Horizontal Mouse Position in CGA and MCGA Modes

When you use the 320 by 200 CGA or MCGA graphics modes and a mouse, the horizontal coordinates returned by the driver are from 0 to 638 in increments of two.

The mouse driver returns a horizontal coordinate, which is twice the actual screen coordinate. However, the vertical coordinates correspond directly to the actual position on the screen.

Because the virtual screen for the mouse is in the range of 640 by 200 to 640 by 480, the virtual coordinate that maps to the actual screen coordinate must therefore be double the actual. The vertical coordinates in each case are 200 pixels, so there is a one-to-one mapping of the virtual and actual coordinates.

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