A No-Frills Introduction to Lua 5.1 VM Instructions

This is a no-frills introduction to the instruction set of the Lua 5.1 virtual machine. Compared to Perl or Python, the compactness of Lua makes it relatively easier for someone to peek under the hood and understand its internals. I think that one cannot completely grok a scripting language, or any complex system for that matter, without slitting the animal open and examining the entrails, organs and other yucky stuff that isn't normally seen. So this document is supposed to help with the peek under the hood bit.

This introductory guide covers Lua 5.1 only. Please see the older document for the guide to Lua 5.0.2 virtual machine instructions. This is intentional; the internals of Lua is not fixed or standardized in any way, so users must not expect compatibility from one version of Lua to another as far as internals are concerned.

Output from ChunkSpy (URL: http://luaforge.net/projects/chunkspy/), a Lua 5 binary chunk disassembler which I wrote while studying Lua internals, was used to generate the examples shown in this document. The brief disassembly mode of ChunkSpy is very similar to the output of the listing mode of luac, so you do not need to learn a new listing syntax. ChunkSpy can be downloaded from LuaForge (URL: http://luaforge.net/); it is licensed under the same type of MIT-style license as Lua 5 itself.

ChunkSpy has an interactive mode: you can enter a source chunk and get an immediate disassembly. This allows you to use this document as a tutorial by entering the examples into ChunkSpy and seeing the results yourself. The interactive mode is also very useful when you are exploring the behaviour of the Lua code generator on many short code snippets.

This is a quick introduction, so it isn't intended to be a comprehensive or expert treatment of the Lua virtual machine (from this point on, "Lua" refers to "Lua 5" unless otherwise stated) or its instructions. It is intended to be a simple, easy-to-digest beginner's guide to the Lua virtual machine instruction set, it won't do cartwheels or blow smoke rings.

The objective of this introduction is to cover all the Lua virtual machine instructions and the structure of Lua 5 binary chunks with a minimum of fuss. Then, if you want more detail, you can use luac or ChunkSpy to study non-trivial chunks of code, or you can dive into the Lua source code itself for the real thing.