Images and Boot

A PureDarwin image is a partitioned raw disk file: a FAT EFI system partition holding the bootloader, a root filesystem, and on the full x86_64 images an APFS partition kept around for driver testing.

image.nix builds it. nix/image-contents.nix decides what goes in it, and defines the several flavours the project ships.

• Image flavours - every image package, its geometry and its baked boot-args.
• Boot chain - firmware to shell prompt, step by step.
• Kernel collections - how the kernel and its kexts are linked together.
• Running in QEMU - the runner apps and their knobs.
• Running on real hardware - what changes off QEMU.

The shape of an image:

GPT
+-- p1  EFI system partition (FAT)
|     \EFI\BOOT\BOOTX64.EFI      xnu-loader
|     \EFI\BOOT\boot-args.txt    boot arguments, read by the loader
|     the kernel collection
+-- p2  root filesystem (ext4, or HFS+ on image-hfs)
|     the base system, plus whatever extra packages the flavour includes
+-- p3  APFS test partition (full images only)

On arm64 the EFI binary is BOOTAA64.EFI. Some arm64 targets boot a RAM disk instead of a root partition; see Image flavours.

Nearly all of /etc is written by image.nix at image build time: passwd, master.passwd, group, hosts, fstab, services, protocols, resolv.conf, shells, ttys, profile, zshenv, zprofile and zshrc, plus the fontconfig and XDG environment defaults.

If you are wondering why an edit to a config file in the guest does not survive a rebuild, this is why: change image.nix, not the image.