Video Memory Cache Mode – BIOS Optimization Guide

Video Memory Cache Mode - BIOS Optimization Guide

Video Memory Cache Mode

Common Options : USWC, UC

 

Quick Review

Video Memory Cache Mode is yet another BIOS feature with a misleading name. It does not cache the video memory or even graphics data (such data is uncacheable anyway).

This BIOS feature allows you to control the USWC (Uncached Speculative Write Combining) write combine buffers.

When set to USWC, the write combine buffers will accumulate and combine partial or smaller graphics writes from the processor and write them to the graphics card as burst writes.

When set to UC, the write combine buffers will be disabled. All graphics writes from the processor will be written to the graphics card directly.

It is highly recommended that you set the Video Memory Cache Mode option to USWC for improved graphics and processor performance.

However, if you are using an older graphics card, it may not be compatible with this feature. Enabling this feature with such graphics cards will cause a host of problems like graphics artifacts, system crashes and even the inability to boot up properly.

If you face such problems, you should set this BIOS feature to UC immediately.

 

Details

Video Memory Cache Mode is yet another BIOS feature with a misleading name. It does not cache the video memory or even graphics data (such data is uncacheable anyway). It is actually similar to the USWC Write Posting BIOS feature.

Current processors are heavily optimized for burst operations which allows for very high memory bandwidth. Unfortunately, graphics writes from the processor are mostly pixel writes which are 8 to 32-bits in nature. Because they do not fill up an entire cache line, such writes are not burstable. This results in poor graphics write performance.

To correct this deficiency, processors now come with one or more internal write combine buffers. These buffers are designed to accumulate graphics writes from the processor. These partial or smaller writes are then combined and written to the graphics card as burst writes.

The use of these internal write combine buffers provides many benefits :-

  1. Partial or smaller graphics writes from the processor are now combined into burstable writes. This greatly increases the performance of the processor and AGP (or PCI) buses.
  2. Graphics writes will require fewer transactions on the processor and AGP (or PCI) bus. This improves the bandwidth of those buses.
  3. The processor will only need write to its internal write combine buffers, instead of the processor bus. This improves its performance by allowing it to work on other tasks while the write combine buffers handle the actual write transaction.

Because the write combine buffers allow speculative reads, this feature is known as the USWC (Uncached Speculative Write Combining) feature. The older method of writing all processor writes directly to the graphics card is known as UC (UnCached).

This BIOS feature allows you to control the USWC (Uncached Speculative Write Combining) write combine buffers.

When set to USWC, the write combine buffers will accumulate and combine partial or smaller graphics writes from the processor and write them to the graphics card as burst writes.

When set to UC, the write combine buffers will be disabled. All graphics writes from the processor will be written to the graphics card directly.

It is highly recommended that you set the Video Memory Cache Mode option to USWC for improved graphics and processor performance.

Please note that this feature must also be supported by the graphics card, the operating system and the graphics driver for it to work properly.

All Microsoft operating systems from Windows NT 4.0 onwards support USWC, so you do not need to worry if you are using a Windows NT 4.0 or newer operating system from Microsoft. As this feature has been around for some time, drivers of USWC-compatible graphics cards will fully support this feature.

However, if you are using an older graphics card, it may not be compatible with this feature. Older graphics cards make use of a FIFO (First In, First Out) I/O model which can only support the UnCached (UC) type of transaction. Enabling this feature with such graphics cards will cause a host of problems like graphics artifacts, system crashes and even the inability to boot up properly.

If you face such problems, you should set this BIOS feature to UC immediately.

 

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