Mark Cerny Explains How the PS4's 8 GB GDRR5 RAM and Bus Work and Why They Were Chosen

[haxxiy]

Pemalite said:

The latency issue is true, I'll repost what I posted several days ago as it's still relevent:

However, since datarates on Ram have been increasing over the years so has the latency in clock cycles, but the absolute latency has been relatively static.

Grab some DDR3 1600mhz memory, that's 800mhz IO, which has a typical CAS latency of 8, that means it has a latency of 10ns.
Grab some DDR2 800mhz memory, that's 400mhz IO, which has a typical CAS latency of 4, this is also 10ns.

Now with GDDR5 the data rates are 4x faster than the IO clock instead of 2x, I.E. 5ghz GDDR5 is 1.25ghz x4 and would have a CAS Latency of 15.
15/(1.25 GHz) = 12 ns

So the latency of GDDR5 is only 20% higher than DDR3.

Did Rambus ever released their XDR2 DRAM? That must have been the epithome of high latency but super fast bandwidth, probably as good as you can get without stacking the RAM on top of the chip itself. Data rates could go up to 16x IO clock if I recall correctly.