reg1-4 defined in stddef.h by default; memcmp_se: for memcmp with side effect (on registers) which is faster
only allow memcmp_se to be used after defining ENABLE_MEMCMP_SE; otherwise, a compilation error printed
memcmp implementation use asm scmp for efficiency
SIO_INVALID macro defined to indicate the invalid sio_id
add comments to sio.h to explain the sio APIs