winapi - Why do WM_APPCOMMAND LPARAM have to be multiplied by 65536 -

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i trying control master volume. able succesfully this:

hwnd mainhwnd = createwindow(szwindowclass, _t("window-noit-ext-profilist"), 0, 0, 0, 0, 0, hwnd_message, null, wcex.hinstance, null); if (!mainhwnd) {     messagebox(null, _t("profilist: call createwindow failed!"), _t("window-noit-ext-profilist"), null);     return 1; }   sendmessage(mainhwnd, wm_appcommand, (wparam)mainhwnd, (lparam)(appcommand_volume_mute * 65536)); // mute  sendmessage(mainhwnd, wm_appcommand, (wparam)mainhwnd, (lparam)(appcommand_volume_down * 65536)); // vol down  sendmessage(mainhwnd, wm_appcommand, (wparam)mainhwnd, (lparam)(appcommand_volume_up * 65536)); // vol

why have multiply 65,536? docs not state this. if don't multiply, doesn't work.

for wm_appcommand, lparam parameter packs 3 values in single integer.

the lower 16bit word, dwkeys, indicates whether various virtual keys down.

the higher 16bit word packs 2 fields: highest 4 bits, udevice, specifies input device generating input event. lower 12 bits, cmd, contains application command.

multiplying 65536 same bit shifting 16 bits left (because 65536 = 0x10000 in hexadecimal). so, when send message appcommand_volume_up * 65536, specifying cmd appcommand_volume_up, , udevice , dwkeys both zero.


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