Date: 2023-01-15 Git: https://gitlab.com/mort96/blog/blob/published/content/00000-home/00015-fast-interpreters.md In this post, I hope to explore how interpreters are often implemented, what a "virtual machine" means in this context, and how to make them faster. Note: This post will contain a lot of C source code. Most of it is fairly simple C which should be easy to follow, but some familiarity with the C language is suggested. What is a (virtual) machine? For our purposes, a "machine" is anything which can read some sequence of instructions ("code") and act upon them. A Turing machine reads instructions from the cells of a tape and changes its state accordingly. Your CPU is a machine which reads instructions in the form of binary data representing x86 or ARM machine code and modifies its state accordingly. A LISP machine reads instructions in the form of LISP code and modifies its state accordingly. Your computer's CPU is a physical machine, with all the logic required to read and execute its native machine code implemented as circuitry in hardware. But we can also implement a "machine" to read and execute instructions in software. A software implementation of a machine is what we call a virtual machine. QEMU is an example of a project which implements common CPU instruction sets in software, so we can take native machine code for ARM64 and run it in a virtual ARM64 machine regardless of what architecture our physical CPU implements. But we don't have to limit ourselves to virtual machines which emulate real CPU architectures. In the world of programming languages, a "virtual machine" is usually used to mean something which takes some language-specific code and executes it. What is bytecode? Many programming languages are separated into roughly two parts: the front-end, which parses your textual source code and emits some form of machine-readable code, and the virtual machine, which executes the instructions in this machine-readable code. This machine-readable cod...
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