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EGA.SYS is a device driver that allows different programs to be aware of the current state of the registers on an EGA video card. It is necessary because many registers on an EGA card can be written to by an application, but there is no way to read the registers to determine the current settings. This is a problem for programs such as MS-DOS Shell that must save and restore the computer screen.

The solution to this problem is to require programs to record the current state of the EGA video registers in the EGA.SYS file. A program that changes the state of the video display can determine current settings by checking EGA.SYS.

If EGA.SYS is not installed on a system that has an EGA video card, the screen may be unreadable after the user switches between two different graphics-oriented programs from within Shell.

The MS-DOS 5.0 Setup program installs the EGA.SYS driver in the CONFIG.SYS file on any computer that it detects as having an EGA video card.