Just some thoughts.
As said there is a difference between a "riding warrior" and a knight in the proper sense. The knight was a member of a social class growing around the period of the Crusades, we can indicate XI century as the period of the birth of the Knighthood in the traditional sense.
Regarding the rule, the code of conduct of a knight we have to keep in mind the substantial diversity among lay and religious orders.
The great religious orders [Hospitallers, Templars, Teutons ...] had a rule which had canonical legal value, so that the knights of those orders tent to respect those rules with great attention. Even if, we know very well that in some circumstances they forced the rules.
For the lay knights there was this social code of the knighthood which had a certain civil value, but actually it was more about the control of the lords who knighted the warriors to grant a certain [just a certain] stability in the behaviors of the lay knights.
At the end, the moment in which "knights" appeared on history, was just when the first ones were "knighted" by lords, kings, dukes ... To be knighted meant the generation of a particular linkage with the lord of reference [while riding warriors of the past were members of regular army units, or clans, or tribes ...]. This linkage with the lord was the "key factor" to create the "Knight" in the traditional sense.
Knighthood was already well established since the IX century. For example Godefroid de Bouillon (the first crusader )was trained by his uncle as a knight. Guillaume Le Conquérant brought hundred Norman knight with him during his conquest of England. I thing that you make some misleading. It's the concept of chivalry (as an informal social code) which was introduced by the crusades at the end of the 11 century.
Knight => European low rank nobility title.
Chivalry : code of valour ruling the behaviour of knight during medieval times