macUSBoot

macUSBoot is a classic-BIOS bootloader that loads the original Microsoft BOOTMGR from compatible FAT32 Windows installation media.

Important

macUSBoot is installed by macUSB as part of its supported Windows-media workflow. It is not intended for standalone installation.

Boot path and compatibility

The classic BIOS starts the macUSBoot Master Boot Record (MBR), which checks the disk layout and BIOS disk access before loading StageTwo. StageTwo reads the FAT32 partition, loads the original Microsoft BOOTMGR, restores the Windows boot context, and transfers control to Windows Boot Manager.

The distributed macUSBoot .bin file is a versioned container intended for use by macUSB, not a raw disk image. It contains separate MBR and StageTwo components and must never be written directly to a USB device.

Note

macUSBoot is designed for classic BIOS systems. Native UEFI is outside this boot path, and operation through firmware-provided Legacy or Compatibility Support Module (CSM) modes is not guaranteed.

Building

Build the Release artifact on macOS with:

make release

Troubleshooting

For boot errors and recovery guidance, see Troubleshooting.

License

Licensed under the MIT License.

Copyright © 2026 Krystian Pierz