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PCI Express x16 Graphics Bus

I don’t think you seriously assume that the AGP 8x bus doesn’t provide sufficient bandwidth for contemporary graphics cards. According to our experience, all contemporary graphics accelerators store all the data they need in the local graphics memory that is why the data transfer rate along the bus connecting the graphics card with the chipset is not that important. However, Intel gave up AGP 8x bus in the new generation platforms in favor of the new and more promising PCI Express x16.

The thing is that transition to this bus is more likely to be the reflection of the industry tendencies rather than a move caused by some practical issues. For the past few years we have been witnessing the gradual replacement of the parallel PC buses with the serial ones. This way we ca not only simplify the connections configuration but also speed up the data transfer. The transition from AGP 8x to PCI Express x16 is exactly this type of transition from the parallel to serial bus. However, this transition will also bring us a bunch of positive side effects such as higher bandwidth, isolated read and write channels, etc.

Without going too deep into details I would like to note that PCI Express x16 boasts the speed of 2.5 gigatransfers per second in each direction. Depending on the bus width (in our case it is the number of data transfer channels, i.e. there are 16 of them in PCI Express x16) you can transfer from 1 to 32bits of info in each direction within a single transfer operation. Taking into account that the data is transferred along the PCI Express bus with 8/10 compressing coding involved (they use 10 bits to code 8 bits of the initial data), and that the data and instructions are transferred along PCI Express along the same signal lines, the bandwidth of the new PCI Express x16 reaches 4GB/sec in each direction, which makes the total of 8GB/sec. This way, the introduction of the PCI Express x16 bus makes the bandwidth of the bus between the graphics card and the chipset 4 times higher compared with what the AGP 8x provided us with.

Moreover, the shift to PCI express x16 also provides a number of other advantages. First of all I would like to mention the independent channels for transferring the data both ways. PCI Express x16 guarantees the 4GB/sec bandwidth for data transfers in either of the directions or in both of them. The AGP 8x bus didn’t have any dedicated channels that is why the data could be transferred either one way or another only.

PCI Express x16 slot, which will now become a common this for many mainboards based on i925 and i915 chipsets is of the similar physical size than the AGP 8x one.

You will not be able to install AGP 8x graphics cards into the PCI express x16 slot neither mechanically nor logically because these two slots use completely different protocols for data transfers. That is why you will have to get a new graphics card with PCI Express x16 interface for the new i925X and i915 based mainboards.

The major graphics chip developers, ATI and NVIDIA, have already got ready for the transfer to the new bus interface. In the nearest future we will see a lot of solutions for the new bus standard based on the chips from both companies. However, the support of the new PCI Express x16 bus will be of pretty “transitional” type still, because ATI and NVIDIA haven’t yet redesigned their chips completely for the new graphics interface.

However, ATI’s and NVIDIA’s approaches to this matter appeared radically different. NVIDIA actually provides its existing graphics chips supporting  AGP interface with a new additional bridge, which will be converting data packs transferred along the PCI Express x16 bus into AGP 8x data format. NVIDIA uses an external HSI (High Speed Interconnect) chip for this purpose, which can be added to any of the already existing solutions.

ATI approached this problem from a different angle and simply replaced the interface part of the already existing chips, having introduced PCI Express x16 instead of the good old AGP 8x.

 
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