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boot: one hand-off contract, defined once

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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>
This commit is contained in:
Nexoniarz 2026-08-10 21:37:36 +02:00
commit 0500e4382d
7 changed files with 89 additions and 56 deletions
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10
boot/bios/cdboot.S
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@ -18,9 +18,7 @@
# info block at 0x30000 that stage2.S fills in a moment from now.
#define STAGE2_ADDR 0x7E00
#define BOOT_MEDIA_FLAG_SEG 0x3000
#define BOOT_MEDIA_FLAG_OFF 0x101
#define BOOT_MEDIA_CD 1
#include "../../include/schboot.h"
.code16
.global start
@ -34,10 +32,10 @@ start:
# "You were booted from a CD." stage2main.c reads this to decide between
# ATA and ATAPI; without it the loader would read the kernel off whatever
# happens to be sitting at 0x1F0, which on a CD boot is usually nothing.
movw $BOOT_MEDIA_FLAG_SEG, %ax
movw $SCHBOOT_MEDIA_SEG, %ax
movw %ax, %es
movw $BOOT_MEDIA_FLAG_OFF, %di
movb $BOOT_MEDIA_CD, %al
movw $SCHBOOT_MEDIA_OFF, %di
movb $SCHBOOT_MEDIA_CD, %al
movb %al, %es:(%di)
# stage2.S expects zeroed segments and an inherited stack, same as it gets