---
name: schos-boot-split
description: boot/ is split into bios/ and uefi/; the hand-off contract is what makes a second loader cheap
metadata:
  node_type: project
---

**`boot/bios/`** holds the MBR (`bootasm.S`), the El Torito CD stub
(`cdboot.S`) and the real-mode stage 2 (`stage2.S`, `stage2main.c`).
**`boot/uefi/`** is empty and documented.

Separate directories rather than `#ifdef`s, agreed with Nexoniarz 2026-08-09,
because the two share almost nothing: a BIOS loader lives in 16-bit real mode,
reaches the firmware through software interrupts, and walks itself into long
mode; a UEFI loader is a 64-bit PE 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, written down in
`boot/README.md`** — ELF segments placed, long mode with interrupts off, a
memory map of 24-byte entries at `E820_MAP_ADDR`, framebuffer geometry at
`VBE_INFO_ADDR`, verbose flag at `BOOT_VERBOSE_FLAG_ADDR`. A second loader is
correct exactly when it satisfies that.

**The trap for the UEFI work:** `GetMemoryMap` returns `EFI_MEMORY_DESCRIPTOR`s,
which are not E820 entries. Translate them into what
`kernel/arch/x86_64/e820.c` already parses rather than teaching the kernel a
second format.

clang can emit PE/COFF directly (`--target=x86_64-unknown-windows`,
`-fuse-ld=lld-link`), so UEFI needs **no new toolchain** — the same LLVM install
already builds every other target here. Write the handful of UEFI structures
from the specification rather than importing gnu-efi or EDK II: that is both a
licensing question ([[schunix-licensing]]) and a large dependency for something
small.

Build output paths under `$(BUILD)/boot/` were deliberately left unchanged when
the sources moved; only source paths differ.