Ben Cantrick (mackys) wrote,
Ben Cantrick

DRAM sucks. ZRAM rocks.

Z-RAM, short for "zero capacitor RAM" is a new type of computer memory in development by Innovative Silicon Inc. Z-RAM offers performance similar to the standard six-transistor SRAM cell used in cache memory but uses only a single transistor, and therefore offers much higher densities. It is also denser than conventional one-transistor, one-capacitor DRAM used for the majority of a modern computer's main memory, although it is unlikely to appear in this role in the short term due to the requirement to be built on only the newest generation fabs.

Z-RAM relies on an effect known as the floating body effect, which was first encountered in CPU design based on the new silicon on insulator (SOI) process introduced in the early 2000s. This effect causes capacitance to form between the transistor and the underlying insulating substrate, and was a problem that needed to be solved in conventional designs. The same effect, however, allows a DRAM-like cell to be built using the transistor only, the floating body effect taking the place of the conventional capacitor. Consisting of only one part instead of two, Z-RAM offers twice the density of DRAM, and five times that of SRAM.

The small cell size leads, in a roundabout way, to Z-RAM being faster than even SRAM with SRAM normally much faster than DRAM. SRAM's large cell size means that any "reasonable" amount of SRAM cache takes up a large portion of the CPU die. The long traces needed to carry current into the cells have a capacitance of their own, and requires the driver circuitry to "slow down" in order to allow the charge to settle. Although Z-RAM's individual cells are not as fast as SRAM, the lack of the long lines allows a similar amount of cache to be run at roughly the same data rates by avoiding this delay while taking up less space. Response times as low as 3ns have been stated.

Imagine if all 4gig of your RAM was as fast as your L1 cache is now.
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