## Display Device Driver Reference for OS/2

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### WrtToScrn/Panic Write Support

Before the video subsystem is loaded, and when the system is about to abnormally terminate, messages are sent to the screen by the WrtToScrn function, also known as Panic Write. This function switches to real mode and executes the INT 10h function to set the video mode to a text mode ( BIOS mode 3). It then uses BIOS INT 10h, "Write TTY", to display the message.

However, adapters such as the 8514/A have native modes that cannot be changed by INT 10h. In such cases, the BVH must include a Level 0 device driver that hooks INT 10h to provide extended set mode support. This hook must force the adapter out of its native mode, and then pass control to the previous INT 10h support. If these conditions are not satisfied, the adapter should not drive the power up display.