Boot the image

The boot image runs from RAM. The workflow starts the image and leaves the installed boot partition unchanged.

Prerequisites

• An unlocked bootloader with fastboot.
• A device with a UEFI port that matches the board configuration.
• A built boot image with a payload, or an empty ramdisk for the boot manager smoke test.
• A serial console when the board exposes a UART.

Start the image

fastboot devices
fastboot boot out/<board>/boot/boot.img

fastboot boot transfers the image and starts it one time. The stock boot partition and the installed OS stay unchanged.

The boot image contains:

Part	Contents
Kernel	gzip of BootShim plus the UEFI firmware volume
Ramdisk	UniXNU payload FAT volume
Appended data	Device tree

The stock Android bootloader loads the image, verifies the ARM64 header in BootShim, and jumps to the shim. BootShim starts the firmware volume.

Boot manager

UniXNU prints its version and configuration summary to the serial port and draws the menu. The default configuration uses a timeout, a default entry and a countdown.

Action	Key
Move the highlight	Up and Down
Boot the selected entry	Enter
Stop the countdown	Any key

The menu entries come from \EFI\UniXNU\unixnu.toml. An entry can start an EFI image, a Linux image with an EFI stub, an action, or a Darwin kernel through the loader protocol.

If UniXNU finds no configuration, it scans every volume for \EFI\BOOT\BOOTAA64.EFI and adds those entries in front of the built-in actions.

Darwin boot

For an entry of type xnu, DarwinBoot reads these files from the entry directory:

File	Required	Contents
kernel.macho	yes	Prelinked XNU kernel
kernel.macho.adt	yes	Apple-format device tree sidecar
kernel.macho.trustcache	no	Trust cache bundle
the entry initrd	no	Data ramdisk seed

DarwinBoot copies the kernel segments into a contiguous physical window, builds boot_args, installs the ADT and the trust cache addresses in the /chosen/memory-map placeholders, and exits boot services.

The default window is 256 MiB. The ADT property /chosen/darwinboot-memory-size can request a larger window.

First boot

The first boot needs a userdata image. The image supplies the OS and SystemOS volumes and the root volume UUID that the kernel matches.

fastboot flash userdata out/<board>/userdata/userdata-root.img
fastboot reboot bootloader
fastboot boot out/<board>/boot/boot.img

The userdata image is large. Build it with unixnu build userdata and flash it from the device repository report.

The Data seed supplies the writable Data volume. The --skip-setup option marks Setup as done in a copy of the seed, so the Hello pages do not run. The procedure and the preference changes are in Skip Setup.

Logs

The UEFI firmware logs to EFI_SERIAL_IO_PROTOCOL when the board exposes a UART. UniXNU prefixes its lines with UNIXNU:.

The kernel logs to the serial port when the command line selects one, for example serial=3. The boot progress layer writes checkpoints to the framebuffer and the serial port.

Useful logs:

Source	Location
Build driver	out/<board>/logs/*.log
Boot manager	Serial console, lines with UNIXNU:
DarwinBoot	Serial console, lines with DARWINBOOT:
XNU	Serial console, kernel output

Recovery

The image runs from RAM. Reset the device to return to the stock bootloader and the installed OS.

fastboot reboot bootloader

If the firmware stalls before the menu, reset the device and start the same image again. A firmware stall leaves the device in the bootloader; a second fastboot boot uses the same image.

macOS

The boot pipeline is driven by the IPSW. A macOS restore image provides KernelCache, DeviceTree, OS, Cryptex1,SystemOS, StaticTrustCache and Cryptex1,SystemTrustCache in the same member layout.

To boot macOS:

1. Point [ipsw] in the board file at a macOS IPSW and set the product, version, build, device class, chip ID and board ID of that image.
2. Pin the member hashes in [ipsw.sha256].
3. Set [userdata] windows to OS and Cryptex1,SystemOS.
4. Build the kernel, payload, boot image and userdata image.
5. Flash the userdata image and start the boot image.

The device tree and the platform drivers supply the hardware description. The userland comes from the macOS disk images.

Next steps

• Userdata image describes the partition image.
• Boot arguments lists the kernel arguments.
• Troubleshooting lists failure conditions and checks.
• Devices lists device specific commands.