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copied!<p>I'll answer your question from a different tack: what is a VM? A VM is just a specification for "interpreter" of a lower level language than the source language. Here I'm using the black box meaning of the word "interpreter". I don't care how a VM gets implemented (as a bytecode intepereter, a JIT compiler, whatever). When phrased that way, from a design point of view the VM isn't the interesting thing it's the low level language.</p> <p>The ideal VM language will do two things. One, it will make it easy to compile the source language into it. And two it will also make it easy to interpret on the target platform(s) (where again the interpreter could be implemented very naively or could be some really sophisticated JIT like Hotspot or V8). </p> <p>Obviously there's a tension between those two desirable properties, but they do more or less form two end points on a line through the design space of all possible VMs. (Or, perhaps some more complicated shape than a line because this isn't a flat Euclidean space, but you get the idea). If you build your VM language far outside of that line then it won't be very useful. That's what constrains VM design: putting it somewhere into that ideal line. </p> <p>That line is also why high level VMs tend to be very language specific while low level VMs are more language agnostic but don't provide many services. A high level VM is by its nature close to the source language which makes it far from other, different source languages. A low level VM is by its nature close to the target platform thus close to the platform end of the ideal lines for many languages but that low level VM will also be pretty far from the "easy to compile to" end of the ideal line of most source languages.</p> <p>Now, more broadly, conceptually any compiler can be seen as a series of transformations from the source language to intermediate forms that themselves can be seen as languages for VMs. VMs for the intermediate languages may never be built, but they could be. A compiler eventually emits the final form. And that final form will itself be a language for a VM. We might call that VM "JVM", "V8"...or we might call that VM "x86", "ARM", etc.</p> <p>Hope that helps.</p>

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