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boot: separate BIOS from UEFI

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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.
This commit is contained in:
Nexoniarz 2026-08-09 16:00:03 +02:00
commit 08a1a9033a
19 changed files with 125 additions and 33 deletions
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4
docs/layout.md
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@ -155,7 +155,7 @@ blob's build location and those symbol names change; update the Makefile to
match or the kernel fails to link.
**`STAGE2_SECTORS` now lives in exactly one place: the root `Makefile`.** It used
to be written out by hand in five — `boot/bootasm.S`, `boot/stage2main.c`'s
to be written out by hand in five — `boot/bios/bootasm.S`, `boot/bios/stage2main.c`'s
`readseg()`, the `dd ... seek=` offset, and twice more inside `tools/posix/mkiso.sh`
(its stage2 size assertion and `-boot-load-size`). This note used to warn you to
keep three of them in step and did not know about the other two.
@ -170,6 +170,6 @@ Makefile always wins.
Two things still fail the build rather than silently misbooting, and should stay
that way: `tools/padstage2.pl` errors if stage2 overflows its budget instead of
truncating it, and `boot/stage2main.c` static-asserts that stage2 cannot reach
truncating it, and `boot/bios/stage2main.c` static-asserts that stage2 cannot reach
the multiboot kernel-search buffer at `0x10000` — which puts the hard ceiling at
65 sectors, since stage2 loads at `0x7E00`.