The motherboard doesn't "hold" drivers, it uses drivers for the chipset and onboard devices. A modem controller codec is integrated into these boards that can be accessed by a simple rizer card (CNR or AMR). The "modem" is enabled/disabled in BIOS, and the driver for it comes with the motherboard when you buy the motherboard seperately. But if you buy a system it often doesn't include the driver CD that came with the motherboard, which usually means you'll have to figure out which motherboard you have in order to download the driver from the motherboard manufacturer's website.
Codecs aren't "real" hardware, they're simple digital/analog converters that rely on the CPU to perform the other functions. Hence AMR and CNR modems have higher CPU overhead than "hardware" modems or even most "software" modems.
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