Boot modes: BIOS, UEFI, and the A/B slot menu

The ISO is hybrid

build/opencube.iso (the release asset) carries two boot paths:

• Legacy BIOS: an El Torito boot image (GRUB i386-pc, built by tools/build_iso.sh with grub-mkimage + xorriso).
• UEFI: an EFI boot image (x86_64-efi) on the same ISO.

Both paths load the same kernel ELF and attach the kernel itself as a multiboot2 module tagged self - this is the mainstream installer pattern: the install and abdisk commands copy the exact kernel that is currently running onto a target disk, so an installed system always matches its installer.

The GRUB menu

* Open Cube OS (slot ISO)
  Open Cube OS (slot A, force)
  Open Cube OS (slot B, force)

• slot ISO - the default. Boot the kernel from the boot medium and read the A/B boot flag to decide which disk slot to chain (falls back to the ISO kernel when no flag exists).
• slot A / slot B force - ignore the flag and boot that disk slot directly. This is the recovery path when a bad update leaves the active slot unbootable: pick the other slot from the menu.

The menu auto-boots the highlighted entry after 5 seconds.

What the kernel does after GRUB

Boot order (this is exactly the serial log):

multiboot2 handshake OK
long mode entry OK
framebuffer 800x600x32 OK
console grid / 8x16 font engine OK
IDT + GDT + TSS (256 gates) OK
8259 PIC remap (IRQ0-15 -> vec 32-47) OK
PIT @ 100 Hz + tick counter OK
PS/2 keyboard OK
COM1 serial RX OK
console input line editor OK
interrupts enabled (sti) OK
exception self-test: 3/3 (#DE/#UD/#PF) OK
PMM / VMM / heap OK
... drivers, VFS, network ...
shell command system (233 commands) OK
boot complete OK
crash recovery armed (panic + dump + crashlog) OK

The multiboot2 parser (kernel/arch/x86_64/arch_multiboot2.c) reads the framebuffer tag, the memory map, the command line and the ACPI RSDP tags (types 14/15) that GRUB forwards from firmware - on UEFI boots the RSDP comes through this path, on BIOS boots the ACPI driver can also scan memory.

Boot arguments

GRUB passes the kernel command line; grub.cfg uses it to pass BOOT_SLOT=ISO|A|B plus console preferences. Inside a running system, uname -a shows the version and build, and abcfg prints the exact grub.cfg files that abdisk/install generate.

Persistence: the /etc volume

The system looks for a FAT32 volume to mount at /etc. When you boot the plain ISO there is none - configuration is ephemeral. install creates one on the target disk (it holds opencube.conf, SSH host keys, kernel.log*, crash dumps). If you only want persistence without installing, create the volume by hand:

oc> abdisk <dev>       (A/B disk with /etc volume - destroys data!)

Booting real hardware

Write the ISO to a USB stick (dd if=opencube.iso of=/dev/sdX or your favourite tool). The requirements are modest: x86_64 CPU, 512 MiB RAM. Boot through BIOS or UEFI firmware as you prefer - both images are on the stick.