b428a50145
Records the four bugs between "boots to cpu0: starting" and "boots to an interactive shell", the new ioremap rule for device registers, and what the disc now contains -- including that it is still not a live CD, and what that would take. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
89 lines
4.8 KiB
Markdown
89 lines
4.8 KiB
Markdown
---
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name: schunix-iso-boot
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description: "`make iso` builds a bootable CD; it needs El Torito no-emulation plus ATAPI because the loader never uses INT 13h"
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metadata:
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node_type: memory
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type: project
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originSessionId: a665e14b-86bd-4b41-a228-51377f3e9225
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modified: 2026-08-09T20:03:23.603Z
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---
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**`make iso` produces a bootable `schunix.iso`** (added 2026-08-04). Always a
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RAMROOT build — a disc that needed a second disk for its root would not be an
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install medium, and read-only media cannot host a writable root anyway.
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**Why it is built the way it is.** Schunix's loader never calls INT 13h: it
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drives the IDE registers directly, and stage2 has already switched to
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protected mode before it reads the kernel. That rules out both El Torito
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emulation modes (hard-disk emulation exists only through the BIOS) and floppy
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emulation (2.88 MB cap). So:
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- `boot/cdboot.S` — a separate stage 1 for optical media. `tools/mkiso.sh`
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passes `-boot-load-size 25`, so the BIOS loads cdboot *and* all 24 sectors of
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stage2 before anything of ours runs, putting stage2 at 0x7E00 exactly where
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`bootasm.S` would have. cdboot only records the medium and jumps.
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- The medium flag is one byte past the verbose flag, at `VBE_INFO_ADDR + 0x101`.
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**`bootasm.S` must clear it** or a warm reboot from a disc sends a hard-disk
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boot looking for an optical drive.
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- `stage2main.c` gained an ATAPI reader (PACKET + SCSI READ(12), 2048-byte
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blocks with a one-block cache) and finds where it was loaded from by reading
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the El Torito boot catalog: LBA 17, pointer at offset 0x47, then Load RBA at
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catalog offset 40.
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- **Do not pass `-boot-info-table`** to xorriso — it patches 56 bytes over
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offset 8 of the boot image, which is the middle of cdboot's code. That is
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why the boot catalog is parsed instead.
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**The trap that cost the most time here:** ATAPI's 0x14/0xEB signature is only
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valid until something issues a command, and the BIOS has just finished reading
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the disc to load us — so on a CD boot the signature is *always* stale.
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`atapi_reset()` (SRST via the control port at `io + 0x206`) is what makes the
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probe work.
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Two unbounded waits were hiding behind that, both of which hung with no
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message on a machine with no ATA disk: `waitdisk()` in stage2main.c and
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`idewait()` in kernel/dev/ide.c. Both are bounded now, `ideinit()` tolerates an
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empty channel, and anything that *probes* a disk must call
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`ide_disk_present()` first — `iderw()` rightly panics on an absent drive.
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Check it with `pwsh -File tools/windows/bootcheck.ps1 -Iso -Fb`, which boots
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with no hard disk attached at all. See [[schunix-verify-by-driving-shell]].
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## 2026-08-09: two boot entries, a real hierarchy, and a repair
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The disc used to hold exactly one file (`boot/schos.bin`) with 8.3 uppercase
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names and a single El Torito entry. Now:
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|---|---|
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| `/boot/schos.bin` | BIOS entry, unchanged |
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| `/boot/efiboot.img` | UEFI entry: FAT image built with mtools |
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| `/EFI/BOOT/BOOTX64.EFI` | the loader, also a visible file |
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| `/bin /etc /usr ...` | the staged sysroot, as an ordinary tree |
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- **Rock Ridge + Joliet** for real names, permissions and symlinks. **Keep
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`-cache-inodes`**: the sysroot is a toybox multicall binary with ~240 hard
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links to it, and without inode caching each link becomes its own copy —
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28 MB becomes ~200 MB.
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- **`-isohybrid-gpt-basdat`** so `dd` to a stick boots under UEFI. UEFI *only*,
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and not an oversight: BIOS isohybrid needs syslinux's `isohdpfx.bin`, and even
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with it this loader reads the kernel over ATAPI, which a stick presented as a
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hard disk cannot serve. `usbstick.img` is the BIOS-from-USB answer.
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- **The FAT image carries the kernel too.** `BOOTX64.EFI` opens `\schos\kernel`
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on the volume it was loaded from, which for a UEFI boot is that FAT image and
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not the ISO 9660 tree. Without it the loader halts with `EFI_NOT_FOUND`. The
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kernel is therefore on the disc twice; the alternative is teaching the loader
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to search other volumes, which is a feature, not a packaging tweak.
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**`make iso` and `make ramroot` had been broken and nobody noticed.**
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`build/fs-ram.img` was still built by the v1 `mkfs` while the kernel accepts
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only SchFS as root, so the ramdisk was a format it could not mount:
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`schfs root: no SchFS volume on the root device`, in forkret. It is an mkschfs
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image now, which needed **`--exclude` added to mkschfs** — dropping the exclude
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list was not an option, because the ramdisk is linked into the kernel binary and
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what goes into it is a licensing question ([[schunix-ramdisk-root-sizing]]).
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`RAMDISK_NINODES` is gone: SchFS has no preallocated inode table.
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**Not a live CD.** The disc boots from the ramdisk embedded in the kernel, not
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from itself. Mounting it would need an ATAPI block driver (`kernel/dev/ide.c`
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speaks only ATA — 0x20/0x30/0xC4, no PACKET) plus an ISO 9660 filesystem behind
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`struct fs_ops`. Only the *bootloader* does ATAPI, in real mode.
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