Information for Western Digital Chipset Users : WD90C24 features
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## 3. WD90C24 features

These next three sections apply only if you have a WD90C24, WD90C24a, or WD90C24a2 and don't specify some other chipset in your XF86Config file. The SVGA pvga1 driver now recognizes the wd90c24 family as different from the WD90C30 and seems to resolve most of the problems people encountered when these chips were treated as WD90C3X. The new code has the following features:

- Locks the shadow registers at appropriate times; This should prevent scrambled displays after exiting X with dual scan screens when simultaneous or LCD display mode is selected. The code does depend somewhat on the behavior of the BIOS regarding when it locks the shadow registers, etc.
- Allows (forces) the use of a full 1 Meg VRAM for dual scan systems when the server is started while external CRT only display is in operation. This allows 1024x768x8 resolution.
- If the `XF86Config` file specifies a virtual screen size which requires more than 512 K VRAM when the server is started on a Dual Scan LCD, the driver will force the virtual size to 640x480. This eliminates the need to edit the `XF86Config` file when you switch from 1024x resolution on the CRT, to or from the LCD screen. If no virtual size is specified, the result will be 800x600 virtual in LCD modes and 1024x768 in CRT only mode (so you have a choice).
- Note that on dual scan systems, you must still exit X, switch displays, and restart X to change to/from CRT only with 1 Meg videoram. This is because once the server starts, you can't change the virtual screen size. There is no way around this with the current server and the WD90C24 with dual scan displays. The WD90C24 requires half the videoram be used for a ``Frame buffer'' when the dual scan LCD is in use.
- The new server uses the accelerated features of the WD90C24a. It is not clear from the data book if the WD90C24 also supports ALL the required features. Several people have stated that the WD90C24 is not accelerated, but the differences section of the WD90c24a data book implies that they ARE all three accelerated. The differences documented with regard to acceleration are with the type of line drawing the hardware does; Only the newer chips support the type of line drawing that MS windows wants. This may be what has caused the confusion since the accelerated windows drivers may only support the WD90c24a chips. If this turns out to be a problem with the WD90C24, acceleration can be disabled by adding the line:

            Option "noaccel"

  to the Device section of the `XF86Config` file.
- Although the new server does not support programmable clocks in the same way as some of the other servers, 8 of the 17 clocks may be set to (almost) any value via the Clocks line. It also supports options for adjusting the VRAM clock.

Information for Western Digital Chipset Users : WD90C24 features
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Next: WD90C24 clocks