To offer burnt offerings unto the LORD upon the altar of the burnt offering continually morning and evening, and to do according to all that is written in the law of the LORD, which he commanded Israel;
Continually morning and evening - as the law enjoined (Exodus 29:38; Numbers 28:3; Numbers 28:6).
And to do according to all that is written in the law - (see Numbers 28:1-31). Thus, in the time of David, the worship was performed at two places, where the sacred things that had been transmitted from the age of Moses were preserved. Before the ark in Jerusalem, Asaph and his brethren officiated as singers, Obed-edom and Hassah served as door-keepers, Benaiah and Jahaziel blew the trumpets; while at the tabernacle and burnt offering in Gibeon Heman and Jeduthun presided over the sacred music, the sons of Jeduthun were door-keepers, and Zadok, with his suite of attendant priests, offered the sacrifices.