The System-On-a-Chip (SOC) market has been around for years. Isn't it obvious that on-chip DRAM would reduce access times and increase performance? I asked my friend and fellow analyst Jim Handy of Objective Analysis, who's been following semiconductors for decades, why chip vendors don't.
I summarize his reasoning here. The answer is simple but the reasons are illuminating.
DRAM is commonly built on a logic process that is customized for its special requirements. For example, DRAM needs a good capacitor that is characterized for leakage to know how often the DRAM needs refreshing.
That's a lengthy and expensive process. It's easier to put Static RAM (SRAM) on the CPU instead of DRAM because SRAM doesn't need finicky capacitors.
Logic processes - those used for CPUs - are also more expensive. A logic wafer might cost $3500 vs $1600 for DRAM. Intel's logic wafers may cost as much $5k. That's costly real estate.
Another cost difference is that the cell (bit) size will be larger if you don't use a process customized for DRAM. So putting DRAM on a logic chip is a double-whammy: larger cell sizes on more expensive wafers. Not a winning combination.
SRAM actually requires more transistors per bit than DRAM as it uses 4-6 transistors vs 1 transistor and a capacitor for DRAM. But since you're already putting a couple of billion transistors on the chip, that isn't much of a problem. And SRAM has another huge advantage.
While SRAM is easier to build on a logic wafer, it also has one other huge advantage: speed. Access times are roughly 10 to 20 times faster than DRAM.
Jim summarized the answer very simply:
There's a reason people don't put main memory onto their chips, and that's because it's always significantly cheaper to use separate memory chips.
The Storage Bits take
Cost pressures are ferocious throughout the storage hierarchy - which is why we have a storage hierarchy. If the fastest and most reliable storage was also the cheapest, the "hierarchy" would be only one layer deep.
Flash memory had no impact on computer storage for decades until it got cheaper than DRAM. Tape - which once dominated computer storage - hangs on because it is cheap.
As long as the speed and cost correlation continues, we'll have a storage hierarchy. And no DRAM on CPUs.
Comments welcome, as always.