#!/bin/sh
# Build a bootable Schunix ISO.
#
#   tools/mkiso.sh [output.iso]
#
# The disc boots through El Torito's "no emulation" mode, which is the only
# one of the three that fits how Schunix actually loads itself.
#
#   floppy emulation      caps the image at 2.88 MB; the ramdisk-root kernel
#                         alone is bigger than that.
#   hard disk emulation   presents the image through the BIOS's INT 13h, and
#                         Schunix's loader does not use INT 13h -- it drives
#                         the IDE controller's registers directly, which
#                         cannot reach a BIOS-emulated drive.
#   no emulation          the BIOS loads a chosen run of sectors at 0x7C00
#                         and gets out of the way. That is this.
#
# So the layout below is deliberate: -boot-load-size is 1 + STAGE2_SECTORS,
# which makes the BIOS place both boot/cdboot and the whole of stage2 in memory
# before anything of ours runs. That puts stage2 at 0x7E00 -- byte for byte the
# arrangement bootasm.S produces on a hard disk. From there the only difference
# is that stage2main.c reads the kernel back with ATAPI instead of ATA.
#
# STAGE2_SECTORS comes from the environment, set by the Makefile, because this
# same number also lives in boot/bootasm.S, boot/stage2main.c and the dd offsets
# in schunix.img's recipe. It was previously written out by hand in three
# separate spots in this file alone, and raising it for the boot logo is what
# turned that from a latent hazard into a broken image.
#
# NOT using -boot-info-table on purpose: it patches 56 bytes over offset 8 of
# the boot image, which is the middle of cdboot's code. stage2main.c finds
# where it was loaded from by reading the El Torito boot catalog instead.

set -e

BUILD=${BUILD:-build}
OUT=${1:-schunix.iso}
STAGE2_SECTORS=${STAGE2_SECTORS:-48}
STAGE2_BYTES=$((STAGE2_SECTORS * 512))
LOAD_SIZE=$((1 + STAGE2_SECTORS))
STAGING="$BUILD/iso"

CDBOOT="$BUILD/boot/cdboot"
STAGE2="$BUILD/boot/stage2"
KERNEL="$BUILD/kernel/kernel"

for f in "$CDBOOT" "$STAGE2" "$KERNEL"; do
    [ -f "$f" ] || { echo "mkiso: missing $f -- run make first" >&2; exit 1; }
done

# The three pieces must be contiguous and at these exact sizes: stage2main.c's
# readseg() finds the kernel at sector (1 + STAGE2_SECTORS) of the boot image,
# so a short stage2 would silently shift the kernel out from under it.
cdboot_sz=$(wc -c < "$CDBOOT")
stage2_sz=$(wc -c < "$STAGE2")
[ "$cdboot_sz" -eq 512 ]            || { echo "mkiso: cdboot is $cdboot_sz bytes, expected 512" >&2; exit 1; }
[ "$stage2_sz" -eq "$STAGE2_BYTES" ] || { echo "mkiso: stage2 is $stage2_sz bytes, expected $STAGE2_BYTES (STAGE2_SECTORS=$STAGE2_SECTORS)" >&2; exit 1; }

rm -rf "$STAGING"
mkdir -p "$STAGING/boot"

cat "$CDBOOT" "$STAGE2" "$KERNEL" > "$STAGING/boot/schunix.bin"

# xorriso if it is here, mkisofs otherwise. Both accept the El Torito flags
# below unchanged -- xorriso's -as mkisofs mode exists precisely to be
# argument-compatible with it. The fallback is not academic: there is no xorriso
# package for Windows in scoop, where this is developed, but cdrtools provides
# mkisofs.exe.
if command -v xorriso >/dev/null 2>&1; then
    MKISOFS="xorriso -as mkisofs"
elif command -v mkisofs >/dev/null 2>&1; then
    MKISOFS="mkisofs"
else
    echo "mkiso: need xorriso or mkisofs on PATH" >&2
    echo "       Windows: scoop install cdrtools" >&2
    echo "       Debian/Ubuntu: apt install xorriso" >&2
    exit 1
fi

$MKISOFS \
    -quiet \
    -V SCHUNIX \
    -A "Schunix" \
    -b boot/schunix.bin \
    -no-emul-boot \
    -boot-load-size "$LOAD_SIZE" \
    -o "$OUT" \
    "$STAGING"

echo "==> $OUT ($(wc -c < "$OUT") bytes)"
echo "    qemu-system-x86_64 -cdrom $OUT -boot d"