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A few other tricky issues
Any tech as radically new and different as HBM is likely to come with some potential downsides. The issues for HBM don't look to be especially difficult, but they could complicate life for Fiji in particular as the first HBM-enabled device.

One problem with HBM is especially an issue for large GPUs. High-end graphics chips have, in the past, pushed the boundaries of possible chip sizes right up to the edges of the reticle used in photolithography. Since HBM requires an interposer chip that's larger than the GPU alone, it could impose a size limitation on graphics processors. When asked about this issue, Macri noted that the fabrication of larger-than-reticle interposers might be possible using multiple exposures, but he acknowledged that doing so could become cost-prohibitive.

Fiji will more than likely sit on a single-exposure-sized interposer, and it will probably pack a rich complement of GPU logic given the die size savings HBM offers. Still, with HBM, the size limits are not what they once were.

Another possible issue with HBM's tiny physical footprint is increased power density.  Packing more storage and logic into a smaller area can make cooling that solution difficult because the cooling solution must transfer more heat through the available surface area. AMD arguably had a form of this problem with the Radeon R9 290X, whose first retail coolers couldn't always keep up, leading to reduced performance. 

Fortunately, Macri told us the power density situation was "another beautiful thing" about HBM. He explained that the DRAMs actually work as a heatsink for the GPU, effectively increasing the surface area for the heatsink to mate to the chips. That works out because, despite what you see in the "cartoon diagrams" (Macri's words), the Z height of the HBM stack and the GPU is almost exactly the same. As a result, the same heatsink and thermal interface material can be used for both the GPU and the memory.

(Notice that Macri did not say Fiji doesn't have a power density issue. He was talking only about the HBM solution. The fact remains that leaked images of Fiji cards appear to have liquid cooling, and one reason to go that route is to deal with a power density challenge.)

The final issue HBM may face out of the gate is one of the oldest ones in semiconductors. Until HBM solutions are manufactured and sold in really large volumes, their costs will likely be relatively high. AMD and its partners will want to achieve broad market adoption of HBM-based products in order to kick-start the virtuous cycle of large production runs and dropping costs. I'm not sure Fiji alone will sell in anything like the volumes needed to get that process started, and as far as we know, AMD's GPU roadmap doesn't have a top-to-bottom refresh with HBM on it any time soon. Odds are that HBM will succeed enough to drive down costs eventually, but one wonders how long it will take before it reaches prices comparable to GDDR5.

When I asked Macri this question, he avoided getting into the specifics of AMD's roadmap, but he expressed confidence that HBM will follow a trajectory similar to past GDDR memory types.

I don't think his confidence is entirely misplaced, even if HBM may take a little longer to reach broad adoption. The tech AMD and its partners have built is formidable for a host of reasons we've just examined, and what's even more exciting is the way HBM promises to scale up over time. According to Macri, HBM2 is already on the way. It will "wiggle twice as fast" as the first-gen HBM, giving it twice the bandwidth per stack. The memory makers will also move HBM2 to the latest DRAM fabrication process, giving it four times the capacity. The stack itself will grow to eight layers, and Macri said someday it may grow as large as 16. Meanwhile, JEDEC is already talking about what follows HBM2.

Whatever happens that far down the road, we appear to be on the cusp of a minor revolution in memory tech. We should know more about what it means for graphics in about a month, when AMD will likely unveil Fiji to the world.

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