It came to my attention some emulators and virtual machines use dynamic recompilation. How do they do that? In C i know how to call a function in ram using typecasting (although i never tried) but how does one read opcodes and generate code for it? Does the person need to have premade assembly chunks and copy/batch them together? is the assembly written in C? If so how do you find the length of the code? How do you account for system interrupts?
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system interrupts and how to (re)compile the data is what i am most interested in. Upon more research i heard of one person (no source available) used js, read the machine code, output js source and use eval to ‘compile’ the js source. Interesting.
Yes, absolutely. That is why parts of the Java Virtual Machine must be rewritten (namely, the JIT) for every architecture.
When you write a virtual machine, you have a particular host-architecture in mind, and a particular guest-architecture. A portable VM is better called an emulator, since you would be emulating every instruction of the guest-architecture (guest-registers would be represented as host-variables, rather than host-registers).
When the guest- and host-architectures are the same, like VMWare, there are a ton of (pretty neat) optimizations you can do to speed up the virtualization – today we are at the point that this type of virtual machine is BARELY slower than running directly on the processor. Of course, it is extremely architecture-dependent – you would probably be better off rewriting most of VMWare from scratch than trying to port it.