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jamixir-vm

The Polkadot Virtual Machine (PVM) in Elixir, implemented from the JAM Gray Paper. It was the first PVM in Jamixir, an Elixir JAM client, before the node moved to the Rust version (jamixir/pvm-rust) for speed. It is kept here as a readable reference, with comments pointing to the Gray Paper formulas.

Written by Luke Fishman and Daniel Cukier.

Example

defmodule Example do
  use PVM.Instructions
  alias PVM.{Encoder, Registers, State, VM}

  def run do
    # load_imm r7, 42 ; trap
    program = <<op(:load_imm), 7, 42, op(:trap)>>
    # one bit per byte of code, set where an instruction starts (bytes 0 and 3)
    bitmask = <<0b10010000>>

    blob = Encoder.encode_program(program, bitmask, {}, 1)
    {exit_reason, state} = VM.execute(blob, %State{gas: 100})

    [r7] = Registers.get(state.registers, [7])
    {exit_reason, r7, state.gas}
  end
end

Example.run()
# => {:panic, 42, 98}

VM.execute/2 returns {exit_reason, state}, where the exit reason is :halt, :panic, :out_of_gas, {:fault, address} or {:ecall, host_call_id}. A trap instruction ends in :panic.

Layout

  • lib/codec: program blob encoding and decoding
  • lib/instructions: opcode table and dispatch, grouped by argument shape
  • lib/memory: paged memory with access rights
  • lib/single_step.ex, lib/vm.ex: the step function and the run loop
  • lib/init_program.ex, lib/arg_invoc.ex, lib/host_call.ex: standard program initialisation, argument invocation and host calls
  • lib/trace: execution traces as text or HTML

Tests

git clone https://github.com/jamixir/jamtestvectors ../pvm-test-vectors
mix deps.get
mix test

The vector tests read the PVM test vectors from ../pvm-test-vectors/pvm/programs.

License

Apache-2.0