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Virtual Screen

    The mouse software operates on the computers screen as if it were a
    virtual screen of individual points arranged in a matrix of horizontal
    and vertical points. The "virtual screen" column in the screen mode
    table gives the number of horizontal  and vertical points in the matrix
    for each supported screen mode. Whenever interrupt 10h is called to
    change the screen mode, the mouse software intercepts the call and
    determines which virtual screen to use. The mouse software also reads
    the screen mode and chooses the appropriate virtual screen whenever
    mouse function 0 is called to reset default parameter values in the
    mouse software.

    Regardless of the screen mode, the software uses a pair of virtual
    screen coordinates to locate an object on the screen. Each pair of
    coordinates defines a point on the virtual screen. The horizontal
    coordinate is given first.

    Many mouse functions take virtual screen coordinates as input, or return
    them as output. Whenever You refer to a pixel or character in a mouse
    function, make sure that the horizontal and vertical coordinates are the
    correct values for the given screen mode. If You supply an incorrect
    value, the function rounds the value down before continuing. The mouse
    functions always return correct values for the given screen mode.

    Graphics modes 6, Eh, Fh, 10h, and 30h:

    In graphics modes 6, Eh, Fh, 10h, and 30h, and for the Hercules
    Monochrome Graphice Card, each point in the virtual screen has a
    one-to-one correspondance with each pixel on the screen. In these modes,
    the full range of coordinates in the "virtual screen" column is
    permitted.

    Graphics modes 4, and 5:

    In graphice modes 4, and 5, the screen has half the number of
    pixels that is has in the other graphics modes. To compensate, the mouse
    software uses only even-numbered coordinates. This means that every
    other point in the virtual screen corresponds to a pixel.

    Text modes 2, 3, and 7:

    Text modes 0, and 1:

                To be completed...