AMD 780G demystified - 01
Let's disseminate what's new and worthwhile with a look at the trusty block diagram.
AMD introduces the 780G northbridge and SB700 southbridge - and both are new. As such, the 780G northbridge, packing in a whole slew of features, is not pin-for-pin compatible with the well-received 690G, but you will see why in just a moment.
AMD 780 northbridge
Going from the top, the 780G - as per the 7-series chipsets before it - is, naturally, an AMD CPU-supporting motherboard that can accommodate all recent AM2 and AM2+ processors. Note that AMD's quad-core, and soon-to-be-released tri-core, Phenom CPUs ship in an AM2+ form-factor that provide a speedy HT3.0 link to the northbridge and multifarious power and granularity enhancements over dual-core AM2.The speed of the HyperTransport link from AMD processor - up to 5.2GT/s for an AM2+ CPU's HT3.0 - to the IGP-based chipset is vitally important, as it regulates just how much data-throughput the CPU can provide to the GPU. Further, IGP-based memory accesses need to traverse the HT bus, so the faster the better.
A whole DX10 Radeon HD 2400 GPU in the IGP, you say?
Up until now, the traditional approach by ATI and NVIDIA has been to architect IGPs with, roughly-speaking, a GPU core that's half of the slowest present discrete generation - power considerations and manufacturing complexity combined to prohibit a complete (discrete) GPU from being melded directly on to a northbridge.
Knowing that AMD's processors carry an integrated memory controller and other gubbins directly on their die, freeing up IGP space for a more-powerful graphics core, AMD has literally taken an entire, discrete-based, Radeon HD 2400 GPU, shrunk the manufacturing process down to 55nm (like the 3400-series'), and placed the DX10-supporting GPU on the IGP. Known as Radeon HD 3200, you can appreciate its heritage from the nomenclature. Intel has been touting the world's first DX10 IGP for a while, we noted, but actual driver-based support will not be available until sometime later this year.
The upshot of doing so is that the 780G northbridge carries a fully DX10-compliant IGP core that's imbued with 40 stream processors - there's no lopping off of specifications in the transition from discrete to IGP. Yes, the Radeon HD 2400 wasn't able to play the latest games at 1,280x,1,024, but its performance, meagre by discrete standards, was an order of magnitude faster than, say, Intel's G33 and NVIDIA's GeForce 7150 IGPs. Discrete is discrete, after all.
IGP-based boards of old generally eschewed extensive tweaking options in favour of lockouts that didn't compromise stability, but AMD's OverDrive utility will be updated to provide control over the IGP's frequencies - and if our internal testing is a harbinger of things to come, 750MHz+ engine speed should be a relative cinch to achieve.
The absence of a memory-controller hub - as per Intel's current Core 2-derived CPUs - and low-power manufacturing process, along with some clever behind-the-scenes plumbing, means that AMD can apportion the vast majority of the 780G northbridge's transistor count, amounting to some 202M, to the core, which operates at 500MHz (40 GFLOPS MADD rate, incidentally) and grabs its memory bandwidth from the system's. As usual, higher-speed system memory benefits IGP performance, and partners have the choice of adding an on-board display cache (SidePort) for a little extra performance and lower power utilisation.
That said, AMD quotes an idle TDP of around 3W and an under-load rating of 15W, making 780G a little toastier than the 690G. Successful integration of a full-fat graphics core of the 780G has only been made possible due to the benefits of a 55nm half-node manufacturing process, we reckon - any higher and thermal considerations would prohibit the inclusion of such a core.
UVD 2.0, too
We also know that the Radeon HD 2400 integrates a special ASIC, UVD (Unified Video Decoder), that's not strictly part of the graphics' architecture, to hardware-accelerate the decoding of computationally-expensive high-definition codecs such as VC-1 and H.264. Have a special-purpose ASIC for the task makes sense; decode process doesn't parallelise particularly well.
AMD integrates the newer and better-specified UVD 2.0 - as found on the Radeon 3000-series, for stutter-free 1080p decoding, right on to the northbridge, too.
Displays and more displays
The 780G supports dual independent displays but one cannot use two digital outputs concurrently. Digital outputs can include DVI, audio-carrying HDMI v1.2 (via Realtek codec and driver), and DisplayPort. The eclectic choice of HDCP-certified outputs will be physically integrated to the I/O section on a per-manufacturer basis, so expect to see varying degrees of outputs depending upon board size (mATX, ATX) and partner.
The IGP-driven dual display can be complemented by a discrete ATI card's to offer a four-monitor setup, under SurroundView.
The external plumbing, Hybrid CrossFire, and some special sauce
The 780G northbridge hooks up to the all-new SB700 via AMD's A-Link Express II (up-to PCIe x4) interconnect that's been around since RD480 days. Out of the 22 PCIe 2.0 lanes available, 16 are allocated to a graphics-card slot and a further six for peripherals, with the exact take-up open to the partners. The x16 PCIe slot can be used exclusively for a discrete card, and should it be taken up, the IGP can be switched off.
Now, knowing that erstwhile IGPs have carried castrated GPU cores and noting that the 780G integrates a full-fat Radeon HD 2400, practically analogous to the discrete model, AMD's Hybrid CrossFire technology can, for all intents and purposes. team up a low-end 3400-series card, placed in the x16 PCIe slot, with the IGP to offer significant scaling.
In other words, adding a discrete, low-end Radeon 2/3000-series card can practically double the IGP's performance. We'll be putting this assertion to the test in our benchmark section. We suppose the benefit of such an approach is to provide fluid, smooth gameplay at 1,024x768 with greater image quality, for an additional outlay of around £25.
Reading between the lines at the AMD press conference, the DX10 IGP is a little more than just a shrunken-down RV610, though, as some 'special sauce' is applied to pave the way for esoteric multi-GPU setups, harnessing three or four discrete cards for rendering and, say, the IGP for physics. AMD wouldn't talk much when pushed on the subject. 780N chipset, anyone?
