SchOS/docs/architecture/boot.md

Boot

Firmware to the first shell prompt. This is the shortest path through the whole system, which makes it the best thing to read first.

The loader's side of the hand-off is specified in boot/README.md — that file is the contract, and this one is the narrative.

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1. Firmware

UEFI only. There was a BIOS path and it was removed: everything since roughly 2012 boots UEFI, and CSM — the firmware-side BIOS emulation that let legacy loaders run — has been going away since 2020. The cost of keeping it was not the code but the tax on everything it touched, and the asymmetry is total. A BIOS loader lives in 16-bit real mode and calls the firmware through software interrupts; UEFI hands a loader long mode with paging already on and a table of function pointers.

The loader is boot/uefi/, a 64-bit PE application built by the same clang install as everything else (--target=x86_64-unknown-windows). It imports neither gnu-efi nor EDK II — the handful of structures it uses were written from the specification, which is both a smaller dependency and no licence question.

2. The loader

Six steps, in this order, and the order is load-bearing:

1. Find the kernel on the volume this image was loaded from — \schos\kernel, or \kernel.
2. Load its ELF PT_LOAD segments to their physical addresses.
3. Record the framebuffer geometry from the Graphics Output Protocol.
4. Translate GetMemoryMap() into the 24-byte E820-shaped entries kernel/arch/x86_64/e820.c already parses — not a second format for the kernel to learn.
5. ExitBootServices.
6. Build the page tables the kernel's 64-bit entry expects, and jump.

Nothing allocates after step 5 and nothing calls a firmware service after it, because there are none. Everything the jump needs is computed before.

The hand-off block

One page of low memory at 0x30000, defined once in include/schboot.h and included by both the loader and entry.S so the two cannot disagree.

Offset	What
+0x000	VBE-shaped mode info: LFB address, stride, width, height, depth
+0x100	Verbose-boot flag
+0x101	Boot medium (1 = CD; 0 = not stated)
+0x200	E820 entry count, then up to 40 × 24-byte entries
+0x600	ACPI RSDP physical address, or 0

Two of these exist because of specific failures worth knowing about.

The RSDP is passed explicitly because the kernel's own search scans the EBDA and 0xE0000–0x100000, which is a BIOS convention. UEFI publishes the RSDP in its configuration table and is under no obligation to leave a copy anywhere; OVMF does not. Without it the kernel finds no MADT, so no IOAPIC, so no interrupt routing — and the machine boots to a prompt on polled serial output and then ignores every keystroke, which looks like a broken console rather than a missing table.

The E820 cap is 40 entries, not 32. Real UEFI firmware produces far more descriptors than QEMU's, and the space was already there.

The entry point

The kernel publishes a sch64_header — magic SCH64BOO, 8-byte aligned within the first 16 KB, followed by a 64-bit physical entry address — found by scanning, exactly as a multiboot loader scans for its own magic.

It exists because the firmware has already put the machine in long mode. Dropping back to protected mode to re-enter through the 32-bit multiboot path is a mode switch whose failure mode is a triple fault with no console. The 32-bit entry is still present in entry.S and now has no caller.

3. kmain()

kernel/core/main.c. The ordering here is not arbitrary and several steps are where they are because of a bug:

  e820_init()          memory map first — PHYSTOP is a variable, not a constant
  kinit1()             the allocator, given everything below the DMA arena
  dma_init()           the arena, carved off the TOP of physical memory
  kvmalloc()           the kernel page tables
  acpi_init()          CPU enumeration from the MADT; mpinit() only as fallback
  lapicinit() tvinit() seginit() ioapicinit()
  irq_init()           before any driver that registers a handler
  inputinit()          before the keyboard and mouse start producing events
  consoleinit() uartinit() mouseinit()
  platform_register_devices(fb)   the fixed furniture of a PC, plus the framebuffer
  pci_init()           enumerate, and bind drivers
  pinit() binit() fileinit() sockinit()
  time_init()          after the timer runs, before userspace
  ideinit() ramdiskinit()
  startothers()        the other processors
  kinit2() sinit()     the rest of memory, then PID 1
  netinit()            after sinit — it starts a kernel thread
  mpmain()             enter the scheduler; never returns

The DMA arena is taken before kalloc gets the rest. A device needs physically contiguous memory and kalloc's free list makes no such promise — two pages from it are adjacent only by chance. Taking it first is the only way to be sure the region exists.

ACPI is asked before the legacy MP table, and getting this backwards is invisible under QEMU. Modern firmware publishes a complete MADT and an MP table containing one processor, so asking the older interface first means never seeing more than one CPU. VMware published the real count, which is why the resulting triple fault only ever appeared there.

main() names no display driver. Whichever driver owns the framebuffer the loader left running publishes it with gfx_attach_scanout(), and the console and crash screen draw on that. See graphics.md.

Memory sizing

PHYSTOP is a variable read from the firmware's map, not a constant. It used to be 0xE000000 — 224 MB — inherited from xv6, and it was not a limit on what the kernel used but an assertion that exactly that much RAM existed. Less and the allocator walked off the end of memory; more and it was invisible.

Three details survive that fix:

• It takes the end of the usable region containing the kernel, not the highest usable address anywhere. A memory map has reserved holes, and freeing a single range from the kernel's end to a global maximum puts firmware tables on the free list — failing later, elsewhere, looking like anything but a memory map problem.
• The usable regions are also kept as a list, and kinit1() frees each one. One range ending at a top address is what a BIOS map looks like and what UEFI never does: OVMF drops an ACPI NVS region at 0x800000, so the run containing the kernel ends at 8 MB and everything above the hole was invisible.
• The ceiling is 3 GB, because kvmalloc() maps 0–4 GB as four 1 GB pages and marks the top one uncached for PCI BARs. The floor is 24 MB, and below it the kernel prints why and stops rather than triple faulting in silence.

4. SMP

startothers() wakes each application processor through the local APIC, pointing it at a trampoline copied to 0x7000. Each AP runs mpenter() → mpmain(), reports cpu%d: starting, and enters the scheduler.

This was broken from the project's beginning and is fixed. Five distinct bugs, each hidden behind the one before it — the details are in ../history/kernel.md, and the one worth carrying forward is in kernel.md: __thread is wrong for per-CPU kernel data.

APs are counted on the boot processor after each one reports in, so the count cannot include a processor that started and died.

5. The first process

sinit() builds PID 1 by hand: a page directory, initcode mapped at 0, 32 pages of stack, a trapframe pointing at user mode, and session and process group both set to 1.

It does not mount the root filesystem, and cannot: mounting reads blocks, bread() sleeps, and sleeping needs a process context. So the mount happens in forkret(), the first time any process is scheduled — guarded by a static int first.

That is also where PID 1's working directory is re-pointed at the real root, and the reason is worth keeping:

sinit() sets cwd = namei("/") before any filesystem is mounted. The old v1 root survived that because iget(ROOTDEV, ROOTINO) is two compile-time constants and touches no disk. SchFS has no constant to offer — the root object id comes off the uberblock — so that early namei("/") builds an inode from an unmounted volume. Nothing complains, because absolute paths never consult cwd: an absolute exec works fine. It is the first relative lookup that fails, and in init that is open("console"). No console means no fd 0/1/2, so every shell it spawns exits immediately and init respawns it forever. The symptom is a silent boot that reads like a hang and is not one.

initcode then execs /bin/schinit, and init.md takes over. There is no fallback path and deliberately no /bin/init any more: the older hardcoded fork-a-shell PID 1 was unreachable from the day that string changed, and an unreachable fallback rots rather than saves.

6. What the screen shows

The kernel log is on screen from the first line that can draw. kmain attaches the loader's framebuffer and clears cons_serial_only before it does anything else, so pci: 7 devices and schfs: mounted are visible on the machine itself.

This is the reverse of what the flag was added for, and the reversal was earned. The original argument was that a boot which prints a developer's log to the screen looks like a hobby project, and it is a good argument — right up until a Chromebox with no serial port froze at boot showing nothing at all. The whole diagnosis had to be made from a photograph of a screen that had been told to stay quiet. A tidy boot that cannot be debugged on the hardware it fails on is not a trade worth making.

cons_serial_only still exists for the case it is now the right answer to: no framebuffer from the loader. Then everything goes to the UART and console_takeover() clears the flag when userspace starts.

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What it does not do

• No BIOS path. A machine without UEFI does not boot SchOS.
• The root device is compiled in. Booting from a USB disk needs the root device to be selectable, which it is not; the USB mass-storage driver works and a superblock reads back through bread(), but nothing can mount it.
• make ramroot is the only way to boot with no data disk. It links a filesystem image into the kernel binary, which costs boot time and carries a licensing constraint — see ../licensing.md §4.
• The 32-bit multiboot entry is dead code. It is left in place deliberately: deleting kernel entry code is its own piece of work with its own way of failing silently.
• make qemu-uefi under OVMF reports less memory than the machine has in some configurations. The loader's map is correct; the fix went in with the region list above, and any remaining case is a fragmentation pattern not yet seen.