| Filename | Latest commit message | Latest commit date |
|---|---|---|
64e4835524
The first piece of the two loaders that is genuinely common code rather than a contract they both honour. It can be, because the difference between the firmwares is entirely in how a framebuffer is *obtained* -- VBE in real mode, or GOP -- and not in what you do with one afterwards. boot/common/ui.c takes a base, a size and a stride and draws; it knows nothing about disks, memory maps or mode setting, which is what lets it compile into a 16/32/64-bit BIOS loader and a PE/COFF UEFI application without either leaking into it. The look is deliberately quiet: dark ground, the mark centred, one thin bar beneath it, nothing on screen that is not load-bearing. The whole group is laid out from the centre outwards rather than from the top down, so it sits identically at 800x600 and at 1600x1200 instead of drifting up the screen as the mode grows. UEFI had no graphical splash at all -- it printed text to a console the QEMU window is usually not even showing. It draws the same screen as the BIOS path now, with the bar tracking bytes of kernel actually written rather than a segment count, which says nothing about how long each takes. Two ordering bugs found by looking at the pixels rather than the code: setup_framebuffer ran *after* the kernel was read, so every progress tick went to a framebuffer that did not exist yet and the splash then painted over the result; and that also put the resolution prompt after the slowest step instead of at the start. The display is set up first now. Verified from the host with screendump: background is exactly SCH_BG, the bar is 4px tall and width/4 wide at y=495 on a 1280x800 screen, which is where centring the group puts it. UEFI still boots to cpu0; BIOS bootcheck 27/27. Not yet shared: the BIOS loader still draws with its own copy of this code. Deleting that copy in favour of this one is the next step and the point of the exercise -- until then this is one loader using shared code and one duplicating it. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> |
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| .. | ||
| bios | ||
| common | ||
| 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.