MSYNC_RB0
mbox = (xord & MSYNC_RBLW) ? MSYNC_RB0 : MSYNC_RB1;
mbox = (xord & MSYNC_RB0) ? MSYNC_RB0 : MSYNC_RB1;
mbox_page = (mbox == MSYNC_RB0) ? QUEUE_OLD_RB0 : QUEUE_OLD_RB1;
#define MSYNC_RB_MASK (MSYNC_RB0 | MSYNC_RB1)