It allows you to run your RAM faster or slower than your CPU. Many of your components including CPU and RAM run off of the external clock of the system. When you raise or lower that frequency, the components move proportionally with it. For example, in my system, the stock external clock speed is 200 MHz. Both the RAM and CPU run at that speed. If I choose to run a divider such as 4:5 (CPU:RAM), the CPU will run at 200 MHz while the RAM will run at 200 x 5 / 4 = 250 MHz. Similarly, a 4:3 divider would result in the RAM running at 200 x 3 / 4 = 150 MHz. Generally 1:1 provides optimal performance, but some systems benefit from pushing RAM harder than the CPU.
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