SchOS/boot

Booting SchOS

One firmware path:

boot/
  uefi/    the loader: a 64-bit PE application
  common/  the user interface, shared with nothing else now

There were two. boot/bios/ held an MBR, an El Torito CD stub and a real-mode stage 2, and it was a separate directory rather than #ifdefs because the two shared almost nothing: a BIOS loader lives in 16-bit real mode and talks to the firmware through software interrupts, while UEFI hands a loader long mode with paging already on and a table of function pointers to call.

That asymmetry is what removed it. Everything since roughly 2012 boots UEFI, CSM — the firmware-side BIOS emulation that let legacy loaders run at all — has been going away since Intel dropped support for it in 2020, and the cost of the second path was not the code but the tax on everything that touched it: stage 2 had a 65-sector ceiling that it silently overflowed, and the resolution menu could not be shared because VBE mode setting is INT 10h and needs real mode.

The contract below survives it. It was written to let a second loader exist, and it is worth keeping in that shape: it is what the kernel expects, stated once, rather than a description of whoever happens to be calling.

What the loader must hand the kernel

Whichever path runs, the kernel expects the same things. This is the contract; a second loader is correct exactly when it satisfies it.

	Where	Notes
Kernel image	loaded per its ELF program headers	entry at entry64
CPU state	see below -- the two paths differ here
Memory map	E820_MAP_ADDR, 24-byte entries, count at E820_COUNT_ADDR	kernel/arch/x86_64/e820.c reads it
Framebuffer	VBE_INFO_ADDR	address, pitch, width, height, depth
Verbose flag	BOOT_VERBOSE_FLAG_ADDR	set by the F8 menu

The addresses are defined once, in include/arch/memlayout.h and include/arch/schboot.h. The kernel does not care which firmware produced them.

The entry point

UEFI hands a loader long mode with paging already on. Dropping back to protected mode only to climb out again is a mode switch whose failure mode is a triple fault with no console, so the kernel publishes an entry that takes the machine as it already is. kernel/arch/x86_64/entry.S carries a sch64_header — magic SCH64BOO, 8-byte aligned within the first 16 KB, followed by a 64-bit physical address — found by scanning. include/arch/schboot.h holds the magic so loader and kernel cannot disagree.

The 32-bit multiboot entry is still in entry.S and now has no caller: the BIOS loader was the only thing that used it. It is left in place rather than removed in the same change that removed the loader, because deleting kernel entry code is its own piece of work with its own way of failing silently.

What that entry expects:

• long mode, interrupts off
• CR3 pointing at tables that identity-map the low 4 GB and map the same 4 GB at 0xffff800000000000. entry64low's first instruction is a movabs to a high address, so identity alone triple faults immediately.
• RBX = the framebuffer physical address, and never 1 — that value marks an application processor in entry64high.

The kernel loads its own GDT at that entry (entry64uefi), rather than the loader doing it. A loader arriving in long mode is still on the firmware's descriptors, and the first thing the kernel does that depends on its own selectors takes a #GP against a GDT the firmware no longer owns — it surfaced as unexpected trap 13 ... cs=38, a long way from the cause.

e820.c reads a BIOS memory map. UEFI hands out a different structure (EFI_MEMORY_DESCRIPTOR, from GetMemoryMap), so the UEFI loader has to translate it into the same 24-byte entries rather than inventing a second format for the kernel to learn.

What booting looks like now

make esp stages EFI/BOOT/BOOTX64.EFI and schos/kernel, and QEMU is pointed at OVMF with that directory as a virtual FAT volume — make qemu. make iso builds a disc with a single El Torito entry pointing at a FAT image holding the same two files.

Neither is an installable disk. A real GPT image with a real FAT partition is the installer's job, and until it exists there is nothing here that writes to a disk a machine could boot from unaided.

Secure Boot is not addressed either: the loader is unsigned, so a machine enforcing it will refuse to run this.