FWIW it took me a while to work out what a 'split lock' was. I suspect because I was thinking of kernel locks, not the instruction lock prefix. It also isn't really obvious that 'split' refers to crossing cache lines. Referring to it as a 'misaligned lock' might be more easily understood. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)