REX_P0REG_XSTARTI
rex_write(dc, REX_PAGE0_SET, REX_P0REG_XSTARTI, to_x);
rex_write(dc, REX_PAGE0_SET, REX_P0REG_XSTARTI, from_x);
rex_write(dc, REX_PAGE0_SET, REX_P0REG_XSTARTI, x);