... Routines for accessing I/O ports are in /usr/include/asm/io.h (or linux/include/asm-i386/io.h in the kernel source distribution). ... Before you access any ports, you must give your program permission to do so. This is done by calling the ioperm() function (declared in unistd.h , and defined in the kernel) somewhere near the start of your program (before any I/O port accesses). ... For example, ioperm(0x300, 5, 1) would give access to ports 0x300 through 0x304 (a total of 5 ports). ...