## Display Device Driver Reference for OS/2

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### EGA.SYS and INT 2Fh Screen Switch Notification

For some DOS EGA applications, OS/2 is not able to switch from a DOS session to an OS/2 session and then back again. Upon return to the DOS application, the screen will be incorrect. The DOS EGA applications that do not run successfully are:

Applications that download fonts into a character generator block other than Block 0. Character Generator Block 0 is supported.

Graphic mode applications that use more than one display page.

Advanced graphics mode applications that write directly to the registers on the EGA adapter.

To supplement OS/2 screen switching support, a DOS application can be written to use the EGA register interface. Alternatively, a DOS application can be notified on a screen switch through multiplex interrupt 2Fh, AH = 40h. These two mechanisms are described in the following sections.

Note: On an IBM PS/2 personal computer the registers on the adapter are both readable and writable. For these configurations, OS/2 reads and saves the registers on a screen switch away from a DOS session, and restores the registers upon return to a DOS session.

For configurations including the IBM PS/2 Display Adapter 8514/A when the 8514/A display adapter is in an advanced function mode, the OS/2 operating system does not save the physical display buffer when switching away from a DOS session. Therefore, end users are cautioned to complete any 8514/A advanced function mode application before switching to OS/2 mode.