| Filename | Latest commit message | Latest commit date |
|---|---|---|
0500e4382d
Groundwork for the unified loader: before the two paths can share code, they have to agree on what they are producing, and until now they only agreed by coincidence. Every field of the loader/kernel hand-off was #defined in whichever file happened to need it -- kernel/core/main.c, kernel/dev/vga.c, kernel/arch/x86_64/e820.c, boot/bios/cdboot.S, boot/bios/stage2main.c and boot/uefi/main.c, six copies of the same addresses. schboot.h existed for exactly this and held only the entry magic and the RSDP slot, with a comment describing the rest of the block it did not define. The whole block lives there now: the VBE info fields, the verbose and media flags (with the real-mode segment:offset spelling cdboot.S needs), the e820 count, map and cap, and the RSDP. Every one of those files includes it instead of restating it. This is not a tidy-up. It is the same hazard as defining the entry magic twice, only quieter -- nothing breaks until one copy moves, and the two loaders diverging is precisely what produced the EFER.SCE and RSDP bugs, where a fix on one path silently did not exist on the other. Verified on both firmwares: BIOS boots to schinit with the same e820, ACPI and VGA lines as before; UEFI reaches cpu0 with 1018 MB in 7 regions and its own 1280x800 mode. `make iso` still builds. bootcheck 108 [ok]. Next: the loaders become starters that gather firmware-specific facts and hand to one common stage. That needs the BIOS side to climb to long mode first, since common code can only be compiled for one CPU mode -- which is the risky part, and is why the contract went first. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> |
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| .. | ||
| bios | ||
| uefi | ||
| README.md | ||
Booting SchOS
Two firmware paths, kept apart on purpose:
boot/
bios/ legacy BIOS: MBR, El Torito CD stub, real-mode stage 2
uefi/ UEFI application (not written yet)
They are separate directories rather than #ifdefs in one because they share
almost nothing. A BIOS loader lives in 16-bit real mode, talks to the firmware
through software interrupts, and has to walk itself up into long mode. A UEFI
loader is a 64-bit PE executable that the firmware has already placed in long
mode with paging on, and it asks for services by calling through a table of
function pointers. The only things genuinely in common are the logo bitmap, the
menu's appearance, and the hand-off contract below.
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/memlayout.h and
boot/bios/stage2main.c. A UEFI loader must fill the same structures — the
kernel does not care which firmware produced them, and that is what keeps the
split cheap.
Two entry points, because the two firmwares hand over differently
The BIOS path enters the kernel in 32-bit protected mode at the multiboot
entry, with EAX holding the framebuffer address; mboot_entry then builds page
tables and climbs into long mode itself.
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 a second entry instead.
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, exactly as the BIOS loader scans for the multiboot magic.
include/schboot.h holds the magic so loader and kernel cannot disagree.
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 amovabsto 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.
The BIOS path today
bios/bootasm.S— the literal MBR sector. Signed to 512 bytes bytools/sign.pl, loads stage 2 from LBA 1 and jumps.bios/cdboot.S— the same job for optical media, as an El Torito no-emulation image. Separate because the loader never uses INT 13h and so needs a real ATAPI read.bios/stage2.S— collects the E820 map, sets a VBE mode, enters protected mode.bios/stage2main.c— the boot menu and splash, then loads the kernel and enters long mode.
STAGE2_SECTORS is defined once, in the Makefile, and passed to both the
assembler and the dd that writes the image. It used to be written out by hand
in five places and silently truncated the loader when they disagreed.
Why UEFI is worth doing
Everything since roughly 2012 boots UEFI by default, and Secure Boot machines will not run the BIOS path at all without firmware settings most people never touch. The BIOS loader stays regardless: it is what QEMU's default machine and older hardware use, and it costs nothing to keep once the two are separated.