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Bit b10 of the address port is reset as the FDC is seen as an expansion, even if it is an internal chip. Bit b7 is reset to select the FDC. Bits b8 and b0 are used to select the specific mode of operation. [https://www.cpc-power.com/cpcarchives/index.php?page=articles&num=48 Source]
The [[Vortex Disc Drives|Vortex disc interface]] uses other ports. See its dedicated wiki page.
