[ielillo]

IIRC correctly the original Java VM was a stack based machine. That made sense when it was first created since a stack based machine is the simplest system you can create that run code and since it only need three registers, one for the instruction, one for the first data and one for the top of the stack for the other data. The problem is that you need to push and pop a lot from the stack during runtime which means more memory accesses and more time spent on gathering the data than on doing actual operations. That also underutilizes the processor registers since on a normal processor you would be using two data registers at most. This was one of the early issues with java running slowly on android and the reason of the creation of the Dalvik VM which was a register one.

[geokon]

Naiive question.. If the opcodes are the same, how can you go from a stack machine to a register one?

[jerf]

You compile the stack based code into register code. It is, of course, easier to say than to do, but it is within the range of a skilled team, not absurdly complicated.

You can think about it on a really small scale:

    PUSH 1
    PUSH 2
    PUSH 3
    ADD
    PUSH 4
    MULT
    ADD

is not that hard to conceptually rewrite into

    STORE 1, r1
    STORE 2, r2
    STORE 3, r3
    ADD r2, r3 INTO r2
    STORE 4, r3
    MULT r2, r3 INTO r2
    ADD r1, r2 INTO r1

Of course, from there, you have to deal with running out of registers, then you're going to want to optimize the resulting code (for instance, generally small numbers like this can fit into the opcodes themselves so we can optimize away all the STORE instructions easily in most if not all assembly languages), but, again, this is all fairly attainable code to developers with the correct skills, not pie-in-the-sky stuff. Compiler courses do not normally deal directly with this exact problem, but by the time you finish one you'd know enough to tackle this problem since the problem since the problem that compiler courses do deal with is a more-or-less a superset of this problem.