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Building a Blockchain: Subroutines in the Virtual Machine

Angel Java Lopez
2 min readAug 30, 2020


Usually, every day I practice programming using TDD (Test-Driven Development) workflow: few minutes adding a minimal feature to one of my personal projects. One of them is writing a blockchain in Java. Based on ideas from Ethereum blockchain, it has a virtual machine to execute smart contracts. Today, I implemented subroutines in that virtual, adding three opcodes and their behavior, according Ethereum Improving Proposal 2315.

The relevant commits:

You can see the incremental coding, adding new expected behavior, writing test in red, then adding code to make the tests pass.

The main additions were:

  • Add a return stack to virtual machine implementation, that it is not exposed and don’t have any influence in consensus.
  • Add three opcodes: JUMPSUB to jump to a subroutine altering the program counter and saving the return address into the return stack; BEGINSUB to mark the beginning of a subroutine (a never executed opcode); RETURNSUB retrieving the return program counter from return stack
  • Add gas costs to those new opcodes
  • Check the expected behavior in normal use and then, also in edge cases

It was easy to add this feature: using TDD, the initial code is simple and it can be tested by code without much effort (previous virtual machine tests were available to serve as a basis for the new tests); and writing explicitly each expected behavior tests facilitated the write of the corresponding production code.

Pending work: add some additional tests about executing nested subroutines, and minor refactors.

Angel “Java” Lopez