These system variables tell you which platform FoxPro is running on. In Visual FoxPro for Windows, _WINDOWS is true and the rest are false. In VFP/Mac, _MAC is .T. If we ever see Visual FoxPro for the other platforms, we expect we'll see the matching system variables set true, but we're not holding our breath.
lIsDos = _DOS
lIsMac = _MAC
lIsUnix = _UNIX
lIsWindows = _WINDOWSBack when cross-platform meant more than Windows 95 and Windows NT, these variables were useful for cross-platform development. They allowed you to bracket platform-specific code so it didn't cause errors on the other platforms.
DO CASE
CASE _WINDOWS
MODIFY WINDOW screen FONT "Arial",10
CASE _MAC
HIDE WINDOW screen
ENDCASE