AGP Read Synchronization – BIOS Optimization Guide

AGP Read Synchronization - BIOS Optimization Guide

AGP Read Synchronization

Common Options : Enabled, Disabled

 

Quick Review

This BIOS feature ensures proper synchronization of data transferred on the AGP bus.

When enabled, the chipset will wait until all writes in the Global Write Buffer are completely written to the system memory before it initiates any writes to the AGP graphics card.

When disabled, the chipset will allow data from the system memory to be written to the AGP graphics card, even if the Global Write Buffer has not completed its data transfers to the system memory.

While it may seem that AGP Read Synchronization should be enabled, this is actually not true. To avoid data synchronization problems, the chipset actually allows the Global Write Buffer to be snooped. If the data is found in the Global Write Buffer, it is read directly from the buffer and then the buffer is flushed.

However, it is not possible for both methods to be enabled simultaneously. One of these two methods must be enabled for proper synchronization of data, but not both at the same time.

For performance reasons alone, AGP Read Synchronization should be disabled. In addition, many motherboard BIOS setup utilities do not allow you to disable the snooping of the Global Write Buffer. Therefore, if you enable AGP Read Synchronization, you will experience problems with your graphics card, especially when data is written to the AGP aperture.

 

Details

When the graphics processor writes data to the AGP aperture, it doesn’t directly write the data to the system memory. Doing so will tie up the graphics processor for a long time as the AGP bus (as well as system memory) is many, many times slower than the local memory buffer.

Instead, the graphics processor writes the data to a Global Write Buffer. This allows the graphics processor to be quickly released for other duties. The Global Write Buffer then writes the data to the system memory, while the graphics processor is working on something else.

Unfortunately, the use of the Global Write Buffer means that data synchronization may be a problem. If the graphics processor writes data to the AGP aperture and then requests the same data before the write buffer completes the write process, the graphics processor will receive outdated or incorrect data.

This is where the AGP Read Synchronization BIOS feature comes in. It ensures proper synchronization of data transferred on the AGP bus.

When enabled, the chipset will wait until all writes in the Global Write Buffer are completely written to the system memory before it initiates any writes to the AGP graphics card.

When disabled, the chipset will allow data from the system memory to be written to the AGP graphics card, even if the Global Write Buffer has not completed its data transfers to the system memory.

While it may seem that AGP Read Synchronization should be enabled, this is actually not true. To avoid data synchronization problems, the chipset actually allows the Global Write Buffer to be snooped. If the data is found in the Global Write Buffer, it is read directly from the buffer and then the buffer is flushed.

However, it is not possible for both methods to be enabled simultaneously. If AGP Read Synchronization is enabled, no writes to the AGP graphics card can occur until the Global Write Buffer is emptied. But for the Global Write Buffer to be snooped, a write must first be initiated by the chipset. This is a Catch-22 situation which is logically not allowed by the chipset.

One of these two methods must be enabled for proper synchronization of data, but not both at the same time. Snooping the Global Write Buffer provides some performance advantage when data is found in the buffer, because the graphics processor can read directly from the buffer. On the hand, waiting for the Global Write Buffer to complete writing its data, before the chipset can initiate writes to the AGP graphics card, reduces performance.

For performance reasons alone, AGP Read Synchronization should be disabled. In addition, many motherboard BIOS setup utilities do not allow you to disable the snooping of the Global Write Buffer. Therefore, if you enable AGP Read Synchronization, you will experience problems with your graphics card, especially when data is written to the AGP aperture.

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