To maximize the efficiency and throughput of the AGP bus, the address bus can be separated (demultiplexed) from the data bus. This is accomplished with the addition of 8 extra sideband lines. These 8 sideband lines are collectively referred to as the Sideband Address (SBA) Port. Although the SBA port is only an 8-bit wide interface, it allows the graphics controller to issue new AGP requests and commands simultaneously while data continues to flow via the main 32 address/data (AD) lines.

Actually, the AGP bus can transmit AGP requests through either the AD bus or the SBA bus but not both at the same time. However, if the SBA bus is used to transmit those requests, it will prevent the graphics controller from clogging up the AD bus with data requests. While this will not increase overall AGP bandwidth, it allows main AD bus to be used solely for data transfers, thus improving overall AGP data throughput.

In our ceaseless search for ways to enhance the performance of our systems, it is inevitable that we will try to overclock the AGP bus as well. Curiously, for cards that support only the AGP1X transfer mode, the AGP bus showed real promise for overclocking. The cards could usually support AGP speeds of over 112MHz. That's 70% over the specified limit.

However, when it came to AGP2X cards, they would often crash when the AGP speed exceeded 75MHz. That means the AGP bus could only be overclocked by 14%, which is only about as much as the PCI bus could be overclocked as well. Why only 14%?

Well, the AGP2X protocol itself isn't the reason. With a little experimentation, I discovered that SBA was the reason.

While the AD bus itself is highly overclockable, the SBA bus is much more timing sensitive and thus, prone to failure when overclocked too much. That's why users of AGP2X cards that don't support sidebanding can overclock the AGP bus as if they are AGP1X cards. So, if you disable sideband support, you will be able to further overclock the AGP bus without losing the use of the AGP2X protocol.

Naturally, the most ideal situation would be one in which the AGP bus could be highly overclocked while still using the sideband support. Unfortunately, we are stuck with an either-or situation. We can either overclock the AGP bus or enable its sideband support. So, the question now would be - which is the better solution?

Overclocking the AGP bus appears to be the most logical step in improving the performance of the AGP bus since it brings about a substantial increase in AGP bandwidth. For example, overclocking the AGP bus to 112MHz increases the AGP bus' maximum bandwidth to 900MB/s (using the AGP2X transfer mode), which is way over the usual AGP2X's maximum bandwidth of 534MB/s.

On the other hand, sidebanding improves AGP performance indirectly by removing the need for the main AD lines of the AGP bus to service AGP requests and commands. Instead, the AD lines are reserved for data transfers only. Both methods increase practical AGP bandwidth so which offers more? Let's find out.