Canon EOS-1DX Instruction Manual page 174

s: Shutter-Priority AE
Shutter Speed Display
The shutter speeds from "8000" to "4" indicate the denominator of the
fractional shutter speed. For example, "125" indicates 1/125 sec. Also, "0"5"
indicates 0.5 sec. and "15"" is 15 sec.
174
If the maximum aperture blinks, it indicates
underexposure.
Turn the <6> dial to set a slower shutter
speed until the aperture stops blinking or
set a higher ISO speed.
If the minimum aperture blinks, it indicates
overexposure.
Turn the <6> dial to set a faster shutter
speed until the aperture stops blinking or
set a lower ISO speed.