Review Index:

AMD's 790GX: I'm Down with ACC

Author: Josh Walrath
Subject: Motherboards
Manufacturer: AMD

Some ACC for Summertime

The 790GX and SB750

    The 790GX product is still primarily based on the 780G and SB700, but it adds a few new wrinkles to the equation.  The northbridge/integrated chip is still based on the 780G, with nearly identical specifications.  The two big differences are the clockspeed that the IGP is at, and SidePort support is mandatory.  The 790GX is clocked at an impressive 700 MHz and the SidePort memory is the GDDR-3 variety running at 1333 MHz (giving it an extra 2.6 GB/sec of bandwidth).  The SidePort can be in either 64 MB or 128 MB varieties.  While this is not exactly a staggering amount of bandwidth, it does provide the chip with plenty of low latency access to a fair sized memory chip.  When we combine the dual channel memory performance of the Phenom (approximately 10 GB/sec of realworld bandwidth) the graphics portion certainly has plenty of bandwidth to serve its needs.  The 790GX also supports the full AM2+ specification, so it is a HT 3.0 enabled device.  This allows a tremendous amount of bandwidth to pass between the CPU and the northbridge, and is handy when dealing with integrated graphics performance.

Even with the 200 MHz boost in clockspeed, the 790GX still does not need any excessive cooling as compared to the earlier 780G.  This is accomplished by both the design of the chip and the use of TSMC’s 55 nm process.  It also does not pull all that much more power than the lower clocked 780G.  We can assume that either AMD designed a lot of headroom into the 780G/790GX, or we are seeing cherry picked chips that can reach these speeds with few problems.

This slide was created in July, and the price drops on the AMD side have made their position that much stronger against Intel.  The Phenom 9850 now retails for around $169.

     The 790GX has a grand total of 26 PCI-E 2.0 lanes to divvy about at its leisure.  One should immediately recognize that 26 is less than 32, which is the number required at a minimum to support a full 2 x 16X PCI-E setup for CrossFire functionality.  The 790GX achieves CrossFire support by splitting one 16X connection into 2 x 8X connections.  This is PCI-E 2.0 though, so the overall bandwidth of an 8X PCI-E slot is equal to that of the older PCI-E 1.0 16X slot.  If a single card is used then the primary slot will automatically configure itself as a 16X connection, but if CrossFire is enabled then it will automatically switch to 2 x 8X.  Four of those PCI-E lanes are used to connect the northbridge to the southbridge, so a grand total of 22 are available for graphics and other slots/peripherals.

    The SB750 does have a few advantages over the SB700.  The SB750 still shares the same number of SATA and USB ports, as well as all of the same I/O connections needed for a modern motherboard implementation.  The differences are that the SB750 supports RAID 5 functionality, and the inclusion of ACC.

We can see how AMD has implemented ACC in two very different ways.  At the top end it enables higher overclocks, while for those users not interested in burning hot CPUs can run ACC and turn down the voltage to allow for lower heat and power draw.

    ACC stands for Advanced Clock Calibration, which is an overclocking tool aimed at increasing the upper overclocking spectrum of current Phenom processors.  ACC utilizes six previously unutilized pins on the Phenom chip, but what exactly it does is a secret.  Many have deduced that ACC actually changes around some of the internal timings to the Phenom CPU to allow for greater overclocking headroom, or for those interested in lowering their voltage and underclocking the processor.  AMD does not make concrete promises as to how much this can improve an individual overclock, but they are not hesitant to say that they are seeing an average of an extra 200 MHz over a standard overclock using the older 790FX chipset with SB600 southbridge.  Considering the latest version of the Phenom 9950’s are now 125 watt products, we should expect to see a bit more headroom in overclocking these quads.  As I have mentioned above, underclockers might be quite interested in this technology.  By using ACC AMD also claims that they see stock speeds running with 1v, and by going down to sub 2 GHz speeds they are seeing 0.9v.

    The 790GX is not meant to be an entry level product, but rather straddle the space between the 780G/770 products and the 790FX.  Prices as of now are between $99 and $150, depending on the model and manufacturer.