uintptr_t startup_io_map(unsigned size, 
                         paddr_t phys);

Same as mmap_device_io() in the C library — provide access to an I/O port on the x86 (for other systems, startup_io_map() is the same as startup_memory_map()) at a given physical address for a given size. The return value is for use in the in*/out* functions in the C library. The value is for use during the time the startup program is running (as opposed to callout_io_map(), which is for use after startup is completed).