Q31149: Mouse-Cursor State and EGA Memory

Article: Q31149
Product(s): See article
Version(s): 6.x
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_basic
Last Modified: 28-JUN-1988

The Microsoft Mouse driver EGA interface maintains the mouse-cursor
state in the unused 8K of video memory available at the top of the
four 64K buffers that comprise the 256K of total video memory
available on a loaded EGA adapter. Normally this is of little
consequence in programming such systems; however, if you resize the
video buffer by reprogramming the CRT Controller's Offset register,
problems may occur.
   You will know there is a problem if garbage appears on the display
after panning through an enlarged virtual screen.
   To work around this problem, do not use Mouse Function 1 to show
the mouse cursor. Instead, use an alternative method of monitoring the
mouse's screen location (e.g. XOR some graphical object to the
screen).

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