Most modern slot machines are designed to look and feel like the old mechanical models but they actually operate quite differently from the original models. Instead of the motion of reels used by the mechanical models, a central computer system located inside the machine controls the new models. The computer uses step motors to turn and stop each reel at a predetermined point. Whereas standard electric motors are driven by a fluctuating electrical current, step motors are driven by short digital pulses of electricity that are computer-controlled. The motor gets moved incrementally and very precisely with each pulse. Even though the computer tells the reels when to stop, the games are not preprogrammed to pay out at a certain time. Rather, a random number generator in the computer makes sure that each pull has an equal opportunity at hitting the jackpot.
Whenever the slot machine is running, the random number generator (RNG) sends out whole numbers (usually between one and several billion) hundreds of times each second. The minute a player presses the start button or pulls the arm back, the computer records the next few numbers from the random number generator. Next, it feeds these numbers through a simple program that determines when the reels should stop spinning.

What this means is that the game is essentially over by the time the reels are spinning because the RNG has already selected the stops. Thus, the reels in modern slot machines are displayed for cosmetic purposes only or as a courtesy to the player. There is technically no reason for them any longer. In fact the reels on slot machines have been made obsolete by the computer– a player could just put his money in and the machine could simply tell him whether he won or lost.

Because the reels are weighted differently, some symbols have a greater chance of hitting than others. For example, the slot may pick a random number from 1 to 27, which might correspond to the symbols in the following hypothetical example:

Selected Number Symbol Picked
1-66 Blank (positions between reels)
67-77 Single Bar
78-85 Double Bar
Etc. Etc.
126-127 Jackpot Symbol
This chart, while not an actual slot chart, does illustrate the operating concept behind the slot machines. As you can see, if there are 22 stops on the wheel, including the 11 symbols and 11 blanks, you have a 1in 64 chance of hitting a jackpot, not a 1 in 22 chance as it first appears. If casinos were more honest they would hang a disclaimer on each of their machines pointing this out.
Perhaps a better way to demonstrate this concept is to take the example of a bag containing three items: a piece of coal, a piece of fruit and a piece of gold. Obviously, your chance of getting the piece of gold is one in three. This is not, however, the way the slot machine works because the slot machine would have a lot more pieces of coal and fruit than it would pieces of gold. More accurately, the slot machine would have about forty pieces of coal, twenty pieces of fruit and only one piece of gold. Your chance of picking the piece of gold is almost zero. You stand a much greater chance of getting a piece of fruit or a piece of coal and of the two, you stand a greater chance of getting the piece of coal.In addition, each reel is weighted different. As a result, a player has a much greater chance of getting jackpot symbols on the first and second reels (which builds the player’s suspense) than on the third reel, where chances are very slim.

To help illustrate how the process works with the standard three-reel machine, assume that you are at a slot machine and have pressed the start button or pulled the handle to get the machine started. The computer will then record the next three numbers from the random number generator. The first number will be used to determine the position of the first reel, the second number to determine the position of the second reel and the third number to determine the position of the third reel. In this particular example, we will set the first number at 123,456,789.

To determine the position of the first reel, the computer divides the first random number by a set value, usually 32, 64,128, 256 or 512. In this example we will say that the computer will divide by 64. The computer then records the remainder of the quotient after it has divided the random number by the set value. In the above example, the computer would find that 123, 456,7 89, can be divided by 64 a total of 1,929,012 times with a remainder of 21.

Obviously, the remainder can’t be less than zero or more than 64, which means that there are only 64 possible end results for this calculation. The 64 end results represent the stops on a large virtual reel and each of these stops corresponds to the 22 stops on the actual reel. The computer then determines how far to move the actual reel for a particular value on the reel by consulting a table. Also, some of the actual stops must be linked to one virtual stop because there are many more virtual stops than there are actual stops.