find_rsdp() scanned the EBDA and 0xE0000-0x100000. That is a BIOS
convention, and UEFI firmware is under no obligation to leave a copy
there -- OVMF does not. So the kernel found no RSDP, no MADT and no
IOAPIC, and nothing routed IRQ 4. Serial output is polled, so the machine
booted all the way to a shell prompt and then ignored every key, which
reads as a broken console rather than a missing table.
The UEFI loader reads the RSDP out of the EFI configuration table,
preferring ACPI 2.0 for its XSDT, and leaves it at SCHBOOT_RSDP_ADDR --
the same low-memory hand-off block the e820 map already uses, now
described in include/schboot.h so the two sides cannot disagree.
The kernel validates it rather than trusting it: the BIOS path never
writes that slot, so there it holds whatever was in memory, and following
a wild pointer faults before there is anything to report the fault with.
Signature and checksum do that. It deliberately does not require 16-byte
alignment -- that rule is about searching the BIOS area, and OVMF's RSDP
sits at 0x3f77e014, so demanding it threw away the one address this code
exists to use.
UEFI now reports "acpi: 1 cpu, lapic at ..., ioapic id 0" and the ISO
boots to an interactive shell: uname, ls /bin (241), writing and reading
a file, ps, /proc/schfs -- all driven over serial.
BIOS unchanged: bootcheck 108 [ok], same ACPI line as before.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Loads the kernel from the ESP, places its ELF segments, records the
framebuffer, translates the UEFI memory map into E820 entries, exits boot
services, builds the page tables the kernel expects and jumps. Verified
under OVMF: the kernel starts, brings up PCI, VGA, e1000 and the RTC, and
reaches "cpu0: starting".
UEFI structures are written from the specification rather than importing
gnu-efi or EDK II -- a large dependency for two hundred lines of use, and
a licensing question the project does not need to answer. clang emits
PE/COFF directly, so this needs no new toolchain.
The kernel gained a second entry point. The multiboot entry is 32-bit
code that climbs into long mode itself, and firmware that already put us
there would have to drop back down only to climb out again -- a mode
switch whose failure mode is a triple fault with no console. So entry.S
publishes its 64-bit entry the same way it publishes the multiboot
header: a magic to scan for, SCH64BOO, with include/schboot.h owning the
constants so the two cannot disagree. That header goes *after* the
multiboot header, never before: header_addr says the multiboot header
lives at mboot_load_addr, and the BIOS loader computes the kernel's file
offset from it.
The kernel loads its own GDT at that entry rather than the loader doing
it. Arriving in long mode means arriving on the firmware's descriptors,
and the first thing depending on the kernel's own selectors faulted --
"unexpected trap 13 ... cs=38", a long way from the cause.
KNOWN LIMITATION, diagnosed and not yet fixed: e820_init takes the usable
run *containing the kernel*, which under OVMF ends at 8 MB -- the
firmware places an ACPI NVS region at 0x800000, so the run is genuinely
fragmented. The map is truthful and untruncated (21 entries); the
assumption of one contiguous run is BIOS-shaped. The fix is for the
kernel to free each usable region rather than one range, which touches
kalloc and deserves its own change rather than the tail of this one.
BIOS boot is unaffected: 27 checks, no failures.
boot/bios/ now holds the MBR, the El Torito stub and the real-mode stage
2; boot/uefi/ is empty and documented.
Separate directories rather than #ifdefs, because the two share almost
nothing. A BIOS loader lives in 16-bit real mode, reaches the firmware
through software interrupts, and walks itself up into long mode. A UEFI
loader is a 64-bit PE that the firmware has already placed in long mode
with paging on, calling services through a table of function pointers.
What they do share is the hand-off contract, which boot/README.md now
writes down: ELF segments placed, long mode, a 24-byte-entry memory map
at E820_MAP_ADDR, framebuffer geometry at VBE_INFO_ADDR.
That contract is the point of documenting it now. UEFI's GetMemoryMap
returns EFI_MEMORY_DESCRIPTORs, and the right move is to translate them
into the entries kernel/arch/x86_64/e820.c already parses rather than
teach the kernel a second format.
Build output paths under $(BUILD)/boot are unchanged; only sources moved.
