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ISA (or AT Bus) Clock Speed / Divisor
This setting controls the speed of the ISA bus, in one of two ways. The less common way is for the setting to allow a direct setting of the ISA clock speed; this would include options of "6 MHz", "8 MHz", etc. The most common way is for the ISA clock speed to be set as a fraction of the PCI clock speed. The settings in this case will usually look something like "PCICLK/3", "PCICLK/4", "PCICLK/6", etc.
The setting to choose is the one that puts the ISA clock speed as close as possible to 8.33 MHz, which is the accepted standard maximum clock speed for the bus. Anything higher than this is considered overclocking. 8.33 MHz means that the correct option for a 33 MHz or 30 MHz PCI machine would be "PCICLK/4", while for a 25 MHz PCI machine it would be "PCICLK/3".
On some BIOSes there is an "Auto" setting as well that will pick the right fraction depending on the PCI bus speed it detects, but this is somewhat less common in my experience than other automatic settings elsewhere in the BIOS.
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