You could split A into multiple components (1st to 4th byte, 5th to 8th byte), and multiply/divide those components separately. That would still fit into 64bit.
Yeah, I did try that, but performance was lacking. Java, if you stick with primitives and use a C-like programming style (miniminal objects) was pretty close to C performance when I tried it, except for this one gap.
Partial example is here: https://codereview.stackexchange.com/questions/72286/multipl...