REX_P0REG_YSTARTI
rex_write(dc, REX_PAGE0_SET, REX_P0REG_YSTARTI, ystarti);
rex_write(dc, REX_PAGE0_SET, REX_P0REG_YSTARTI, from_y);
rex_write(dc, REX_PAGE0_SET, REX_P0REG_YSTARTI, y);