DRAM Almost as Fast as SRAM

An anonymous reader writes "IBM said it has been able to speed up the DRAM to the point where it's nearly as fast as SRAM, and that the result is a type of memory known as embedded DRAM, or eDRAM, that helps boost the performance of chips with multiple core calculating engines and is particularly suited for enabling the movement of graphics in gaming and other multimedia applications. DRAM will also continue to be used off the chip."

Comments

Here's a better explanation

[Wesley Felter]

EE Times article. Today SRAM is used for processor caches, but new multicore chips need massive (i.e. expensive) cache. Because eDRAM is much denser than SRAM, it allows chip designers to fit much more cache in the same size chip, increasing overall performance. IBM and AMD use silicon-on-insulator (SOI) technology, while the rest of the industry uses bulk CMOS; eDRAM for bulk has been available for a while (it's used in Xbox 360 and BlueGene/L for example), but now IBM has developed SOI eDRAM that can be used in IBM's future processors (and maybe AMD's).

Re:To those wondering

[TheRaven64]

To add to this:

Cache misses are expensive. Really expensive. There are two ways of getting around this:

1. More hardware contexts so that you can switch to another thread instantly when a cache miss happens.
2. More (SRAM) cache.

The first one is better if you have highly parallel software, but isn't so good for single-threaded applications. The second is the more common approach. While SRAM uses six transistors per bit, DRAM uses one transistor and one capacitor. This could give something around three times the density, allowing CPU manufacturers to triple the amount of cache without increasing die size. Bigger cache means fewer cache misses, which means less time spent doing nothing.

For reference, a cache miss typically costs something around 1-200 cycles.

Re:eDRAM is quite old

[stevesliva]

Some big examples? PS2, Nintendo Gamecube, Wii, Xbox 360. All these consoles use eDRAM for their GPU's on-chip framebuffers to enhance their performance, and that goes back to at least the year 2000 when the PS2 came out.

Some will be quick to say "no, the Nintendo consoles use 1T-SRAM, not DRAM". Yeah, right, but even 1T-SRAM (despite its name) is a form of embedded-DRAM.

First, it is news because IBM is announcing that the performance is on par with SRAM, and because they have integrated their deep-trench eDRAM process with the SOI process used for their Power CPUs. The result? 3x the cache on the die. IBM has offered embedded DRAM with bulk technologies for a few generations, but this is the first real SOI annoucement.

Second, the consoles that have issued PR about using "embedded DRAM" with their GPUs don't actually embed DRAM on the GPU die. The "embedded DRAM" is a process offered by NEC that is separate from the Sony and TSMC processes used to fab the GPUs that supposedly have "embedded DRAM." I am pretty sure that all of the consoles you mention include a separate custom DRAM chip in the same package as the GPU. I am certain this is the case for the XBox 360. I am unsure about Sony. That DRAM process substantially modifies the back end wiring to make room for a MIM cap between the FETs and the first level of metal.