The system relied on a Kernel Mode Driver (typically named PortIO.sys or similar). This driver sat at Ring 0 (the highest privilege level) of the processor. It had the permission to execute hardware I/O instructions.
: A kernel-mode driver that bypasses the standard Windows protection layers to interact directly with hardware registers. Port95nt.exe
The driver bypasses Windows’ normal hardware protection mechanisms, granting user-mode programs direct read/write access to I/O memory ranges. This was historically necessary for: The system relied on a Kernel Mode Driver