08a1a9033a
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.
61 lines
3.6 KiB
Markdown
61 lines
3.6 KiB
Markdown
---
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name: schunix-boot-splash-logo
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description: "The boot logo pipeline, why the loader draws it rather than the kernel, and the STAGE2_SECTORS trap that consolidating it exposed."
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metadata:
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node_type: memory
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type: project
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originSessionId: 9d8a2aef-bcf0-47a0-96c6-f706efab4a95
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modified: 2026-08-08T12:04:49.027Z
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---
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The boot splash shows the `Assets/Schunix Small Logo.png` mark (an SX monogram in
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a ring) above the `S C H U N I X` wordmark and its accent rule, all centred as
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one group. Verified rendering correctly on 2026-08-08.
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**The logo is drawn by the bootloader, not the kernel.** This is the
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counter-intuitive part and it is a timing fact, not a preference: the kernel does
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not reach `vga_init()` until roughly the last 300ms of a ~2.8s boot, so a
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kernel-drawn logo appeared for about one frame before `console_takeover()` wiped
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it. Drawn from `splash()` in `boot/bios/stage2main.c` it is up for the whole boot.
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`kernel/dev/vga.c` keeps the same drawing code against the same mask
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(`vga_draw_logo`, `vga_logo_centered`) for things that want the mark *after*
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boot — the crash screen, eventually `schinit`. Don't "fix" the duplication by
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deleting the loader's copy.
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**Pipeline:** `tools/mklogo.py` (host Python, has its own PNG inflate +
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un-filter) crops to the mark, box-downscales, normalises to full strength, packs
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to a 4-bit coverage mask, and writes `include/logo.h`. Run `make logo` — it is
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*not* a build dependency, and `include/logo.h` is committed, on the same
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reasoning as `include/font.h`. The mask is coverage, not colour, so the palette
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stays in [[schunix-console-tty]]'s `include/theme.h`; the loader composites it in
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`SCH_FG` because the source art is a silver gradient and the accent is reserved
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for the one rule. 4 bits rather than 8 because the header is included by the
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*bootloader*, where size is a boot constraint.
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**`STAGE2_SECTORS` was encoded in five places, not the three `docs/layout.md`
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claimed.** Raising it 24 → 48 to fit the 8 KiB mask proved it: the `dd seek=25`
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offset stayed behind and the kernel got written straight over stage2's tail.
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The five were `boot/bios/bootasm.S`, `boot/bios/stage2main.c`, the `dd seek=`, and *twice*
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inside `tools/mkiso.sh` (its size assertion and `-boot-load-size`). It now lives
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only in the root Makefile, which passes `-DSTAGE2_SECTORS` and exports it to
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mkiso; the `#define`s left in the boot sources are `#ifndef` fallbacks. Hard
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ceiling is 65 sectors — stage2 loads at `0x7E00` and must not reach the
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multiboot kernel-search buffer at `0x10000` — and `stage2main.c` now
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static-asserts that. See also [[schunix-iso-boot]].
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**`tools/splashshot.ps1`** captures a burst of QEMU screendumps across early
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boot and reports each frame's distinct-colour count, because the splash window
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is too short to hit with a single guessed delay. It uses `-snapshot` plus
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`file.locking=off` so it runs while an interactive QEMU has the images open —
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booting from copies also works but means duplicating all 153 MB of
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`build/fs.img` per run. Colour counts are the
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assertion: ~2 means text console or blank, 18 means the 4bpp logo is up. If it
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ever reads 156 again, something is drawing an 8-bit mask.
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**Two traps, both hit while doing this:**
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- `$(shell expr $(N) \* 512)` in a Makefile does not work — make passes the
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backslash through, sh globs the `*`, and expr receives a directory listing.
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Make silently carries on with nonsense. Use `$$(( ))` in the recipe.
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- `make iso` was failing when this was written, for unrelated reasons since
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fixed — see [[schunix-ramdisk-root-sizing]]. `-boot-load-size 49` is now
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verified: the ISO boots from optical media only.
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