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A first look at AMD's Radeon HD 8790M

Graphics Core Next goes mainstream on mobile

AMD's Graphics Core Next architecture premiered around this time last year on the Radeon HD 7000 series. The architecture brought with it a slew of improvements, which we covered at length here. Before long, GCN had spread through much of AMD's desktop graphics lineup, providing fierce opposition to the competition from Nvidia.

When AMD got around to updating its mobile offerings, though, it did something a little disappointing. The company decided to use its older TeraScale architecture to power mid-range and low-end members of the Radeon HD 7000M series, from the 7600M on down. Those offerings were built using TSMC's 40-nm fab process, as well, instead of the finer 28-nm process used to fab newer, GCN-powered chips.

AMD did eventually introduce GCN-driven Radeon HD 7700M, 7800M, and 7900M series products to service the higher end of the mobile market. However, cheaper and thinner gaming notebooks have essentially been stuck with re-branded last-gen Radeons for the better part of a year.

Happily, that's all about to change. AMD officially announced the first members of the Radeon HD 8000M series earlier this week, and those offerings are due out early next year. All of them are based on the GCN architecture and built using TSMC's 28-nm fab process, and AMD promises some meaty performance gains over the 40-nm TeraScale parts they're supposed to replace.

We've been able to get one of those newcomers, the Radeon HD 8790M, in our lab today. As we reported a few days ago, AMD has rejiggered its mobile branding somewhat, so the Radeon HD 8700M series is meant to succeed the Radeon HD 7600M series. As luck would have it, we have representatives of both lineups in our labs today: a 7690M and an 8790M.

Left: the Radeon HD 7690M. Right: the new Radeon HD 8790M.

A cursory look at the picture above shows that the 8790M's graphics chip, code-named Mars, is quite a bit smaller than Thames, the 40-nm slab of silicon that powers the 7690M. Mars measures about 77 mm² by our count, while Thames takes up 118 mm². That difference is at least partly due to the fact that Mars is built using the finer 28-nm process, so its transistors are smaller.

We'd love to compare transistor counts at this point, but unfortunately, AMD is refusing to disclose complete specifications for the Radeon HD 8000M series until January 7. What little we've been able to glean about our test samples is listed in the table below:

Memory Fab.
Radeon HD 7690M 480 600 MHz 800 MHz 128-bit 1GB GDDR5 40 nm 118 mm²
Radeon HD 8790M 384 900 MHz 1000 MHz 128-bit 2GB GDDR5 28 nm ~76.5 mm²

The 8790M has fewer shader ALUs than its predecessor, but at least in our test sample, its clock speeds are higher—and it comes packed with more memory. Also, of course, the 8790M is based on AMD's latest GPU architecture. Interestingly, AMD tells us it's made no refinements to the GCN architecture in the Radeon HD 8000M series. No new features have been added, either, beyond those that were already present in proper, GCN-powered 7000M-series parts.

The new architecture and higher clocks bode well for performance, despite the 8790M's lower ALU count. AMD's internal benchmarks show an increase of 20-50% from the 7670M to the 8770M, and one would expect the 7690M to trail the 8790M by a similar margin. Since we have the latter two GPUs our disposal, we're going to put that assumption to the test.

A novel approach to mobile GPU testing
Traditionally, testing mobile GPUs involves using pre-assembled laptops. That can make comparisons with other offerings tricky, especially when you want to benchmark a previous-gen part that may only be available inside older systems with outdated CPUs.

With the help of AMD, we've tried a different approach this time. AMD supplied the aforementioned GPUs—the Radeon HD 8790M and 7690M—as bare MXM modules, each one with its own, dedicated cooler. For our test platform, AMD sent us an MXM to PCI Express x16 adapter alongside an off-the-shelf desktop processor, motherboard, and memory. Using this setup, we're able to test mobile GPUs free from the confines of notebooks or proprietary qualification hardware.

What you see here is an Intel Core i7-3770K processor, a Gigabyte Z77X-UD3H motherboard, four gigs of AMD memory, AMD's MXM adapter, and our two mobile Radeons. AMD also threw in a 500GB Seagate hard drive, but we swapped that for a Crucial solid-state drive from our own supply of test hardware. Benchmarking games is a lot quicker on an SSD.

Since we're testing with a very fast desktop CPU, we're able to ensure that the processor isn't a primary performance bottleneck. That means our GPUs should be free to fulfill their performance potential. We follow this same approach when testing desktop graphics cards, and it has served us well. Of course, in this particular case, the processor we're testing is a fair bit quicker than what you'd find in even a top-of-the-line gaming notebook. Our scores may be a little higher as a result.

Benchmarking mobile GPUs with a desktop platform also rules out battery testing, for obvious reasons. However, elegant solutions to that problem are rare in any event. Every notebook is bound to be different—some will have smaller batteries, some will have larger displays, and others will couple gaming GPUs with slim enclosures and power-sipping CPUs. At least with our test setup, we we can offer a look at the power consumption delta between new and old mobile Radeons, and that's exactly what we've done.

Sadly, we're still working on getting a competing mobile GPU from Nvidia. The benchmarks on the next few pages will show you how the new Radeon compares to the old one, but they won't tell you how either stacks up against a rival GeForce. Our apologies. That said, knowing how the two generations compare—not to mention what kinds of games can be played, and at what settings—is very valuable information, especially given the dearth of mobile GPU benchmarks out there. We think we're better off making this information available now and building upon it later, rather than waiting, possibly indefinitely, for the stars to align.