The BEAM virtual machine executes Erlang instructions; ERTS is the larger runtime system around it. That distinction explains what a .beam file contains, how registers and instructions work, and why features such as processes and ETS should be attributed to the runtime rather than to the abstract machine itself.
What is the BEAM virtual machine?
BEAM is the abstract machine that executes compiled Erlang code. It is a register machine: instructions operate on named registers, rather than relying on a stack-based operand model. The term is often used casually to mean the entire Erlang runtime, but the distinction matters. The official Erlang/OTP BEAM primer notes that BEAM itself has no notion of processes, ports, or ETS tables; these belong to the wider runtime environment.
ERTS—the Erlang Runtime System—provides that broader execution context. It loads code and supplies runtime facilities needed by Erlang applications. Thus, “BEAM executes instructions” is more precise than saying that BEAM alone manages every runtime feature.
How does the BEAM VM work?
From Erlang source to loaded code
The Erlang compiler turns source modules into object code, commonly stored in files with the .beam suffix. ERTS loads that code through the code server. The file format and the instructions executed after loading are related, but they are not simply the same thing: the loader can transform generic instructions into specific runtime instructions.
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During the OTP build, the beam_makeops script uses instruction definitions to generate source for the compiler and runtime. Its documentation distinguishes external generic instructions, internal generic instructions, and specific instructions. Generic forms provide a compiler- and runtime-facing representation; the loader maps them to specific forms for execution. The traditional interpreter and BeamAsm use different implementation paths. See the official beam_makeops documentation for OTP 29.1.1.
X and Y registers
BEAM uses two register classes for different roles. X registers hold temporary values and are used to pass function arguments and results. Y registers belong to stack frames and preserve values associated with those frames.
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Arguments are passed from left to right, starting at {x,0}; a function returns its result in {x,0}. The official BEAM primer walks through compiler output for a tail-recursive sum function. In simplified terms, its instruction flow tests the input, branches to a failure label if the expected type or list shape does not match, calls the relevant function when appropriate, and returns the result through the designated register. The generated instructions make control flow and data movement explicit rather than hiding them behind source-level syntax.
How does BeamAsm change execution?
BeamAsm is the just-in-time execution engine documented for Erlang/OTP 29.1.1. It converts BEAM instructions to native code at load time on x86-64 and aarch64, subject to the OTP release and build in use. This changes how loaded instructions are executed, not the compiler’s register-allocation model.
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BeamAsm should not be mistaken for a continuously optimizing, profile-guided JIT. The OTP documentation describes load-time conversion and little optimization across instruction boundaries. It also notes implications for code loading and tracing. For the release-specific description, see BeamAsm, the Erlang JIT (OTP 29.1.1).
| Execution approach | What happens to loaded code | Documented scope |
|---|---|---|
| Traditional interpreter | Executes runtime instruction forms through the interpreter path. | OTP documentation distinguishes this path from BeamAsm; the cited comparison does not establish a universal performance ranking. |
| BeamAsm JIT | Converts BEAM instructions to native code at load time. | OTP 29.1.1 documentation names x86-64 and aarch64; actual availability depends on release and build. |
The OTP 29.1.1 BeamAsm documentation estimates loaded code memory at about 10% above the interpreter’s code memory. That comparison concerns code memory, not total process or node memory, and is not a benchmark of every application. The documentation says earlier prototypes used about twice the interpreter code memory.
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How should you assess BEAM performance?
A JIT label alone does not predict whether an application will run faster. Results depend on the workload, OTP release, architecture, build, runtime flags, and measurement method. Compare representative work on the systems you care about, and record those conditions alongside the result rather than treating a single figure as a general guarantee.
Inspecting BeamAsm code with Linux perf
The official OTP 29.1.1 guide describes profiling JIT-generated code with Linux perf. The workflow requires a build with JIT profiling support; the documentation explains enabling that support, then collecting and inspecting samples with perf record and perf report. Call-graph collection has caveats, particularly around transitions between Erlang and C code, so interpret traces in light of the guide’s limitations. Follow the version-specific instructions in the BeamAsm documentation.
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What happens when Erlang code is replaced?
Erlang/OTP supports module-level code replacement rather than requiring every process to stop at once. Under the code-loading model described in the OTP 27.3.4.18 code-loading guide, current and old code can coexist while processes may still be executing the old version. A fully qualified call can move execution to current code. This is not unlimited version retention: the guide describes current/old code handling and purging when another version is loaded.
How large is an Erlang process?
An Erlang process is a lightweight runtime entity, not an operating-system process. The Erlang/OTP v29.1.1 process guide gives an example in which a newly spawned process uses 327 words, including 233 words for its initial heap area. Those figures describe the guide’s documented runtime example, not a guaranteed cost for every process or configuration. See Processes in the Erlang System Documentation v29.1.1.
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