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A primer on how slot machines reach their result

By Frank Legato


Longtime readers of this magazine will have seen the subject of this column before, but as letters from our readers indicate, many still ask for a description of how the random number generator works to determine the results of each spin on a slot machine.

Therefore, periodically in this space, we like to revisit the basics of how the modern slot machine works, and how the technology developed.

Modern slot machines determine the result of each spin through a method patented in 1984 known as the “virtual reel” system. Before then, slot machines were almost exclusively three-reel mechanical machines, and results were determined by a combination of simple gravity and the number of symbols and spaces on each reel.

It was a system that had endured since the invention of the three-reel slot machine in the 1890s. Originally, reels were spring loaded. Pulling the handle down would wind the springs, and releasing the handle would send the reels spinning. A separate mechanism would stop the reels, and where the reels stopped would determine the win or loss. The process was later modified with an electrical “stepper” motor, which propelled and stopped the reels without the need of coiled springs.

The payback percentage of these old mechanical slot machines was determined by the reel map, which was a layout of the symbols and blanks on each reel strip, known as reel stops. With the mechanical games, there were 22 stops—the position containing a symbol or a blank—on each reel. The laws of probability on where the reels would land, and where the winning symbols and blanks were positioned on the reel map, determined the payback percentage.

One drawback to this system was a comparatively low limit on how large a jackpot could be offered. Since the reel strips were static, the probability of a top jackpot was locked in. The jackpot level had to be low enough for the machine to make money. On a quarter game, it was typical that the top jackpot would be only in the hundreds.

The only ways to offer the enticement of larger jackpots were to raise the required wager, increase the number of stops on each reel, or increase the number of reels. Beginning in the 1970s, slot makers did all these things. Dollar slots offering jackpots in the thousands were a big hit in Northern Nevada. The number of stops on each reel was increased, and four-reel and even five-reel games appeared.

But the jackpot levels were still limited by the physical reels. The virtual reel system would solve that problem. In this system, since the reels are virtual within a computer program, they can have hundreds of virtual stops on each reel. Thus, the math could be worked to offer huge jackpots that would have much longer odds of hitting. For the first time, casino slot machines were able to compete with the life-changing jackpots of the state lotteries.

The key to the system is a virtual duplication of reel stops. Each stop— symbol or blank—is assigned a number in the program. The stops representing higher-paying symbols, including top jackpot symbols, get the lowest amount of assigned numbers in the program. The lower- paying symbols and the blanks get the most numbers in the program.

This allows the programmer to manipulate the laws of probability to determine the overall payback percentage.

The complete set of numbers in the program is only one key to the system. The other is the random number generator (RNG). This is a computer program that cycles through the complete set of numbers continuously. It generates numbers from the program randomly, for each reel, at a speed of 1,000 numbers per second or more.

The RNG continues to cycle through numbers as long as a slot machine is powered up. When the player sits down at a slot machine, the computer is cycling through the entire set of numbers. When the “spin” button is touched, or the handle is pulled (which, in a modern slot, only touches an internal spin button—it’s there for show and tradition only), the computer freezes the set of numbers generated at that instant, and translates it into a reel result.

By law, those number combinations must include the slot’s top jackpot, which must be possible to hit on every spin. The programmers have already set the odds you will hit that jackpot by the duplication of numbers related to the reel stops. After the reel map is created, the mathematicians run tests consisting of millions of simulated spins to verify the theoretical payback percentage, also known as return to player, or RTP. Normally, several programs, each with a slightly different mix of numbers and thus a different RTP, will be offered to casinos for each slot game.

In this system, the physical reels, or video reels, are simply display mechanisms to show the result at which the computer has arrived. The result on the reels comes down to the timing of when the spin is initiated, hopefully with that magical combination of numbers that will yield the big jackpot.

This, by the way, is why no one can “steal your jackpot.” If you stop playing a machine, and the very next player sits down and hits the jackpot, you can be sure that you would not have gotten the same result had you kept playing, because you would have had to hit the spin button at the exact same nanosecond as that other player—nearly impossible.

The virtual reel system is the technology behind all modern slot games, mechanical or video, with paylines or “ways to win” formats. It’s also the basis for video poker. There, the number set is much smaller, representative of every possible result on a 52-card deck of cards (53 with the Joker), with one number for each card on each of the five spots. It’s akin to a poker game in which the deck is reshuffled after every hand.

The system’s not perfect, as evidenced by all the jackpot disputes you hear about. There have been many instances where mechanical reels do not stop where the computer tells them to, or the video display does not match the result generated by the RNG, or a computer glitch causes a jackpot amount to be displayed that is higher than the top jackpot programmed by the machine. That’s why an attendant verifies all jackpots by tapping into the slot’s computer—to verify that the result reflects the RNG result.

The rule is that the RNG result is the result, regardless of what the physical reels display. That’s why all slots say, somewhere on the screen, “Machine malfunction voids all play.” The courts have generally sided with the casinos in these disputes, with a few notable exceptions.

The malfunctions, though, are rare in the overall scheme of things. And no matter how long the odds, the virtual reel system has made its share of millionaires. Just like in the lottery, someone’s going to hit that big jackpot. As slot players, that gives us hope, and a fun goal while we enjoy the other features of the game.

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