Developer mode is a mode that can be invoked via the command-line switch (-devparm) in Vanilla Doom.
When developer mode is activated, pressing F1 captures a screenshot to disk in PCX format. This replaces the help screen that normally appears when F1 is pressed. Furthermore, a set of dots appear at the bottom-left of the screen which can be used to determine the frame rate, using the formula 35 / (number of dots).