# Booting SchOS Two firmware paths, kept apart on purpose: ``` boot/ bios/ legacy BIOS: MBR, El Torito CD stub, real-mode stage 2 uefi/ UEFI application (not written yet) ``` They are separate directories rather than `#ifdef`s in one because they share almost nothing. A BIOS loader lives in 16-bit real mode, talks to the firmware through software interrupts, and has to walk itself up into long mode. A UEFI loader is a 64-bit PE executable that the firmware has *already* placed in long mode with paging on, and it asks for services by calling through a table of function pointers. The only things genuinely in common are the logo bitmap, the menu's appearance, and the hand-off contract below. ## What the loader must hand the kernel Whichever path runs, the kernel expects the same things. This is the contract; a second loader is correct exactly when it satisfies it. | | Where | Notes | |---|---|---| | Kernel image | loaded per its ELF program headers | entry at `entry64` | | CPU state | see below -- the two paths differ here | | | Memory map | `E820_MAP_ADDR`, 24-byte entries, count at `E820_COUNT_ADDR` | `kernel/arch/x86_64/e820.c` reads it | | Framebuffer | `VBE_INFO_ADDR` | address, pitch, width, height, depth | | Verbose flag | `BOOT_VERBOSE_FLAG_ADDR` | set by the F8 menu | The addresses are defined once, in `include/memlayout.h` and `boot/bios/stage2main.c`. A UEFI loader must fill the same structures — the kernel does not care which firmware produced them, and that is what keeps the split cheap. ## Two entry points, because the two firmwares hand over differently The BIOS path enters the kernel in **32-bit protected mode** at the multiboot entry, with EAX holding the framebuffer address; `mboot_entry` then builds page tables and climbs into long mode itself. UEFI hands a loader long mode with paging already on. Dropping back to protected mode only to climb out again is a mode switch whose failure mode is a triple fault with no console, so the kernel publishes a second entry instead. `kernel/arch/x86_64/entry.S` carries a `sch64_header` — magic `SCH64BOO`, 8-byte aligned within the first 16 KB, followed by a 64-bit physical address — found by scanning, exactly as the BIOS loader scans for the multiboot magic. `include/schboot.h` holds the magic so loader and kernel cannot disagree. What that entry expects: - long mode, interrupts off - CR3 pointing at tables that identity-map the low 4 GB **and** map the same 4 GB at `0xffff800000000000`. `entry64low`'s first instruction is a `movabs` to a high address, so identity alone triple faults immediately. - RBX = the framebuffer physical address, and never 1 — that value marks an application processor in `entry64high`. The kernel loads its own GDT at that entry (`entry64uefi`), rather than the loader doing it. A loader arriving in long mode is still on the firmware's descriptors, and the first thing the kernel does that depends on its own selectors takes a `#GP` against a GDT the firmware no longer owns — it surfaced as `unexpected trap 13 ... cs=38`, a long way from the cause. `e820.c` reads a *BIOS* memory map. UEFI hands out a different structure (`EFI_MEMORY_DESCRIPTOR`, from `GetMemoryMap`), so the UEFI loader has to translate it into the same 24-byte entries rather than inventing a second format for the kernel to learn. ## The BIOS path today 1. **`bios/bootasm.S`** — the literal MBR sector. Signed to 512 bytes by `tools/sign.pl`, loads stage 2 from LBA 1 and jumps. 2. **`bios/cdboot.S`** — the same job for optical media, as an El Torito no-emulation image. Separate because the loader never uses INT 13h and so needs a real ATAPI read. 3. **`bios/stage2.S`** — collects the E820 map, sets a VBE mode, enters protected mode. 4. **`bios/stage2main.c`** — the boot menu and splash, then loads the kernel and enters long mode. `STAGE2_SECTORS` is defined **once**, in the Makefile, and passed to both the assembler and the `dd` that writes the image. It used to be written out by hand in five places and silently truncated the loader when they disagreed. ## Why UEFI is worth doing Everything since roughly 2012 boots UEFI by default, and Secure Boot machines will not run the BIOS path at all without firmware settings most people never touch. The BIOS loader stays regardless: it is what QEMU's default machine and older hardware use, and it costs nothing to keep once the two are separated.