AMD's SB750: Enabling Higher Phenom Overclocks?
by Gary Key on July 23, 2008 12:00 AM EST- Posted in
- CPUs
The new SB750, or more like a SB700 with BOTOX-
The SB750: I remember when South Bridges weren't built on flip-chip packages
While technically a new part, the SB750 appears to us to be a point upgrade to the SB700. However, it is a major improvement over the SB600 used on the current 790FX boards. Anyway, let’s get into what has changed and what has not.
The SB700/750 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. We just received a beta set of drivers for this new feature, but at first glance, it appears to work fine at this point.
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 or upcoming product that we dare not mention, 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, um, Kinda
AMD was extremely vague in explaining to us exactly how the new SB750 South Bridge actually improved Phenom overclocking. At first all we were given was these two diagrams:
Clearly.
It makes it faster.
You see, the SB750...yeah, these pictures really meant nothing to us so we dug a little deeper with AMD.
Here's what we do know:
The SB750 now has a direct 6-pin interface to the AM2+ socket on the motherboard, there are now pins on the Phenom CPU that connect directly to the SB750. These pins were previously unused and are now used as a means of communication between the South Bridge and the CPU. The SB750, in combination with an updated BIOS, can now override some of the CPU's internal settings which can potentially increase the overclocking headroom of the chip.
AMD says that the settings tweak doesn't impact performance and doesn't change thermals or voltages, it simply can allow a Phenom processor to clock higher when overclocking. The BIOS exposes the parameter being changed, which AMD refers to as the Advanced Clock Calibration (ACC) value. Typically this value has a range of -2 to 0, on motherboards with the SB750 that support ACC the value can be set from -12 to +12. Higher numbers should allow for higher clock speeds, while lower values should allow for lower voltages/lower power operation.
Even after further pressuring, AMD wouldn't tell us what this value actually adjusts - simply stating that it makes it easier for the CPU to run at higher speeds. Based on AMD's careful choice of words, it would seem that adjusting the ACC somehow changes the acceptable margins of operation for the CPU cores (the value can be changed on a per-core basis in AOD or BIOS). By loosening these margins, however it is able to do so, the SB750 + ACC combo can enable many Phenom processors to operate outside of their normal overclocking margins. In fact, AMD is confident enough about the technology that they are willing to state that on average; a 100MHz to 300MHz increase in clock speeds is attainable by the user after some tweaking.
The SB750/ACC trick doesn't work on anything other than Phenom processors, and it works particularly well on the Black Edition processors. While AMD didn't rule out eventually enabling this on K8 based cores, it is a Phenom-only option for now. Reaching higher clock speeds is more of a top priority for Phenom, and it's unclear whether or not whatever AMD is doing here can even work on K8 if they tried. AMD also committed to enabling similar tweaks for upcoming 45nm based Phenom parts, implying that this was not a short-term solution to the clock speed problem.
At the same time, AMD implied that the tweaks that the SB750/ACC feature enables could be incorporated into the manufacturing chain and actually implemented in hardware. There's something strange going on here, but as you'll soon see - the tweak actually works.
Microprocessors are designed to operate in even the most extreme of conditions, AMD seemed to imply that its ability to adjust the ACC value somehow changes this. Curiously enough, AMD cited "competitive concerns" as a reason why it would not disclose exactly what's going on with this new overclocking feature. We can't help but wonder if it is because AMD is going a little too far in the sacrifices it's willing to make in the quest for higher clock speeds.
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bbqchickenrobot - Monday, August 11, 2008 - link
This article claims the PCIe slots are 1.x when they are in fact 2.0 as stated in the manual, website and other sources.Just wanted to make everyone aware of this. There are four total PCIe 2.0 that can run 2x PCIe 2.0 cards at x16/x16 or 4x PCIe 2.0 cards at x8/x8/x8/x8
To get around this, get an X2 version of the Radeon HD 3870 or 4870 and then crossfire with another one or drop down to a standard 3870/4870
MikeODanyurs - Wednesday, August 6, 2008 - link
Anyone see AMD OverDrive 2.1.2 available for download yet? It's Wednesday the 6th, official release day of 790GX+SB750.CZroe - Monday, July 28, 2008 - link
"it's unclear whether or not whatever AMD is doing here can even work on K8 if they tried."Nope. I've been waiting for a board/BIOS maker to expose this chipset setting since the Athlon XP Mobile Barton CPUs first made me aware of it. My Mobile Barton Athlon XP was basically a completely unlocked bin-sorted Athlon XP 3200+ that was stable at lower voltages expected to be run at much-lower voltages and clock speeds.
With it, I discovered that there was clearly a chipset setting not exposed when my Mobile Barton failed to reach 200/400FSB in an nForce2 Ultra400 board (the Shuttle board inside my SN45G XPC... the FN45-whatever IIRC) while the same CPU could easily do 450+FSB in an Asus A7N8X-E Deluxe and an Abit NF7-S. The solution? Drop a thin wire inside the socket to make this completely unlocked CPU *DEFAULT* to 400FSB and it runs perfectly fine and can even go higher (the L12 trick/mod).
With the default FSB being the only difference (all settings/frequencies, including the FSB, were the same), it was very clear that it was related to a calibration setting that was automatically applied by the chipset based on the default FSB (100MHZ/200FSB for this unlocked CPU). Previously, I could only get to 333FSB before Memtest 86+ would start giving errors. There were some reports of even the overclocking boards (mentioned earlier) getting higher with the L12 trick and later many had BIOS updates and board revisions that seemingly integrated the trick. Regardless, the setting was still not exposed and was likely not even something the board/BIOS makers could expose (though they could spoof the default FSB I'd assume). It looks like AMD, having full control of chipset and board-level options here, has finally let the cat out of the bag.
Although the FSB has become the HT bus, the behavior is the same. Changing the calibration setting allows for higher clocks while actually becoming unstable at lower clocks. Ever accidentally erased the OSCAL calibration value on a PIC microcontroller and had to find a working value through trial and error? I'm guessing that this is why it was automatic before.
initialised - Sunday, July 27, 2008 - link
Leads me to suspect that ACC is a means of adjusting the substrate potential to provide a bias to the bottom of the SOI stack to control the threshold voltage of the transistors.SirYes - Friday, July 25, 2008 - link
To spread the word: http://ubuntu-virginia.ubuntuforums.org/showthread...| To be honest I find it hard to believe that they have a campaign
| against Linux. There would be nothing to gain and would have
| a zero fiscal gain.
|
| I think it is a bug you have found
After looking through the disassembled BIOS for the last several hours, rebooting it, and tweaking it more, I'd say this is very intentional, I've found redundant checks to make sure it's really running on Windows, regardless what the OS tells it it is, and then of course fatal errors that will kernel panic FreeBSD or Linux, scattered all over the place, even in the table path for Windows 9x, NT, 2000, XP, and Vista, and had to correct them (Well, at least divert them off into a segment of RAM I hope to god I'm sure about)
No, this looks extremely calculated, it's like they knew someone would probably go tearing it apart eventually and so tried to scatter landmines out so as to where you'd probably hit one eventually.
So if it is a mistake, or incompetence, then it's the most meticulous, targeted, and dare I say, anal retentive incompetence I've seen.
Will14 - Friday, July 25, 2008 - link
I've never had much faith in Windows OC tools.However if this one actually works and AMD can up their speeds a little then I may switch back for my next system.
I must say my 6400-8400(cheated got an e3110 with 20% discount when first released and they were all $280 or OOS for $170). Upgrade path was seamless and I feel bad going from 1950Pro to 8800GT 512 but I follow my wallet not the company I root for.
A cheap system built off cheap Black Box edition(OC'd easily with their tool) and 4870x2 would be pretty sweet come this December if the new cores are kicking and the utils work(a lot of ifs). Not that I need a new system though, but if one did. I don't see nehalems being affordable anytime soon(unless new AMD core rocks(another if)), although my mobo would support. I like the $200 mark for processors $100 for HDD's $200 for gpu's plus generally only needing 1-4 new parts for a yearly $400ish upgrade which is cheaper and allows better performance than the $1-2k some people build.
Zisyncmon - Friday, July 25, 2008 - link
Does anyone know when they will release more info? Gary? Any estimates of when to expect another article? Or will we have to be in suspense all weekend.geok1ng - Thursday, July 24, 2008 - link
I am really hoping that AMD can make some competition on CPU market as they did on the IGP (man i am really looking forward to the next IGP offer from AMD, the 780G is the king, the next chip will be a market consolidating move for sure)and VGA markets but i am not joking when i ask:Will a 3.1Ghz ultraoverclocked quad core Phenom beat a $80 E2180@3,1Ghz in gaming and common day tasks?
As i remember the C2Ds are 15%-25% faster clock per clock, and the AMD CPUs consume MORE power for the same tasks , because they take LONGER to do the same task of a C2D.
jslusser - Thursday, July 24, 2008 - link
So does this ACC on the SB750 do anything for vanilla Phenom's? Will it unlock the multiplier on the non-black Phenoms or just make them more stable with the HT and memory over clocks?I'd like to see where the 9150e/9350e can get to with ACC and the power usage, since the voltage, etc doesn't change. So many questions...
HazaroudsSmoker - Friday, July 25, 2008 - link
First, you can't unlock the multiplier on the non Black Edition Phenoms. They are locked at the factory during manufacturing and can not be altered.Sounds like there will be an advantage still to having the SB750 with Vanilla Phenoms.
Can't wait for Benchmarks.