I'm curious about "a-" being used as an affix in verb forms you don't see as much, anymore, in English. (As in, "a-walking", "a-going".) Does this have a specific meaning, or syntactical rules that decide which verbs it can go on? As far as I can tell it only goes on present continuous forms, so I can't see how it introduces any extra nonperfective meaning, or anything like that.
Secondly: is there a word for this "a-"?