What exactly are the advantages and disadvantages to using a register-based virtual machine versus using a stack-based virtual machine?
To me, it would seem as though a register based machine would be more straight-forward to program and more efficient. So why is it that the JVM, the CLR, and the Python VM are all stack-based?
Implemented in hardware, a register-based machine is going to be more efficient simply because there are fewer accesses to the slower RAM. In software, however, even a register based architecture will most likely have the "registers" in RAM. A stack based machine is going to be just as efficient in that case.
In addition a stack-based VM is going to make it a lot easier to write compilers. You don't have to deal with register allocation strategies. You have, essentially, an unlimited number of registers to work with.
Update: I wrote this answer assuming an interpreted VM. It may not hold true for a JIT compiled VM. I ran across this paper which seems to indicate that a JIT compiled VM may be more efficient using a register architecture.
Stack based VM's are simpler and the code is much more compact. As a real world example, a friend built (about 30 years ago) a data logging system with a homebrew Forth VM on a Cosmac. The Forth VM was 30 bytes of code on a machine with 2k of ROM and 256 bytes of RAM.