Hi, Please correct me, if I'm wrong, but that's my understanding. include/linux/byteorder/generic.h defines macros, which take both the CPU endianess and the endianess of the peripherial device you want to access into account. e.g. cpu_to_le32 will no matter what's the endianess of the CPU write to a little endian 32 address (do the swapping or not) but there are also in lib/iomap.c macros like iowrite32, which don't seem to take the CPU endianess into account Now looking e.g. at drivers/net/e100.c both kind of macros are used. e.g. ioread32 and also cpu_to_le32 The iowritexxx macros seem to boil down to outxx macros and it looks like iomap needs to be called before - maybe the endianess of the CPU is taken into account there, if so it's not obvious to me;) I'm a bit confused since we seem to have I/O ports and I/O memory. Can you please elaborate a bit when to use what and what happens with the CPU endianess in case of ioread/write macros? My goal is to understand how to write portable drivers which can be used with all kinds of funny CPU / peripheral endianess combinations. Regards, Robert ..."Everything should be built top-down, except the first time." My public pgp key is available at: http://pgp.mit.edu:11371/pks/lookup?op=get&search=0x90320BF1