AMD's SB750 South Bridge Helps Bring Higher Overclocks to Phenom

AMD's SB750 South Bridge Helps Bring Higher Overclocks to Phenom

Many people know that the AMD Phenom is lacking compared to the Intel Core series in both total performance and overclocking ability.  However, with the low price of their "Black Edition" unlocked chips, the entry price into the overclocking elite is much lower compared to Intel's "Extreme Edition" unlocked chips.  Lately, AMD has had much trouble keeping their processors stay stable at above 3.0 GHz while Intel has had overclockers around the world routinely hit 4.0 GHz and beyond using simple air and liquid cooling methods.

AMD hopes to remedy this by releasing a new South Bridge for their enthusiast 790 series chipset.  What does not make sense is that the South Bridge is at the opposite end of the motherboard compared to the processor.  The South Bridge handles SATA, audio, USB, and PCI-E x1 interfaces.  It is the North Bridge of the chipset that really controls the processor.  Let us see what exactly AMD has done to this new SB that changes things.

What Is Different

Currently, the 790FX platform uses an older SB600 series South Bridge.  The SB700/SB750 features six SATA 3.0Gb/s ports, up from four on the SB600, with the ability to reserve up to two of those ports for eSATA connectivity. Drives can be set up in RAID 0, 1, or 10 and RAID 5 now makes its appearance in the SB750.  Still missing in action is a native interface for networking support. AMD continues to use an external PHY and MAC for network operations. Although performance is similar to the NVIDIA and Intel solutions, this setup does incur a cost penalty for the motherboard suppliers.

The major improvement in the SB700/750 series over the SB600 is the increase in USB 2.0 performance and the number of ports available. The new dual-channel controller features 12 USB 2.0 capable ports and 2 specific 1.1 ports for compatibility reasons. USB 2.0 performance is now on par with the Intel and NVIDIA solutions.

A single PATA channel provides native IDE support for up to two drives. This channel supports PIO, multi-word DMA, and Ultra DMA 33/66/100/133. Six PCI lanes are still included although we doubt a board manufacturer is going to offer that many. AMD dropped HyperFlash support on the SB700 to make way for the new Advanced Clock Calibration interface.

The SB700/SB750 features four PCI Express lanes for the A-Link Express II interconnect, but like the current 790FX/780G series those four lanes are based on PCI Express 1.1 specifications. That means the interconnect bandwidth is capped at 2GB/s, half of what it would be in a PCI Express 2.0 configuration.

Finally, we have the High Definition Audio controller carried over from the SB700 that allows up to 16 channels of audio output per stream. The controller supports up to four codecs with sample rates reaching 192kHz at up to 32-bits per sample.

How It Works

Unknown to many people before, all the AM2+ processors feature a 6-pin connection directly to the SB based on the Hyper Transport bus system.  Before the SB750, the connection was unused.  Now with this connection, the BIOS is able to change some internal settings on the processor creating additional headroom for overclocking.  AMD calls the value being changed the Advanced Clock Calibration value or ACC for short.  As far as if the value is some sort of maximum temperature or maximum clock rating, AMD refused to divulge.

AMD expects that this new combination will create an additional 100 MHz to 300 MHz headroom when overclocking.

Test Setup and Results

The test involved using a beta motherboard from FoxConn as the SB750 is still not out for consumer use yet.

Phenom 9850BE - Highest Core Speed / Stock Voltages
Vista 32	Core Speed	HT Ref Clock	HT Link Speed	North Bridge Speed	Memory Speed	CPU VID
Foxconn A79A-S ACC On	3000	200	2000	2000	1066	1.312V
Foxconn A79A-S ACC Off	2800	200	2000	2000	1066	1.312V
ASUS M3A32-MVP No ACC	2800	200	2000	2200	1066	1.312V

At stock voltages and stock HT/NB speeds, the highest the 9850BE sample would reach was 2.80GHz. This was true on both the ASUS M3A32-MVP as well as the Foxconn A79A-S. Turning on ACC let it hit 3.0GHz without adjusting the CPU's voltage at all.

Phenom 9850BE - Highest Core Speed / HT Ref Clock
Vista 32	Core Speed	HT Ref Clock	HT Link Speed	North Bridge Speed	Memory Speed	CPU VID
Foxconn A79A-S ACC Off	3280	205	2050	2255	1093	1.4250V
Foxconn A79A-S ACC Off	2870	205	2050	2255	1093	1.4125V
ASUS M3A32-MVP No ACC	2970	205	2050	2255	1093	1.4000V

This particular CPU acts really strange when trying to increase the HT frequency, anything over 205MHz usually meant decreasing clocks in a hurry, no matter voltages, NB speeds, or HT settings. We found it amazing that turning on ACC all of a sudden allowed a 410MHz increase on the Foxconn board and 310 compared to the SB600 board..all with a minimal increase in CPU voltage.

Phenom 9850BE - Highest Core Speed
Vista 32	Core Speed	HT Ref Clock	HT Link Speed	North Bridge Speed	Memory Speed	CPU VID
Foxconn A79A-S ACC On	3400	200	2000	2200	1066	1.4750V
Foxconn A79A-S ACC Off	3100	200	2000	2200	1066	1.4375V
ASUS M3A32-MVP No ACC	3100	200	2000	2200	1066	1.4250V

If we simply push for highest core speed, ACC makes a 300MHz difference. Our 9850 could hit 3.1GHz on the two reference configurations, but 3.4GHz was possible on the A79A-S with ACC enabled.

Phenom 9950BE - Highest Core Speed
Vista 32	Core Speed	HT Ref Clock	HT Link Speed	North Bridge Speed	Memory Speed	CPU VID
Foxconn A79A-S ACC On	3500	200	2000	2400	1066	1.4750V
Foxconn A79A-S ACC Off	3400	200	2000	2400	1066	1.4750V
ASUS M3A32-MVP No ACC	3400	200	2000	2200	1066	1.4675V

Our latest retail Phenom X4 9950 Black Edition overclocks quite well, 3.4GHz isn't a problem. Here ACC doesn't make much of a difference at all, 100MHz is all the extra it got.

Conclusion

It would seem that the SB750 South Bridge really does make a difference in overclocking.  As compared to the older SB600 on the current 790FX motherboards, the additional features are a welcomed changed that any user would be happy to see.  Seems like AMD is really trying to make its Phenoms comparable to the Core 2 processors from Intel.

However, I feel that the push is just too late.  Intel has the Nehalem chips, now Core i7, coming out in just a few short months.  That would mean the current stock of Core 2 processors would drastically reduce in price ruining AMD's performance/price ratio that it has always enjoyed.  Also, Intel's Nehalem boasts about 30% improvement in performance for the same clock speeds compared to its Core 2 processors.  Since AMD could never compete directly in clock speeds, this is another blow against AMD.

While the new SB may bring AMD closer to matching Intel's high clock speeds, the Nehalem processors will widen the gap even more.

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