AMD 780G Chipset: PCIe and SB700
AMD’s website does not provide a lot of information on the chipset, so we had to pull it from some presentations; we already looked at the 780G chipset in March 2008. AMD has positioned the chipset as a low-power solution with powerful video capabilities, to address the HTPC market and multimedia-savvy users. The release was synchronized with the launch of AMD’s Athlon X2 4850. The test results were impressive: the chipset is capable of playing High Definition Blu-ray video with the CPU power of a small Sempron processor (if operated close to 100% processor load).
From a feature perspective, the 780G provides everything you need on a desktop PC for all sorts of media PC solutions. Up to four DDR2-1066 DIMMs are supported by the AMD processors’ integrated memory controller. There are as many as 26 PCI Express lanes that all comply with the fast PCIe 2.0 standard, providing 500 MB/s bandwidth per lane for each of the upstream and downstream directions.
PCI Express Usage
Of the 26 PCI Express lanes, 16 are used to run a discrete graphics card, although it is technically possible to lay out a motherboard with two x16 PCIe 2.0 slots running eight lanes each. Four more lanes are required to connect to the SB700 southbridge, leaving six to be used for peripheral devices. Remember that AMD does not offer an integrated network controller like Nvidia does; as a consequence, one more PCIe lane has to be used for network connectivity.
Since a two-chip solution to separate the graphics and processor interfaces from the connectivity is very much standard today, we don’t need to spend many words on it. However, we want to point out that AMD uses two 55 nm dies, while Nvidia decided to go for a single chip solution.
We haven’t analyzed the performance of the SB700 south bridge’s USB 2.0 and SATA controllers in detail. Performance levels don’t differ dramatically, and we will follow up in a second part when we also analyze power requirements and performance in hybrid graphics mode, as well as graphics overclocking.
Compared to the aged SB600 southbridge, the SB700 is a more advanced part thanks to the efficient 55 nm manufacturing process. AMD says that the idle power of the SB700 is less than 1 W, but the feature set equals the SB600: it still connects to the northbridge via four PCI Express 1.0 lanes, even though the 780G northbridge could run PCIe 2.0 mode. The SATA controller doesn’t support RAID 5 or multiple RAID arrays on one set of drives as does Intel’s Matrix RAID on all ICH8R, ICH9R and ICH10R south bridges. However, it is the only southbridge part that still offers two fully-featured UltraATA/133 ports for legacy hard drives and optical drives.