| Filename | Latest commit message | Latest commit date |
|---|---|---|
55a585792e
UEFI had no boot menu at all, and asked about resolution in a prompt of
its own before anything else. Two questions at two moments through two
mechanisms is what made the firmwares feel like different programs.
There is one window now: press F8 (or 'o' -- a serial console sends no
scan codes, and that is how this is usually driven) and the same panel
the BIOS loader shows appears, with resolution as a row in it. Selecting
it steps to the next 32bpp mode at or above 800x600 and redraws; the
framebuffer is re-read every pass, because changing mode moves it and
drawing where it used to be is a black screen with no explanation.
Two layout bugs, both visible and both arithmetic:
- TITLE_H was 34 while the heading alone occupies PANEL_PAD + GLYPH_H
= 34, so the rule at TITLE_H - 6 was drawn *through* the lettering
and the first row started on the heading's baseline. TITLE_H is
derived from its parts now and cannot drift from them again.
- the selected row's highlight was offset by a fixed 3px against a
24px row, so it sat on the text rather than around it. It is centred
on the glyph box: (ROW_H - GLYPH_H) / 2 above and below.
Verified from the host: the panel is centred at 640 on a 1280 screen, the
highlight is exactly ROW_H tall and clear of the rule, and Enter boots
through to cpu0. bootcheck 108 [ok], iso builds.
The BIOS resolution chooser is still a separate text prompt, and that is
not an oversight to fix later in the same way. VBE mode setting is an
INT 10h call, so it can only happen in real mode -- before there is any
framebuffer to draw a menu in. Offering it from the graphical menu means
either returning to real mode from long mode, or entering unreal mode to
draw the menu before the switch. Both are real mode-switching work in the
riskiest code in the tree, and neither is a tidy-up.
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.