, which might be a foot of wire away from the processor, have no chance of keeping up.
It’s hard to compare speeds between Serial ATA (for SSD) and memory buses, since SATA is measured in gigabits per second, whereas memory buses are measured in megahertz. The latter measures latency, or how long a bit takes to travel, which is the more important measure for replacing RAM with SSD. But my guess is that SATA 3 is within the “close enough” range, where a computer designer could start to consider throwing out the RAM and just having SSDs.
The iPad takes a step in that direction. The processor is designed to have its single RAM chip stacked on it like a layer cake, in a position which resembles an on-chip cache. (Desktop CPUs often have “on-chip” caches on a separate silicon chip, but packaged in the same plastic “chip.”) Future desktop processors may well have a similar configuration.
Indeed, there’s talk that down the road, supercomputers may need to have their “disks” (SSDs) on the processor chip simply to reduce their power requirements. That’s because if supercomputers continue to become more powerful without becoming more energy efficient, they’ll soon need dedicated nuclear power plants. But on-chip disks would also be useful in phones, tablets, and laptops.