## Display Device Driver Reference for OS/2

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### XGA Design Tradeoffs

It is impossible to anticipate what the DOS application might do when the memory-mapped I/O register is touched; assume the worst. To preserve the integrity of the system by not permitting the DOS application to interfere with system operations, lock down the affected memory regions.

Although there are numerous ways to do this, there are penalties involved with each one. Some of the methods (and their possible drawbacks) are described following this table.

In a multiple XGA configuration system, saving and restoring every XGA probably is desirable. Instead, you can let the DOS application take care of it, or you can selectively save/restore only one XGA device.

Software emulating the XGA hardware operations is not desirable.

The only apparent alternative is to freeze the DOS application when the I/ O ports are accessed in the background. A frozen image can be presented in a window if the complete video state and VRAM are saved for the DOS window session.