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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
Download patch file Download diff file Expand all files Collapse all files

10
boot/bios/cdboot.S
View file

@ -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

31
boot/bios/stage2main.c
View file

@ -34,7 +34,7 @@ _Static_assert(0x7E00 + STAGE2_SECTORS * 512 <= 0x10000,
// by stage2's own tail end. 0x30000 is clear of stage2, the multiboot
// kernel-search buffer (0x10000-0x14000), and the kernel's load address
// (0x100000).
#define VBE_INFO_ADDR 0x30000
#include "../../include/schboot.h" /* the hand-off block */
// Fixed low-memory scratch byte, read by kernel/core/main.c's kmain() before
// any other init: 1 if the user picked "verbose" from the boot menu. Placed
@ -44,21 +44,16 @@ _Static_assert(0x7E00 + STAGE2_SECTORS * 512 <= 0x10000,
// perspective (verbose is read before startothers() runs), so it wasn't an
// active bug, but reusing a documented-elsewhere address is exactly the
// kind of thing that turns into one later.
#define BOOT_VERBOSE_FLAG_ADDR (VBE_INFO_ADDR + 0x100)
// Set by boot/bios/cdboot.S when we were booted from optical media. One byte past
// the verbose flag, for the same reason that one sits where it does: clear of
// the 256-byte VBE mode info block at VBE_INFO_ADDR.
#define BOOT_MEDIA_FLAG_ADDR (VBE_INFO_ADDR + 0x101)
#define BOOT_MEDIA_CD 1
// the 256-byte VBE mode info block at SCHBOOT_VBE_INFO.
// E820 memory map, collected by stage2.S in real mode (the same firmware
// interface GRUB and Linux's boot stub use). Count at +0x200, then 24-byte
// entries: {uint64 base; uint64 length; uint32 type; uint32 acpi_ext}.
// The kernel can read the same addresses later; the bootloader uses it to
// report detected RAM like any grown-up loader does.
#define E820_COUNT_ADDR (VBE_INFO_ADDR + 0x200)
#define E820_MAP_ADDR (VBE_INFO_ADDR + 0x208)
#define E820_USABLE 1
// NOT include/types.h's uint64: that's `unsigned long`, which is 4 bytes
@ -76,8 +71,8 @@ struct e820entry {
static uint32
e820_usable_mb(void)
{
uint32 count = *(uint32*)E820_COUNT_ADDR;
struct e820entry *e = (struct e820entry*)E820_MAP_ADDR;
uint32 count = *(uint32*)SCHBOOT_E820_COUNT;
struct e820entry *e = (struct e820entry*)SCHBOOT_E820_MAP;
e820_u64 total = 0;
if (count > 32) return 0; // garbage count: BIOS didn't fill the map
for (uint32 i = 0; i < count; i++)
@ -142,11 +137,11 @@ static int g_have_fb;
static void
fb_init(void)
{
g_stride = *(ushort*)(VBE_INFO_ADDR + 16);
g_width = *(ushort*)(VBE_INFO_ADDR + 18);
g_height = *(ushort*)(VBE_INFO_ADDR + 20);
uchar bpp = *(uchar*)(VBE_INFO_ADDR + 25);
uint32 fbaddr = *(uint32*)(VBE_INFO_ADDR + 40);
g_stride = *(ushort*)(SCHBOOT_VBE_INFO + 16);
g_width = *(ushort*)(SCHBOOT_VBE_INFO + 18);
g_height = *(ushort*)(SCHBOOT_VBE_INFO + 20);
uchar bpp = *(uchar*)(SCHBOOT_VBE_INFO + 25);
uint32 fbaddr = *(uint32*)(SCHBOOT_VBE_INFO + 40);
g_fb = (uint8*)fbaddr;
g_have_fb = (bpp == 32 && fbaddr && g_width && g_height);
}
@ -587,7 +582,7 @@ stage2main(void)
// Sort out where the kernel is coming from before anything is drawn, so a
// disc we cannot read says so on the splash instead of failing silently
// several seconds later in the middle of the progress bar.
if (*(uchar*)BOOT_MEDIA_FLAG_ADDR == BOOT_MEDIA_CD) {
if (*(uchar*)SCHBOOT_MEDIA_FLAG == SCHBOOT_MEDIA_CD) {
if (atapi_find())
g_cd_base512 = atapi_boot_base() * (CD_SECTSIZE / SECTSIZE);
g_from_cd = (g_atapi_io != 0 && g_cd_base512 != 0);
@ -601,7 +596,7 @@ stage2main(void)
if (wait_for_f8(3000000))
verbose = boot_menu();
*(uchar*)BOOT_VERBOSE_FLAG_ADDR = verbose;
*(uchar*)SCHBOOT_VERBOSE_FLAG = verbose;
if (g_have_fb) {
fill_rect(0, 0, g_width, g_height, SCH_BG);
@ -615,7 +610,7 @@ stage2main(void)
// legible reason is the only useful thing left to do. This is drawn after
// the clear above on purpose: an earlier version put it on the splash,
// where it was wiped a moment later and nobody ever saw it.
if (*(uchar*)BOOT_MEDIA_FLAG_ADDR == BOOT_MEDIA_CD && !g_from_cd) {
if (*(uchar*)SCHBOOT_MEDIA_FLAG == SCHBOOT_MEDIA_CD && !g_from_cd) {
draw_centered(g_height / 2, "No readable boot disc found.", SCH_ACCENT);
draw_centered(g_height / 2 + 20, "The drive did not answer ATAPI.", SCH_DIM);
for (;;)
@ -660,7 +655,7 @@ stage2main(void)
// broke it silently (kernel received garbage instead of the real
// LFB address). Pin it down explicitly instead of hoping.
{
uint32 fbaddr = *(uint32*)(VBE_INFO_ADDR + 40);
uint32 fbaddr = *(uint32*)(SCHBOOT_VBE_INFO + 40);
uint32 entry = hdr->entry_addr;
asm volatile("jmp *%0" : : "r"(entry), "a"(fbaddr));
}

16
boot/uefi/main.c
View file

@ -27,13 +27,9 @@
#include "efi.h"
#include "../../include/schboot.h"
// Where the kernel expects to find things. These are the BIOS loader's
// addresses and they are deliberately unchanged: the kernel should not be able
// to tell which firmware booted it.
#define VBE_INFO_ADDR 0x30000
#define E820_COUNT_ADDR (VBE_INFO_ADDR + 0x200)
#define E820_MAP_ADDR (VBE_INFO_ADDR + 0x208)
#define E820_MAXENTRIES 32
// Where the kernel expects to find things comes from schboot.h, included
// above: the hand-off is one contract and both loaders write the same one.
#define E820_MAXENTRIES SCHBOOT_E820_MAX
#define E820_USABLE 1
#define E820_RESERVED 2
#define E820_ACPI 3
@ -438,7 +434,7 @@ setup_framebuffer(void)
// block as the BIOS path leaves it, and the kernel takes exactly four
// fields out of it -- so those four are what get written, at the offsets
// it uses.
UINT8 *vbe = (UINT8 *)VBE_INFO_ADDR;
UINT8 *vbe = (UINT8 *)SCHBOOT_VBE_INFO;
zero(vbe, 512);
UINT32 bpp = 32; // GOP's 8-bit formats are 32bpp
@ -612,8 +608,8 @@ EfiMain(EFI_HANDLE image, EFI_SYSTEM_TABLE *systab)
__asm__ volatile("cli");
copy((void *)E820_MAP_ADDR, e820, nent * sizeof e820[0]);
*(UINT32 *)E820_COUNT_ADDR = nent;
copy((void *)SCHBOOT_E820_MAP, e820, nent * sizeof e820[0]);
*(UINT32 *)SCHBOOT_E820_COUNT = nent;
*(UINT64 *)SCHBOOT_RSDP_ADDR = rsdp;
// The page tables mboot_entry would have built. entry64low's first act is