It’s less work to port it to a future machine that you don’t know the number and type of registers of yet. If a future CPU has more registers than your VM, you

It’s less work to port it to a future machine that you don’t know the number and type of registers of yet. If a future CPU has more registers than your VM, you have to either cripple your VM by not using all registers of the new hardware or write code to detect and remove register spilling that was necessary on the smaller architecture. If, on the other hand, it has fewer or a different mix, you have to change your VM to add register spilling. Either way, your byte code compiler has done register assignment work that it almost certainly has to discard once it starts running on the target architecture. If you to start at the extreme end of “no registers”, you only ever have to handle the first case, and do that once and for all (sort-of. Things will be ‘a bit’ more complex in reality, certainly now that CPUs have vector registers, may have float8 or float16 hardware, etc) You can also start at the extreme end of an infinite amount of registers. That’s what LLVM does. I think one reason that’s less popular with VMs because it makes it harder to get a proof of concept VM running on a system.