Initial commit: Schunix as it stands before the SchOS rename

An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.

No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.

tools/mkiso.sh

@@ -0,0 +1,91 @@
+#!/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"