Chivalry as a set of ideals and duties changed throughout the Middle Ages to meet new socio-economic realities. Somewhat ironically, our most 'chivalrous king' Edward III supported the trappings of chivalry, and during his reign the rise of heraldry, tournaments and banquets, courtly love and the writing of epic romances flourished.
But on the field of battle, mounted knights and feudal obligation were becoming an anachronism. This was the rise of the age of the hired soldier, and in particular the use of that devastating English weapon of mass destruction, the longbow and the archers 'gens de nul value' who wielded it with such effect.
Although there is no 'authentic' code, discussions of knightly virtues can be found in the writings of knights and bards throughout history; Chrètien de Troyes, Ramon Lull, Geoffrey de Charny, Honoret Bonet, and others.
However a French literary historian Leon Gautier (Emile Theodore Leon Gautier, 1832–1897) decided to try and codify 'Chivalry':
The Ten Commandments of the Code of Chivalry
I. Thou shalt believe all that the Church teaches, and shalt observe all its directions.
II. Thou shalt defend the Church.
III. Thou shalt repect all weaknesses, and shalt constitute thyself the defender of them.
IV. Thou shalt love the country in the which thou wast born.
V. Thou shalt not recoil before thine enemy.
VI. Thou shalt make war against the Infidel without cessation, and without mercy.
VII. Thou shalt perform scrupulously thy feudal duties, if they be not contrary to the laws of God.
VIII. Thou shalt never lie, and shall remain faithful to thy pledged word.
IX. Thou shalt be generous, and give largess to everyone.
X. Thou shalt be everywhere and always the champion of the Right and the Good against Injustice and Evil.
I'm not convinced by I and II and Number VI. seems a little dodgy in the 21st century...but the rest is good stuff!