Information for Tseng Chipset Users : Acceleration issues
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## 11. Acceleration issues

The XFree Acceleration Architecture makes extensive use of the unused video memory on the VGA card. If there is not enough free video memory, some acceleration features will be disabled or crippled, resulting in less performance.

To avoid this from happening, try to keep an absolute minimum of 16 kb of free memory, in addition to the 1kb already reserved by the accelerator.

In practice, this small amount of memory should not be a problem. Most cards nowadays have 2 MB of video memory, and running 1280x1024 still leaves plenty of memory unused. Even a 1600x1200 desktop will leave over 170kb unused, which will then be used by the accelerator to enhance performance.

Most 1MB cards cannot display modes larger than 1024x768 with a decent refresh rate, leaving 256kb unused.

The order in which free memory is used to accelerate certain features is as follows.

If no video memory is unused (i.e. all of it is used for display memory), no acceleration can be used at all -- not even a hardware cursor on the ET6000.

If the hardware cursor is enabled (ET6000 only) and there's at least 1kb of free video memory, 1kb is used for that.

If there is at least 1kb of free memory remaining after this, most acceleration features are enabled as well, reserving an extra 1kb of video memory.

If there's still some free memory, some extra acceleration features are enabled. These require more free video memory, depending on the virtual screen width and the color depth (bpp). The server will print out how much memory it used if it could.

If there's still some free video memory, it is used as a pixmap cache. This way, small patterns and images can be kept in the video memory so that they don't need to be transferred into the video memory each time they're needed. This is beneficial because transferring an image over the bus to the video memory takes a lot more time than letting the accelerator blit it from the pixmap cache to the display memory.

Information for Tseng Chipset Users : Acceleration issues
Previous: Mode issues
Next: ET6000 memory size facts and fiction