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.
49 lines
2.6 KiB
Markdown
49 lines
2.6 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: 2026-08-04T17:28:16.714Z
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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/bios/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/bootcheck.ps1 -Iso -Fb`, which boots with no
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hard disk attached at all. See [[schunix-verify-by-driving-shell]].
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