Fantasm

The Fantastic (dis-/re-)Assembly tools for 6502 code, version 0.18.0.

Full user guide and CLI / API reference: acornaeology.github.io/fantasm

Fantasm is a consolidated suite of tools for working on annotated disassemblies of 6502-based ROMs (originally those from the Acorn / BBC Microcomputer family). It brings together capabilities that previously lived in per-project forks under the acornaeology umbrella, behind a single fantasm command and a programmatic fantasm.api package.

This README covers the absolute basics; for everything else (the fantasm.toml schema, the version graph, command-by-command workflows, the importable fantasm.api) follow the docs.

Install

fantasm is on PyPI. Add it to your project as a regular dependency:

[project]
dependencies = [
    "fantasm>=0.18.0",
]

Or for a one-off install:

pip install fantasm

The disassembly itself is performed by a per-version Python driver script — typically built on dasmos or py8dis, but fantasm only sees the artefacts those scripts emit (.asm, .json) and stays disassembler-agnostic. Round-trip verification needs beebasm on PATH.

CLI

$ fantasm --help
Usage: fantasm [OPTIONS] COMMAND [ARGS]...

  Fantasm — the Fantastic (dis-/re-)Assembly tools for 6502 code.

Options:
  --version                 Show the version and exit.
  --project-root DIRECTORY  Project root directory. Overrides
                            FANTASM_PROJECT_ROOT. If neither is given, fantasm
                            searches upwards from the current directory for a
                            fantasm.toml.
  -h, --help                Show this message and exit.

Commands:
  addresses    Address translation across ROM versions.
  annotations  Cross-version annotation diff and management.
  asm          Assembly-source extraction and inspection.
  audit        Subroutine annotation audit.
  backfill     Propose annotation propagations from SOURCE_VERSION to...
  bytes        Byte-signature search across ROM images.
  cfg          Inter-procedural call-graph queries.
  comments     Comment / annotation consistency checks.
  compare      Compare two ROM versions at byte / opcode / full-instruction...
  context      Code context queries (depth, sub-context, uncommented gaps).
  coverage     Report the disassembly's inline-comment coverage as a single...
  data         Data-declaration review (runs, heuristic reclassification).
  disassemble  Run the version's disassembly driver script to (re-)generate...
  driver       Driver-script housekeeping (sorting, formatting).
  fingerprint  Fingerprint each block of a ROM version's bytes and report...
  hooks        hook_subroutine() discovery and review.
  info         Show the resolved project context.
  labels       Auto-generated label classification and renaming.
  lint         Validate that a driver script's annotation addresses...
  project      Initialise and manage fantasm projects.
  promote      Score labelled code items as candidates for promotion to...
  shared       Find shared 6502 code fragments between a primary ROM and...
  sub          Subroutine workflow helpers.
  verify       Verify a disassembly round-trips: assemble its .asm with...

The top-level fantasm command accepts a --project-root option and also reads FANTASM_PROJECT_ROOT from the environment. When neither is given, fantasm walks upwards from the current working directory looking for a fantasm.toml to identify the project root.

fantasm info

The info sub-command shows the resolved project context for the current invocation. Like every fantasm command that produces structured output, it inherits a uniform --as display | tsv | json story (and friends) from asyoulikeit:

$ fantasm info --help
Usage: fantasm info [OPTIONS]

  Show the resolved project context.

Options:
  Report Output Options: 
    --no-reports              Suppress all report output. The handler still runs
                              (useful for action commands whose reports are
                              incidental); only rendering is skipped. Mutually
                              exclusive with --report and --all-reports.
    --all-reports             Render every report the handler returns,
                              regardless of the command's default_reports.
                              Useful for commands whose default is a subset (or
                              silent) but where you want the full picture this
                              time. Mutually exclusive with --report and --no-
                              reports.
    --report [project]        Report name(s) to display (can be specified
                              multiple times). Shows all if omitted. Valid
                              values: project.
    --header / --no-header    Include column headers in output. Overrides each
                              report's default. Format-specific: TSV prefixes
                              first cell with '#', display omits
                              headers/title/caption, JSON ignores this flag.
    --detailed / --essential  Include detailed columns or only essential
                              columns. Auto-detects based on output format if
                              not specified.
    --as [display|json|tsv]   Output format for tabular data. Defaults to
                              'display' for terminals, 'tsv' for pipes.
  -h, --help                  Show this message and exit.

Sample output (an explicit project root that does not contain a fantasm.toml):

$ fantasm --project-root /path/to/your/project info
              Fantasm project              
┏━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━┓
┃ Key             ┃ Value                 ┃
┡━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━┩
│ project_root    │ /path/to/your/project │
│ config_filepath │ (none)                │
│ config_keys     │ (empty)               │
└─────────────────┴───────────────────────┘
 Resolved project context for the current  
                invocation.

The same invocation through a pipe (or with --as tsv) emits tab-separated output suitable for downstream tooling:

$ fantasm --project-root /path/to/your/project info --as tsv
# Key	Value
project_root	/path/to/your/project
config_filepath	(none)
config_keys	(empty)

Programmatic API

from fantasm.api import ...  # surface filled in as modules are ported

Testing

fantasm pins down observable behaviour with characterisation tests against hand-crafted JSON / ROM fixtures — the original codebase fantasm consolidates had no tests of its own. Modules that emit 6502 assembly source are exercised with round-trip tests that re-assemble the output with beebasm and compare bytes; the beebasm_filepath pytest fixture skips such tests when the assembler is not on PATH. See docs/testing.rst for the patterns and CLAUDE.md for the project-level guidance.

Layout

src/fantasm/             the package
src/fantasm/cli/         Click entrypoint, one module per command group
src/fantasm/api/         programmatic API (re-exports the public surface)
src/fantasm/cli_helpers.py  shared CLI helpers (AnalysisContext, …)
src/fantasm/config.py    project-root resolution + fantasm.toml loading
tests/api/               api-side pytest suite
tests/cli/               cli-side pytest suite
docs/                    Sphinx site (published to GitHub Pages)
scripts/                 README generator + Jinja2 template

Related projects

• dasmos — the tracing disassembler fantasm now builds on top of (a Python rewrite of py8dis with a stable API and stricter classification semantics).
• py8dis (fork) — the predecessor disassembler; driver scripts written against py8dis still work end-to-end with fantasm, since fantasm runs them as a subprocess and only consumes the JSON / asm output.
• The four sibling repositories under acornaeology from which fantasm draws its initial tooling: acorn-6502-tube-client, acorn-adfs, acorn-econet-bridge, acorn-nfs.

This README is generated from scripts/readme_template.md.j2 by scripts/generate_readme.py. Do not edit it directly — edit the template (or the generator, or the source files whose output it captures) and re-run uv run python scripts/generate_readme.py. The pre-commit hook and the readme-check CI job both run the generator's --check mode and will refuse stale READMEs.