April 21, 2025
Ruby is often praised for its expressiveness and ease of use. But behind the scenes, there’s a powerful virtual machine making it all happen: YARV (Yet Another Ruby VM). If you’ve ever wondered how your Ruby code actually runs , this article is for you.
Let’s dive into the inner mechanics using this simple Ruby expression:
1 + 2
Sounds simple enough, right? But here’s what actually happens under the hood.
Let’s Connect!
If you’re as fascinated by Ruby internals as I am, or you’re looking for an experienced Ruby developer to join your team, feel free to reach out!
I’m always open to new opportunities, collaborations, or just a good Ruby chat. 
YARV’s Double Stack System
YARV operates with two main stacks :
- Instruction Stack (Internal Stack): Where the bytecode instructions are executed.
- Ruby Call Stack : Managed via a C structure called rb_control_frame_t, this tracks method calls, blocks, etc.
Here’s a visual breakdown:
Bytecode for 1 + 2
When the code 1 + 2 is parsed and compiled, YARV generates a sequence of bytecode instructions:
== disasm: <RubyVM::InstructionSequence:<main>@test.rb>==========
0000 trace 1 ( 1)
0002 putself
0003 putobject 1
0005 putobject 2
0007 opt_plus <calldata!mid:+, argc:1, ARGS_SIMPLE>
0009 leave
Each instruction manipulates the internal stack :
- putself: pushes self onto the stack.
- putobject 1 and putobject 2: push values.
- opt_plus: pops the top two operands and calls the + method.
- leave: returns the result.
The Control Frame Stack (rb_control_frame_t)
Each frame in the Ruby Call Stack (like method calls or blocks) is stored in a structure called rb_control_frame_t, containing:
- PC (Program Counter): points to the current instruction.
- SP (Stack Pointer): tracks the top of the operand stack.
- self : current object context.
- type : method/block/eval etc.
These frames are linked through a pointer called cfp (Current Frame Pointer), which moves as execution flows through the code.
Why Should You Care?
Understanding this stack mechanism is crucial for debugging , optimizing performance, and writing better Ruby code. For example:
- Memory issues? Look at stack depth.
- Unexpected behavior in blocks or procs? Check how frames are being created and popped.
- Want to write a debugger or trace tool? Start with the rb_control_frame_t.
Tools to Explore Further
Want to dive into your own Ruby code?
Try:
RubyVM::InstructionSequence.compile("1 + 2").disasm
Or explore the C source code in MRI Ruby under vm.c to see the control frame logic.
Final Thoughts
The Ruby VM is a beautifully designed machine, and knowing how it works deepens your appreciation of the language and unlocks new levels of capability as a developer.
If you found this breakdown useful or want to geek out more on Ruby internals, let’s connect! 
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