"It's something that had actually been happening for a long time," Angus tells Rolling Stone as promotes the band's new album "Rock Or Bust." Malcolm's symptoms - lapses in memory and concentration - "had surfaced even before the last project," AC/DC's 2008 album, "Black Ice."

But Angus says Malcolm was "still capable of knowing what he wanted to do. I had said to him, 'Do you want to go through with what we're doing?' And he said, 'Sh*t, yeah.' " Malcolm, Angus points out, "liked to finish what he started."

Angus also reveals that Malcolm was already in treatment during his last tour with AC/DC, from 2008 to 2010. "He got good help, good medical care," Angus says. Malcolm had to "relearn a lot of things."