/DRIVER:UPONLY causes the linker to add the IMAGE_FILE_UP_SYSTEM_ONLY bit to the characteristics in the output header to ...

/DRIVER:UPONLY causes the linker to add the IMAGE_FILE_UP_SYSTEM_ONLY bit to the characteristics in the output header to specify that it is a uniprocessor (UP) driver. The operating system will refuse to load a UP driver on a multiprocessor (MP) system.