64-bitswitch load library: Copy /opt/IBM/mqsi/6.0/sample/xatm/db2swit64 to /var/mqm/exits64/db2swit (This command also renames the file to remove the 64 when installed to exits64.)
Although 64-bit processors typically have wide enough data busses to keep up fairly well, there's still a twofold increase in data to be moved when you switch to 64-bit data.