---
name: schunix-iso-boot
description: "`make iso` builds a bootable CD; it needs El Torito no-emulation plus ATAPI because the loader never uses INT 13h"
metadata: 
  node_type: memory
  type: project
  originSessionId: a665e14b-86bd-4b41-a228-51377f3e9225
  modified: 2026-08-04T17:28:16.714Z
---

**`make iso` produces a bootable `schunix.iso`** (added 2026-08-04). Always a
RAMROOT build — a disc that needed a second disk for its root would not be an
install medium, and read-only media cannot host a writable root anyway.

**Why it is built the way it is.** Schunix's loader never calls INT 13h: it
drives the IDE registers directly, and stage2 has already switched to
protected mode before it reads the kernel. That rules out both El Torito
emulation modes (hard-disk emulation exists only through the BIOS) and floppy
emulation (2.88 MB cap). So:

- `boot/bios/cdboot.S` — a separate stage 1 for optical media. `tools/mkiso.sh`
  passes `-boot-load-size 25`, so the BIOS loads cdboot *and* all 24 sectors of
  stage2 before anything of ours runs, putting stage2 at 0x7E00 exactly where
  `bootasm.S` would have. cdboot only records the medium and jumps.
- The medium flag is one byte past the verbose flag, at `VBE_INFO_ADDR + 0x101`.
  **`bootasm.S` must clear it** or a warm reboot from a disc sends a hard-disk
  boot looking for an optical drive.
- `stage2main.c` gained an ATAPI reader (PACKET + SCSI READ(12), 2048-byte
  blocks with a one-block cache) and finds where it was loaded from by reading
  the El Torito boot catalog: LBA 17, pointer at offset 0x47, then Load RBA at
  catalog offset 40.
- **Do not pass `-boot-info-table`** to xorriso — it patches 56 bytes over
  offset 8 of the boot image, which is the middle of cdboot's code. That is
  why the boot catalog is parsed instead.

**The trap that cost the most time here:** ATAPI's 0x14/0xEB signature is only
valid until something issues a command, and the BIOS has just finished reading
the disc to load us — so on a CD boot the signature is *always* stale.
`atapi_reset()` (SRST via the control port at `io + 0x206`) is what makes the
probe work.

Two unbounded waits were hiding behind that, both of which hung with no
message on a machine with no ATA disk: `waitdisk()` in stage2main.c and
`idewait()` in kernel/dev/ide.c. Both are bounded now, `ideinit()` tolerates an
empty channel, and anything that *probes* a disk must call
`ide_disk_present()` first — `iderw()` rightly panics on an absent drive.

Check it with `pwsh -File tools/bootcheck.ps1 -Iso -Fb`, which boots with no
hard disk attached at all. See [[schunix-verify-by-driving-shell]].