Dear Skalkaz:The following is taken from the Online Etymological Dictionary. It is by no means a complete answer, but perhaps it's a start:
a- (1) in native (derived from O.E.) words, it most commonly represents O.E. an "on" (see a (2)), as in alive, asleep, abroad, ashore, etc., forming adjectives and adverbs from nouns; but it also can be M.E. of, as in anew, abreast (1599); or a reduced form of O.E. pp. prefix ge-, as in aware; or the O.E. intens. a-, as in arise, awake, ashame, marking a verb as momentary, a single event. In words from Romanic languages, often it represents L. ad- "to, at."