http://lxr.linux.no/#linux+v2.6.38/arch/arm/mach-realview/include/mach/hardw... Seems to be used in hardware that can be emulated with QEMU. The offset and mask seem totally arbitrary to me, but multiple mach types use the same ones. I'm trying to use it to access the arm global timer. I can use it to access the CPU local timer, but simply adding the offset to the global timer registers causes QEMU to land in the GIC CPU interface. This isn't for the realview board btw, just a similar example. I feel like there is something fundamental I'm missing that is preventing me from understanding why: __io_address(BASE + 0x600) //local timer, works, no problems __io_address(BASE + 0x200) //global timer, but reads from gic cpu interface? that happens. (I am using QEMU btw) arm docs for offsets here: (bottom table) http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0407f/CACCJFC... Thanks for reading, Chris.