At the start of Siggraph yesterday, AMD announced some additions to its FirePro lineup of workstation-class graphics cards. Nestled quietly in that lineup, onboard the FirePro W7100, is a brand-new GPU. This chip, code-named "Tonga," is widely expected to be released as part of a new consumer-class Radeon graphics card in the coming months.
The best information I've found about Tonga comes courtesy of Damien Triolet at Hardware.fr, who has mapped out this GPU's feature set quite nicely.
The bottom line is that Tonga joins the Hawaii (Radeon R9 290X) and Bonaire (R7 260X) chips as the only members of AMD' s GCN 1.1 series of graphics processors. Tonga looks to be a mid-sized GPU and is expected to supplant the venerable Tahiti chip used in everything from the original Radeon HD 7970 to the current Radeon R9 280.
Here's a rundown of Tonga's feature set, based on info from AMD and some of Damien's own informed speculation.
The most notable change from Tahiti is Tonga's narrower memory interface: 256-bits instead of 384. This choice isn't terribly surprising, since Radeons in this class have long had to compete with GeForce cards based on Nvidia's GK104 chip, which also has a 256-bit interface and has proven to be quite competitive. The narrower memory interface should save AMD and its board partners some production costs and could lead to lower retail prices on the R9 280's forthcoming replacement.
Tonga looks to make up any lost ground in the memory throughput department by offering more capability elsewhere, including quad rasterizers (rather than the dual ones in Tahiti), support for the CrossFire XDMA transfer path, and the DSP block used for TrueAudio acceleration. Tonga also has eight Asynchronous Compute Engines for scheduling parallel computing work. (I believe the ACE engines are also used in AMD's Mantle API to allow more parallel command scheduling in graphics.)
Note that the stream processor count for Tonga in the table above comes with an asterisk. Damien says he suspects Tonga may have 32 GCN compute units (CUs), even though the FirePro W7100 only has 28 of them active. If so, then Tonga could have a total of 2048 stream processors, just like Tahiti. If I were betting, I'd bet on Tonga having 32 CUs, but time will tell.