Understanding Instruction Sequence Disasm

I've been trying to go through some of the MRI internals in order to better follow along with Pat Shaunessy's examples from Ruby Under a Microscope. Pat shows us how to analayze the output from RubyVM::InstructionSequence#disasm, and it's great. When we compile simple programs we can see the instructions that YARV is going to execute. Let's look at an example:

output

Each stack frame is shown in a pretty understandable way. Here is a good article that gives us an quick list of all the instruction set definitions. These definitions can be found in insns.def in the source. I found this quite illuminating.

Anyways, while going through all this I really wanted to know what this was:

So I went into the code and found this. My C is a little rusty but I eventually found my way to:

Which then led to:

Here we see Ruby is using a macro rb_str_catf which calls printf and prints the pos variable. This pos variable is the position of the current instruction being printed.

This has been kind of a dump of information, hopefully it is helpful. Let me know if you like this style of post, it's considerably less formal as I am really just walking you through some problem I'm working on.

Thanks for reading!

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Understanding Instruction Sequence Disasm

Hi, I'm Jon.

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