GDDR Vs DDR

fzkl

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Nov 14, 2004
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Whats the difference between GDDR and DDR apart from the fact that GDDR runs at higher speeds. Also, whats the need for different memory types for a GPU in comparison to a CPU?

If AMD moves to socketed GPU's and they share common memory, how will this affect memory technology like GDDR?
 

acegazda

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May 14, 2006
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gddr is graphic card specific, thus can be dedicated to less tasks than ddr and be more efficiant. Don't know about your 2nd question though...
 

Lord Evermore

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Oct 10, 1999
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There is nothing technical about the chips that would prevent them being used for system memory, all that is required is a memory controller. However they aren't drop-in compatible with standard DDR, and isn't optimized for the general tasks that main system memory needs.

Per Wikipedia, "This memory uses internal terminators, enabling it to better handle certain graphics demands. To improve bandwidth, GDDR3 memory transfers 4 bits of data per pin in 2 clock cycles."

It still runs at a higher voltage (2.5V) than DDR2 (1.8V), which was reduced from DDR1's 2.5V, because they decided it's okay for video memory to run at a little bit higher heat output in order to get those extreme clock speeds.

A GPU must be designed to support the memory it's being connected to. Current GPUs are designed for GDDR3, and some might support plain DDR2, or even DDR1. They have directly integrated memory controllers.

A socketed GPU would need to have a memory controller designed to communicate over whatever bus is used by the socket. For an AMD system that would be HyperTransport. The GPU would use HyperTransport to communicate memory access requests to the system memory controller (integrated in the CPU), which would then do the actual access to the memory. This is what happens already with a PCI-Express or AGP video card which needs to store data in main memory. The big difference would only be that ATI would need to design a GPU with a HyperTransport interface, rather than PCI-Express or AGP, but that could technically even be done with a bridge chip.

With the socketed GPU, there'd be no need for GDDR in the system, however such memory access will still be slower than dedicated video memory, just like PCIe or AGP memory access is slower.
 

Goi

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Oct 10, 1999
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GDDR are soldered on board and are very near to the memory controller/GPU, which keeps the trace lengths shorter, which reduces signal integrity issues, which enables higher clocks. Regular DDR memory on the other hand is mounted on DIMMs which are connector to DIMM slots, which then have to be routed to the memory controller/CPU. The traces are longer, it goes through 2 PCBs and a connector, which causes more SI issues, which means they can't get the clocks up that high.
 

Lord Evermore

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Oct 10, 1999
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Originally posted by: Goi
GDDR are soldered on board and are very near to the memory controller/GPU, which keeps the trace lengths shorter, which reduces signal integrity issues, which enables higher clocks. Regular DDR memory on the other hand is mounted on DIMMs which are connector to DIMM slots, which then have to be routed to the memory controller/CPU. The traces are longer, it goes through 2 PCBs and a connector, which causes more SI issues, which means they can't get the clocks up that high.

DDR memory speeds are limited by the design of the chips. Video cards using DDR2 are limited by those speeds even though they're soldered on with short traces. DDR modules on the other hand do have some limitations because of being mounted on a PCB and connected to a slot, but that's not an inherent limitation of the DDR2 memory chips. GDDR3 memory chips could be put on a PCB and in a slot and would still be faster than DDR2 modules.
 

Goi

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Oct 10, 1999
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Well, I was talking about similar DDR technologies on both the main memory and GPU side, i.e. those with a prefetch of 2 are compared, and those with a prefetch of 4 are compared, but not between the 2 groups, coz that would be apples and oranges.