Dell Latitude C640 - Manual (Page 24)

System Functions
Opens the Windows Security window
Enables and disables the numeric keypad
Enables and disables scroll lock
Display Functions
Switches the video image to the next display in the following sequence: the integrated display,
an external monitor, and both displays simultaneously
Increases brightness on the integrated display only (not on an external monitor)
Decreases brightness on the integrated display only (not on an external monitor)
Power Management
Activates the power management mode of your choice. You can program this keyboard shortcut
on the Advanced tab in the
Power Options Properties window .
Speaker Functions
If no sound comes from the speakers, press and adjust the volume.