Simple virtual machine

If possible, avoid shorts, bytes, and compressed byte code. Most CPUs work with 32 and 64 bit numbers, and they will convert bytes/shorts to 32 bits usually, which takes time (your compiler may do this regardless, depending on optimization, but anyhow it keeps your VM smaller and cleaner). Also, your VM would need to support every permutation of action (add a byte to an int, a int to a short, etc). Also, compressing an instruction into a 32 bit or 64 bit space is adverse to performance. Memory is plentiful enough (both RAM and cache) that you should not bother with this unless you are targeting an embedded device or something.

Use enums for your op codes, and a switch statement (depends on compiler). If you use enums and a switch, the compiler can generate a jump table because it knows the enum will always be in a certain range. Much better performance than a bunch of if/else statements.

It is possible to write a VM that only uses floating point numbers, if you want to keep the VM as small and simple as possible. This means casting the floats for things like addressing, but otherwise I think it might perform well.

"I would like to touch a slightly different application of this technology - protect your software/data from being hacked."

I wrote something similar that I posted to my blog a couple of years ago ( my blog post is titled "An Embedded Mini Interpreter" ).

While I had tinkered with making my own VM's with elaborate file-formats, the only data-structure I used for my VM was an array of integers that acted as indices to functions in a C program and an index to the array element next in line to be interpreted.

The interpreter would begin at the top of the array and would issue the function relative to the number in the current integer in the array. For some functions, I would encode data in the integer array. For the sake of example, I simply encoded a string for a "print string" function as a single character in each successive integer slot.