For a GPU just a few months away it's surprising how little we actually know about AMD's Radeon R9 390 series. Previously there were rumors that 4GB of High Bandwidth Memory (HBM) was the maximum possible but now word is going around that the GPU will utilize a dual-link interposer to stack 2x 4-HI HBM components to achieve a total of 8GB RAM without having to wait for second generation HBM. The leak also states that when 2nd gen HBM arrives from SK Hynix, AMD will be able to shift seamlessly to the HBM2.
The problem arises from the fact that the 1st Generation HBM memory is limited by design to just 4GB worth of capacity. While 2nd Generation memory can support up to 8GB of capacity, its production time line is too far for AMD to be able to use it in its Radeon 300 series designs. And AMD is well aware of the fact that 4GB of memory is no longer enough for a flagship product, therefore the only solution is dual link interposing. With a Dual Link Interposing design, SK Hynix will be able to stack 4x (Dual 1GB HBM modules) via an Interposer (2.5D stacking). The design will have very high performance at low clock speeds and will also be very power efficient as compared to GDDR5.
2x 4-HI HBM1 (which should technically be called 8-Hi-Hi according to nomenclature rules) features a 1024-bit interface, two prefetch operations per IO (dual command) and can push 128GB per second per pin. The tRC is 48nm, tCCD is 2ns (1tCK), and VDD is 1.2V. The 4-Hi HBM2 (generation 2) features a 1024 bit interface, two prefetch operations per IO (dual command), 64 Byte access granularity (=I/O x prefetch) and can push 256 GB per second per pin. The tRC is 48nm, tCCD is 2ns (1tCK), and VDD is 1.2V.