There is a lot of confusion around different types of RAID - hardware, firmware and software.
The only way to offload all processing from the MB and CPU is to use an expensive, dedicated RAID card - not the $20-$50 ones that you see available. They have dedicated processors to do any required data handling and parity calculations.
Otherwise, with both onboard RAID and the cheaper RAID cards, the CPU and chipset to the bulk of the work. This is not necessarily a bad thing, however, especially with most modern systems.
Onboard RAID, and the ones using the cheap cards, are *not* software RAID. They have controllers that handle some of the work, but still need the chipset and CPU for heavy lifting.
Software RAID is used when you have *no* hardware support for RAID at all, and using the OS to provide RAID-like functionality. Only Unix systems can boot from software RAID.
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