We were able to take a peek at AMD NDA information (aimed at engineers) that details the technical features of the Kaveri APU. According to this information, Kaveri features a GDDR5 memory interface consisting of four 32-bit memory channels.
This is perfectly matching the width of a GDDR5 chip which is also 32-bit. However, the memory controller has to be set up in a way so that two 32-bit channels work in tandem, half-channel use is not supported. The total width is 128-bit, so the main advantage comes from higher clock speeds of GDDR5 memory. This is in addition to the 128-bit DDR3 interface that we already know from previous APUs. Usage of DDR3 and GDDR5 is mutually exclusive.
While the information we glanced at is clearly preliminary and could be changed till the release of the chip, the focus should be less expensive GDDR5 chips with moderate clock speeds – not the 6GHz hotness you find in high-end graphics cards like GeForce GTX Titan.
Specifically the document lists 800 MHz QDR and 850MHz QDR (3200MHz and 3400MHz) clocks which would result with 51.2 GB/s and 54.4 GB/s of system memory bandwidth. Compared to current 25.6 GB/s with the DDR3-1600, this is quite the performance bump. The surprises don’t end there – Kaveri will support DDR3 up to 1250 MHz DDR (2500MHz) – it specifically adds 2400MHz and 2500MHz modes over Trinity, which officially supported up to 2133MHz. Nevertheless GDDR5 would provide a tangible bandwidth improvement and might be the smarter choice given that DDR3 above 1866MHz starts to get prohibitively expensive.
We can anticipate one potential downside with GDDR5
Currently most commonly deployed chips only feature 2 GBit capacity, which translates to 256MB per chip. To fully populate the GDDR5 interface, you need four chips, totaling mere 1GB, which can be considered a bit low for system memory. With two chips per channel we get 2GB, which is still low but starts to get workable. We don’t have any hard info on how many chips per channel are possible with GDDR5, on GPUs currently no more than two are used. Either AMD plans to have a crazy amount of GDDR5 BGA soldered to the boards, or they bet on 4 GBit chips which would bring 4GB configurations within reach or the whole GDDR5 support is meant for embedded systems with lower memory requirements. Remember that even Nvidia had to give up on the idea of selling the Tesla K20 card with 12GB memory and had to settle for 6GB. Last year, we talked with representatives from Samsung Memory which refuted the idea of offering higher capacity GDDR5 chips but as the time goes, opinions might change.
The GDDR5 interface inside Kaveri represents a very interesting option. After Sony made their PS4 announcement (8GB GDDR5), this step actually appeares to be a logical evolution of the APU products. Current APUs feature quite capable GPUs who are hamstrung by low memory bandwidth compared to the entry-level discrete GPUs. Even though GDDR5 memory reduces system flexibility (BGA package has to be soldered to the mainboard), the performance gain may more than make up for that.
Other changes in Kaveri include a PCIe 3.0 interface, which brings the APU in line with Intel’s Ivy Bridge and Haswell CPUs. In addition to the 16 lane PCIe 3.0 interface, Kaveri features eight PCIe 2.0 lanes for general purpose use. The PCIe 3.0 interface can be configured as two x8 interfaces, thus enabling Crossfire or Discrete GPU plus Storage Card (Solid State Cards are gaining in popularity). Four of the PCIe 2.0 lanes are used to connect the Southbridge or Fusion Controller Hub (FCH) as AMD calls it, the other four lanes can be used as four x1, two x2 or one x4 connection.
We also tried to clarify if Kabini contains a GDDR5 interface as well, which is scheduled to debut around the Computex timeframe. Given that an PS4 uses significantly modified version of Kabini and utilizes GDDR5 memory, such possibility would not be unreasonable. However, from the documents we were able to look at didn’t contain any information on such a feature. While this is not a 100% confirmation, Kabini with GDDR5 for the PC market probably won’t happen.
Once the DDR4 memory comes along the advantages of GDDR5 start to diminish again. DDR4 will enable higher densities at comparable speeds as well as upgradeable modules which might be more desirable for customers. More importantly, we view Kaveri GDDR5 as a trial run, first generation part. As we all know, GDDR5 will be replaced with GDDR6 in 2014 timeframe, foreshadow of what might await us down the road: APUs in BGA package soldered to the board with memory soldered to the board and next to no upgradeability. While this is not desirable from an enthusiast point of view, this is what the mass market will inevitably move to in the next few years.
Original Author: Marcus Pollice
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