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Mac OS ROM mangler
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tbxi: the Mac OS Toolbox Imager

tbxi is a command-line Python script to help with the inspection and modification of Macintosh Toolbox ROM images. These images contain varying amounts of high-level Mac OS code and low-level boot code.

tbxi has two modes, both with a --help option:

  • tbxi dump converts a ROM image to a tree of self-describing text files and small binaries.
  • tbxi build rebuilds the image as accurately as possible.

In between running the two commands, the directory tree can be modified. The text-file formats produced by tbxi dump are designed to be easily editable using text editors and scripts.


"OldWorld" (pre-USB) Macs contain a real physical ROM. "NewWorld" Macs (iMac and later) have an unusual "ROM in RAM" arrangement, which retains compatibility with the ROM-based architecture of the Mac OS while easing software updates. An almost-complete legacy ROM image is loaded from the "Mac OS ROM" file in the System Folder (type code 'tbxi'), leaving only a minimal Open Firmware "Boot ROM" in the hardware.

During the progression from 68k ROMs to NewWorld PowerPC ROMs, multiple layers of wrapping and abstraction were added. These are reflected in the multi-level output of tbxi dump:

  • Open Firmware bootinfo file (Mac OS ROM => Bootinfo textfile + binaries)
  • structured binary of "Parcels" (Parcels => Parcelfile textfile + binaries)
  • 4 MB PowerPC ROM (MacROM => Configfile-1 textfile + binaries)
  • 3 MB 68k ROM (Mac68kROM => Romfile textfile + binaries)


Some very quirky PowerPC OldWorld ROMs (e.g. PowerBook 1400/2400) cannot be rebuilt correctly. NewWorld ROMs of version 2 or later (Sawtooth G4) will be slightly different due to an uninitialised buffer in the original build process.

ROM images predating before the "SuperMario" ROM (Quadra 660AV/840AV) are not supported, excluding most 68k Mac ROMs.

The resource fork of a NewWorld ROM image is ignored, despite containting a System Enabler that is paired with the data fork contents. Simply copying the resource fork back will cause a crash, because the 'cfrg' resource contains offests to some PowerPC binaries at the end of the data fork.

The tbxi dump format is likely to change. If you keep a collection of dumped ROM images to peruse, re-dump them regularly.

No specific guidance is provided on ROM patches that are known to work, e.g. to boot Mac OS 9 on the PowerPC Mac mini.


Yes please! Bug reports, suggestions and requests are welcome. Open a GitHub pull request or image, or get in contact via the email on the PyPI page.

All are welcome on our retro Mac-hacking mailing list:

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