Many books on slot machines incorrectly say that each reel is equally likely to stop on each position, or stop. This was true of the early days of slot machines before they become electronic. Today this is patently untrue. Modern electromechanical slot machines are governed by random numbers and microchips. Based on evidence from a variety of sources including program specifications for specific slot machines I offer the following explanation of the steps that a three reel slot machines goes through on a typical play.
Player initiates a new play by inserting a coin, pulling the handle, or hitting the 'spin' button.
The machine selects three random numbers. These are chosen from a random number generator that is constantly drawing random numbers at a rate of thousands per second. The numbers chosen at the moment the play is initiated are the ones used to determine the final outcome. The random numbers are drawn in a cycle, to insure that each number is chosen once per cycle, but to the player this would not matter since thousands of random numbers are drawn between plays. The three random numbers chosen will be divided by a fixed number.