IOP_MSGLEN
u_char msg[IOP_MSGLEN];
#define IOP_MSGBUFLEN (IOP_MSGLEN * IOP_MAXCHAN)
offset = IOP_ADDR_SEND_MSG + msg->channel * IOP_MSGLEN;
_iop_upload(ioph, msg->msg, IOP_MSGLEN, offset);
offset = IOP_ADDR_RECV_MSG + chan * IOP_MSGLEN;
_iop_download(ioph, msg->msg, IOP_MSGLEN, offset);
if (msglen > IOP_MSGLEN) return -1;
if (msglen < IOP_MSGLEN) memset(msg->msg, '\0', IOP_MSGLEN);