Jamieson, Fausset and Brown on 1 Samuel 20:25

25. the king sat upon his seat, as at other times ... by the wall--The left-hand corner at the upper end of a room was and still is in the East, the most honorable place. The person seated there has his left arm confined by the wall, but his right hand is at full liberty. From Abner's position next the king, and David's seat being left empty, it would seem that a state etiquette was observed at the royal table, each of the courtiers and ministers having places assigned them according to their respective gradations of rank.

Jonathan arose--either as a mark of respect on the entrance of the king, or in conformity with the usual Oriental custom for a son to stand in presence of his father.