Question

How does a virtual machine generate native machine code on the fly and execute it?

Assuming you can figure out what are the native machine op-codes you want to emit, how do you go about actually running it?

Is it something as hacky as mapping the mnemonic instructions to binary codes, stuffing it into an char* pointer and casting it as a function and executing?

Or would you generate a temporary shared library (.dll or .so or whatever) and load it into memory using standard functions like LoadLibrary ?

Answer 1

You can just make the program counter point to the code you want to execute. Remember that data can be data or code. On x86 the program counter is the EIP register. The IP part of EIP stands for instruction pointer. The JMP instruction is called to jump to an address. After the jump EIP will contain this address.

Is it something as hacky as mapping the mnemonic instructions to binary codes, stuffing it into an char* pointer and casting it as a function and executing?

Yes. This is one way of doing it. The resulting code would be cast to a pointer to function in C.