oaknut-zip 10.0.1

Work with ZIP files containing Acorn computer metadata.

oaknut-zip

A Python tool for extracting ZIP files containing Acorn computer metadata.

Standard unzip tools silently discard the load addresses, execution addresses, and file attributes that Acorn systems (BBC Micro, Master, RISC OS machines) store in ZIP archives. oaknut-zip preserves this metadata, writing it out in your choice of several output formats.

The problem

When software for Acorn 8-bit and 32-bit systems is distributed as ZIP files, the archives often contain platform-specific metadata embedded using mechanisms that non-Acorn unzip tools ignore:

• SparkFS extra fields in the ZIP structure itself, carrying load/exec addresses and RISC OS file attributes.
• Bundled INF sidecar files inside the archive, in either Acorn or PiEconetBridge format.
• Unix filename encoding, where metadata is appended to filenames as comma- separated hex suffixes.

Extracting these archives with unzip or Python's zipfile module on Linux or macOS produces the raw file data but loses all the Acorn metadata. For 6502 or ARM executables, this means you no longer know where in memory a program should be loaded or where execution should begin. For a PiEconetBridge fileserver, the files become unserviceable without their attributes.

oaknut-zip reads all three metadata sources from the ZIP and writes them out in a format your target system can consume.

Prerequisites

oaknut-zip requires only uv. uv handles Python installation, dependency resolution, and virtual environments automatically --- you do not need to install anything else by hand.

Installing uv

macOS (Homebrew):

brew install uv

Linux / macOS (standalone installer):

curl -LsSf https://astral.sh/uv/install.sh | sh

Windows:

powershell -ExecutionPolicy ByPass -c "irm https://astral.sh/uv/install.ps1 | iex"

See the uv installation docs for other methods including pip, pipx, Cargo, Conda, Winget, and Scoop.

Installation

Run without installing

uvx oaknut-zip <command> [options]

This always uses the latest published version, but has a small startup overhead on each invocation.

Persistent install

uv tool install oaknut-zip

Subsequently run as just oaknut-zip:

oaknut-zip <command> [options]

This is faster to invoke, but you need uv tool upgrade oaknut-zip to pick up new versions.

Usage

Commands

$ oaknut-zip --help
Usage: oaknut-zip [OPTIONS] COMMAND [ARGS]...

  Work with ZIP files containing Acorn computer metadata.

Options:
  --version  Show the version and exit.
  --help     Show this message and exit.

Commands:
  extract  Extract a ZIP file, preserving Acorn metadata.
  info     Show summary of Acorn metadata in a ZIP file.
  list     List ZIP contents showing Acorn metadata.

Inspecting an archive

list --- tabular view of all entries

$ oaknut-zip list NetUtils.zip
                             NetUtils.zip                              
┏━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━┳━━━━━━┳━━━━━━━━━┓
┃ Filename   ┃     Load ┃     Exec ┃   Length ┃ Attr ┃ Type ┃ Source  ┃
┡━━━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━╇━━━━━━╇━━━━━━━━━┩
│ Free       │ FFFF0E10 │ FFFF0E10 │ 000001E6 │ LR/R │  F0E │ sparkfs │
│ FSList     │ FFFF0900 │ FFFF0900 │ 00000200 │ LR/R │  F09 │ sparkfs │
│ PSList     │ FFFF0900 │ FFFF0900 │ 000001B5 │ LR/R │  F09 │ sparkfs │
│ Notify     │ FFFF0E23 │ FFFF0E23 │ 0000012A │ LR/R │  F0E │ sparkfs │
│ Remote     │ FFFF0E10 │ FFFF0E10 │ 000001ED │ LR/R │  F0E │ sparkfs │
│ Servers    │ FFFF0900 │ FFFF091A │ 000001CF │ LR/R │  F09 │ sparkfs │
│ SetStation │ FFFFDD00 │ FFFFDD00 │ 00000200 │ LR/R │  FDD │ sparkfs │
│ Stations   │ FFFF08D5 │ FFFF08E1 │ 0000022B │ WR/R │  F08 │ sparkfs │
│ SJMon      │ FFFF1B00 │ FFFF1B00 │ 00000D7E │  LR/ │  F1B │ sparkfs │
│ Users      │ FFFF0E23 │ FFFF0E23 │ 00000139 │ LR/R │  F0E │ sparkfs │
│ View       │ FFFF0900 │ FFFF0904 │ 000001FF │ LR/R │  F09 │ sparkfs │
│ ReadMe     │ FFFFFF52 │ 2FEEAFD0 │ 00000255 │  R/R │  FFF │ sparkfs │
└────────────┴──────────┴──────────┴──────────┴──────┴──────┴─────────┘

The Source column shows where the metadata was found (see the format identifier table below). The Type column shows the RISC OS filetype extracted from the load address (when the top 12 bits are 0xFFF).

info --- summary statistics

$ oaknut-zip info NetUtils.zip
Archive:    NetUtils.zip
Files:      12
Dirs:       0
SparkFS:    12 files with ARC0 extra fields
inf-trad:   0 files with bundled traditional INF
inf-pieb:   0 files with bundled PiEconetBridge INF
Filename:   0 files with encoded filenames
Plain:      0 files without Acorn metadata
Filetypes:  6 distinct
  F08: 1 files
  F09: 4 files
  F0E: 4 files
  F1B: 1 files
  FDD: 1 files
  FFF: 1 files

Extracting an archive

$ oaknut-zip extract --help
Usage: oaknut-zip extract [OPTIONS] ZIPFILE_PATH

  Extract a ZIP file, preserving Acorn metadata.

Options:
  -d, --output-dir PATH           Output directory (default: ZIP filename
                                  without extension).
  -v, --verbose                   Show extraction progress.
  --meta-format [inf-trad|inf-pieb|xattr-acorn|xattr-pieb|filename-riscos|filename-mos|none]
                                  Metadata format: inf-trad, inf-pieb, xattr-
                                  acorn, xattr-pieb, filename-riscos,
                                  filename-mos, or none.
  --no-decode-filenames           Do not decode metadata from filename
                                  suffixes (,xxx or ,load,exec).
  --owner INTEGER                 Econet owner ID for inf-pieb and xattr-pieb
                                  outputs (default: 0 = SYST).
  --help                          Show this message and exit.

oaknut-zip uses a consistent <mechanism>-<flavour> naming scheme for format identifiers. The same identifiers appear as metadata source labels in the list and info commands, and as --meta-format choices for output:

Identifier	As input source	As output format (--meta-format)
sparkfs	SparkFS/ARC0 extra fields in the ZIP structure	--- (not an output format)
inf-trad	Bundled traditional .inf sidecar files	Traditional INF sidecar files
inf-pieb	Bundled PiEconetBridge .inf sidecar files	PiEconetBridge INF sidecar files
filename	Metadata encoded in filenames (,xxx, ,load,exec, ,load-exec)	--- (not an output format)
filename-riscos	---	RISC OS filename encoding (,xxx or ,llllllll,eeeeeeee)
filename-mos	---	MOS filename encoding (,load-exec)
xattr-acorn	---	Extended attributes (user.acorn.*)
xattr-pieb	---	Extended attributes (user.econet_*, PiEconetBridge)
none	---	No metadata (raw extraction)

Format 1: Traditional INF (default)

oaknut-zip extract NetUtils.zip

Each extracted file gets a companion .inf sidecar file:

$ cat SetStation.inf
SetStation  FFFFDD00 FFFFDD00 00000200 5D

$ cat ReadMe.inf
ReadMe      FFFFFF52 2FEEAFD0 00000255 55

The fields are space-separated, with the filename left-padded to 11 characters:

filename    load     exec     length   [access]

Field	Format	Description
filename	ASCII string	Leaf filename, left-padded to 11 chars
load	8-digit hex	32-bit load address; if top 12 bits = FFF, bits 8--19 encode the filetype
exec	8-digit hex	32-bit execution address (entry point for code)
length	8-digit hex	File length in bytes
access	2-digit hex	Acorn file attributes (optional; see attribute bits below)

This is the standard format understood by BBC Micro emulators, disc image tools (BBCIM, Disc Image Manager), and other retro-computing utilities.

Format 2: PiEconetBridge INF

oaknut-zip extract --meta-format inf-pieb NetUtils.zip

Writes .inf sidecar files in the format used by PiEconetBridge:

$ cat SetStation.inf
0 ffffdd00 ffffdd00 5d

$ cat ReadMe.inf
0 ffffff52 2feeafd0 55

The fields, all lowercase hex:

owner load exec perm

Field	Format	Description
owner	short hex	Econet user ID (set with --owner, default 0 = SYST)
load	long hex	32-bit load address
exec	long hex	32-bit execution address
perm	short hex	File permissions byte

PiEconetBridge reads this format from <filename>.inf when extended attribute support is unavailable on the host filesystem (e.g. FAT32). This is the format produced by the parse_acorn_zip.pl utility bundled with PiEconetBridge.

Format 3: Extended attributes (Acorn namespace)

oaknut-zip extract --meta-format xattr-acorn NetUtils.zip

Writes metadata directly into the filesystem's extended attributes using the user.acorn.* namespace, with no sidecar files:

$ xattr -l SetStation
user.acorn.attr: 5D
user.acorn.exec: FFFFDD00
user.acorn.load: FFFFDD00

$ xattr -l ReadMe
user.acorn.attr: 55
user.acorn.exec: 2FEEAFD0
user.acorn.load: FFFFFF52

The attribute names and value formats:

Attribute	Format (sprintf)	Example
user.acorn.load	%08X	FFFFDD00
user.acorn.exec	%08X	FFFFDD00
user.acorn.attr	%02X	03

This is the oaknut-file convention, free of the Econet/PiEconetBridge naming legacy. The user.acorn.attr attribute is written only when attribute information is available; there is no Econet owner field.

Format 4: Extended attributes (PiEconetBridge namespace)

oaknut-zip extract --meta-format xattr-pieb NetUtils.zip

Writes metadata using the user.econet_* namespace defined by PiEconetBridge:

$ xattr -l SetStation
user.econet_exec: FFFFDD00
user.econet_load: FFFFDD00
user.econet_owner: 0000
user.econet_perm: 5D

$ xattr -l ReadMe
user.econet_exec: 2FEEAFD0
user.econet_load: FFFFFF52
user.econet_owner: 0000
user.econet_perm: 55

The attribute names and value formats match PiEconetBridge's C implementation exactly:

Attribute	Format (sprintf)	Example
user.econet_owner	%04X	0000
user.econet_load	%08X	FFFFDD00
user.econet_exec	%08X	FFFFDD00
user.econet_perm	%02X	03

This is the preferred format for PiEconetBridge when the host filesystem supports extended attributes. PiEconetBridge reads xattrs in preference to .inf files when both are present. Use --owner to set the Econet owner ID (default 0 = SYST).

Format 5: RISC OS filename encoding

oaknut-zip extract --meta-format filename-riscos NetUtils.zip

Encodes metadata directly into the output filenames using comma-separated hex suffixes. No sidecar files are created. Filetype-stamped files get a ,xxx suffix (e.g. FILE,fdd); files with literal load/exec addresses get ,llllllll,eeeeeeee (e.g. PROG,00001900,0000801f).

This is the most portable format --- it requires no filesystem-specific support and survives transfers between any systems. Files already carrying the correct suffix are not double-encoded.

Format 6: MOS filename encoding

oaknut-zip extract --meta-format filename-mos NetUtils.zip

Encodes metadata into output filenames using the ,load-exec convention documented in the BeebWiki INF file format specification. This format uses a comma prefix with load and exec addresses separated by a dash, with variable-width hex digits (no zero-padding): ,1900-801f.

This is the convention used by MOS and SparkFS when encoding addresses in filenames. Unlike the RISC OS form (Format 4), it always encodes full load/exec addresses and does not special-case filetype-stamped files.

Format 7: No metadata

oaknut-zip extract --meta-format none NetUtils.zip

Extracts files only, discarding all Acorn metadata. Equivalent to a standard unzip.

Metadata sources in ZIP files

oaknut-zip reads Acorn metadata from three sources within ZIP archives. When multiple sources are present for a given entry, they are used in priority order: SparkFS extra fields, then bundled INF sidecar files, then filename encoding.

SparkFS extra fields (ARC0)

ZIP archives created by SparkFS, SparkPlug, or RISC OS zip tools embed Acorn metadata as a ZIP extra field with header ID 0x4341 ("AC") and an "ARC0" signature.

The extra field layout (all values little-endian), as defined in the Info-ZIP extra field specification by David Pilling:

Offset	Size	Description
0	4 bytes	Signature "ARC0"
4	4 bytes	Load address (uint32)
8	4 bytes	Execution address (uint32)
12	4 bytes	File attributes (uint32)
16	4 bytes	Reserved (zero)

This is the most reliable metadata source, as it is embedded in the ZIP structure itself and survives transfers between any systems.

A note on the 32-bit attributes field

Although the attributes field is stored as a 32-bit little-endian word, the Info-ZIP specification and David Pilling's own SparkFS source define meaning only for the low 8 bits --- the standard RISC OS access byte (R, W, L, public R, public W, plus a couple of reserved bits). Bits 8-31 have no documented semantics. Genuine RISC OS tooling (SparkFS, SparkPlug) writes zero in the upper 24 bits, and archives with host OS 13 (Acorn RISC OS) in the ZIP "version made by" field are clean.

In the wild, however, some non-RISC-OS producers leave junk in the upper 24 bits of the word --- stale memory, tool-private state, or similar. The archives in tests/fixtures/ sourced from MDFS are in this shape: the access byte in bits 0-7 is correct and self-consistent across archives, but bits 8-31 are effectively random. oaknut-zip masks the Attr field to its low 8 bits when populating metadata, so the access byte written to .inf sidecars and extended attributes is always a valid two-digit hex value.

Bundled INF sidecar files

Some ZIP archives include .inf sidecar files alongside the data files they describe. oaknut-zip detects and parses these automatically, supporting both flavours:

• Traditional INF (filename load exec length [access]) --- reported as source inf-trad in the list and info commands.
• PiEconetBridge INF (owner load exec perm) --- reported as source inf-pieb.

When extracting, bundled .inf files are consumed as a metadata source rather than extracted as separate files. The metadata is then written in whatever output format was requested (inf-trad, inf-pieb, xattr-acorn, xattr-pieb, etc.). This allows, for example, converting a PiEconetBridge archive to user.acorn.* extended attributes in a single step.

With --meta-format none, bundled .inf files are extracted as-is (no metadata is consumed).

Unix filename encoding

Because Unix filesystems have no way to store Acorn load/exec addresses natively, a convention exists for encoding the RISC OS filetype into the filename itself as a comma-separated hex suffix:

Pattern	Example	Meaning
filename,xxx	HELLO,ffb	RISC OS filetype FFB

oaknut-zip also supports two additional forms that encode full load and exec addresses for files that are not filetype-stamped:

Pattern	Example	Meaning
filename,llllllll,eeeeeeee	PROG,ffff0e10,0000801f	Load and exec addresses
filename,load-exec	PROG,1900-801f	Load and exec (MOS/SparkFS)

The first form is used by python-zipinfo-riscos and requires exactly 8 hex digits per address. The second form, documented in the BeebWiki INF file format specification, uses variable-width hex digits separated by a dash and is the convention used by MOS and SparkFS.

oaknut-zip strips these suffixes from output filenames during extraction. Use --no-decode-filenames to preserve the encoded filenames as-is.

RISC OS filetype encoding

When the top 12 bits of a load address are 0xFFF, the address does not represent a literal memory location. Instead, bits 8--19 encode a RISC OS filetype:

Load address: FFFtttxx
                 ^^^
                 filetype (3 hex digits)

Example RISC OS filetypes:

Type	Name	Description
FF9	Sprite	Sprite or saved screen
FFA	Module	Relocatable module
FFB	BASIC	Tokenised BASIC program
FFC	Utility	Position independent code
FFD	Data	Arbitrary data
FFE	Command	Command (Exec) file
FFF	Text	Plain ASCII text with LF newlines

RISC OS file attribute bits

The attribute byte encodes access permissions following the RISC OS convention (see RISC OS PRM volume 2, FileSwitch attributes):

Bit	Mask	Meaning
0	0x01	Owner readable
1	0x02	Owner writable
3	0x08	Locked (L)
4	0x10	Public readable
5	0x20	Public writable

A file with attributes 0x03 is owner-readable and owner-writable (WR/). A value of 0x15 adds public read access (R/R), which is the default for files served by PiEconetBridge.

Development

After cloning, install the pre-commit hooks:

uv run --group dev pre-commit install

Running the tests

uv run --group test pytest tests/ -v

The test suite covers all parsing, formatting, and extraction logic, including integration tests against six real-world ZIP fixtures:

• NetUtils.zip --- Econet utilities from MDFS, with SparkFS/ARC0 extra fields. This is the archive that motivated the project.
• NetUtilB.zip --- A second Econet utilities pack, also with SparkFS metadata.
• MASTER.zip --- BBC Master utilities from MDFS, 344 files with SparkFS metadata and 36 distinct filetypes.
• sweh_econet_system.zip --- PiEconetBridge system files from sweh, containing bundled PiEconetBridge-format .inf sidecar files (no SparkFS extra fields).
• testdir-unix.zip --- Test archive from python-zipinfo-riscos, with Unix filename encoding (,xxx suffixes).
• testdir-ro.zip --- The same content as testdir-unix.zip but with SparkFS extra fields instead of filename encoding.

References

Metadata format specifications

• INF file format --- BeebWiki specification for the Acorn .inf sidecar format.
• ZIP extra field registry --- Known ZIP extra field types, including the Acorn/SparkFS entry (0x4341).
• RISC OS filetype --- How RISC OS encodes filetypes in load addresses.
• ZipToInf documentation --- J.G.Harston's ZipToInf utility and INF format description.
• RISC OS filename translation --- Documents the ,xxx filetype suffix convention used on Unix filesystems.
• INF file format --- J.G.Harston's detailed INF file format specification.

Tools and related projects

• PiEconetBridge --- Econet bridge for Raspberry Pi; defines the user.econet_* xattr convention and the alternative .inf format (owner load exec perm).
• python-zipinfo-riscos --- Python library for reading/writing RISC OS extra fields in ZIP archives; source of the ,llllllll,eeeeeeee load/exec filename encoding convention.
• ZipTools --- J.G.Harston's suite of Acorn-aware ZIP utilities including ZipToInf.
• SparkFS --- David Pilling's RISC OS filing system for ZIP/Arc/Spark archives; originator of the ARC0 extra field format.

Forum discussion

• Stardot forum thread --- Discussion of the metadata-loss problem that motivated this tool.
• INF format discussion --- Community discussion on INF format variants and standardisation.