This is a beginning of W.B.Yeats's poem Under Ben Bulben:
Swear by what the Sages spoke
Round the Mareotic Lake
That the Witch of Atlas knew,
Spoke and set the cocks a-crow.
What is the function of a- in the phrase cocks a-crow? What is the general structure and meaning?
I think this function is different from what explained in this link
I guess here a- applied in the meaning of of. The Oxford dictionary indicates this meaning and brings the example of anew.