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Onboard Graphics Shootout - AMD 790GX vs. 785G - The Contenders

After previously testing the 785G chipset's performance compared to the 790GX, today we will solely focus on their graphics sub-systems.

By: | Socket AM3/AM3+ in Motherboards | Posted: Aug 24, 2009 5:21 am



Onboard Graphics Shootout - AMD 790GX vs. 785G


First off on the list we have a re-cap of the AMD 790GX chipset, which by far is a much better option than the high-end and aging 790FX chipset that AMD still calls its flagship chipset.


790GX supports AM2+ and AM3 processors using the Hyper Transport 3.0 link from the CPU to the External Northbridge. This gives the CPU as much bandwidth as it needs to communicate with offboard devices like the GPU and others. The 790GX is a two-chip solution with the Northbridge containing the IGP and PCI Express 2.0 hubs. The Southbridge can be paired with any of the ATI/AMD offerings that use either A-Link or A-Link II for communication, the best of which is the new SB750.


The Northbridge is quite impressive for a mid-range chipset. The 790GX PCI Express controller hub on the Northbridge has a total of 20 lanes. 16 of them are used for the graphics system, while the other four are for the A-Link II. That's right, ATI uses a PCIe x4 link to connect to the SB and the NB, a very smart idea as it allows for modular designs. You can add any SB to any chipset that uses this connection. For the 16 PCIe lanes for discrete GPU use, you can have them as either a single x16 lane or two x8 lanes.


The IGP that the 790GX incorporates is a Radeon HD 3300. This GPU itself is equipped with 40 stream processors and there's also an increase in the 16-bit memory interface from 500MHz on the 780G chipset to 700MHz on the 790GX.


To add a bit of spice to the onboard graphics situation, AMD finally went with adding an optional cache memory chip. It is at the motherboard manufacturer's discretion to add this, but 128MB of GDDR3 memory can be added which is clocked at 700MHz to help reduce the overall use of system memory.


In the 790GX there are three options for the video memory processing; you have SidePort, UMA and Sideport+UMA. In SidePort only mode no system memory is set in reserve for additional frame buffer space, only the 128MB of GDDR3 is used.


In UMA only mode the board functions like any other IGP based system, using the system memory only for its frame buffer. This option is only there for boards without the cache memory integrated.


In Sideport+UMA the IGP is able to access a set amount of system memory for additional frame buffer space to try and help increase the overall performance.


Not only does the IGP get a boost with the cache memory, it still supports ATI's Hybrid Crossfire design, allowing the IGP and a discrete ATI graphics card to be paired up to not only help save power, but also increase the system performance by running Crossfire.


Paired with any number of AMD's Southbridges that support either A-Link or A-Link II, the most common pairing is using the AMD SB750, the most advanced Southbridge AMD currently has on offer.


AMD 785G


Onboard Graphics Shootout - AMD 790GX vs. 785G


Now it's to the newest member of the group from AMD, the 785G. With the naming you may think it's an older and slower chipset compared to the 790GX, which is far from the truth. In fact, it is the 785G chipset that supports something the 790GX cannot.


785G supports the same ability to run two ATI video cards in Crossfire by splitting the 16 lanes on the 785G Northbridge into 2x8 slots on the board, this as well as PCIe 2.0 and all the latest AM2+ and AM3 processors. The only difference lies in the IGP itself.


The IGP is based on an ATI 4xxx series core designed to bring DX10.1 to the IGP market, which ATI has been a little slack about actually. The 785G supports a Radeon HD 4200 GPU which contains the same shader count and memory clock bus and width as well as support for SideWort memory. Only the DirectX driver has been changed to be compliant with DX10.1 games and applications.


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