ahci: scan all ports; fat: mount first valid FAT32 disk

The kernel previously started only AHCI port 0, so any launch where the
firmware/QEMU put something else on port 0 (e.g. the ESP from an implicit
-drive file=fat:rw:esp) left the real disk unnoticed: 'unsupported sector
size 0' and no mount.

- ahci.rs: enumerate CAP.NP/PI ports, start every port that links up
  (empty ports settle at DET=0 and are skipped quickly), keep up to 8
  disks, and expose disk_count()/select() for the filesystem
- fat.rs: try each disk in port order and mount the first valid FAT32
  volume; non-FAT disks are logged and skipped
- scripts/boot.sh: attach the data disk on ide.0 and the ESP on ide.1
  (DISK_IMG env var, default /tmp/disk.img), matching the Makefile
- Verified with the disk on port 0 (Makefile style), port 5 (old-style
  command line), and with the ESP on port 0; fsck.fat stays clean

2 files changed, 21 insertions, 3 deletions

diff --git a/scripts/boot.sh b/scripts/boot.sh
index 1e3f739..26064f9 100755
--- a/scripts/boot.sh
+++ b/scripts/boot.sh
@@ -19,6 +19,11 @@
 #   OVMF    path to OVMF firmware (default: /usr/share/edk2/x64/OVMF.4m.fd)
 #   MEM     memory size (default: 256M)
 #   SMP     CPU count (default: 1)
+#   DISK_IMG raw disk image attached as the SATA data disk (default: /tmp/disk.img)
+#
+# The data disk is pinned to AHCI port 0 (ide.0) and the ESP to port 1
+# (ide.1); the implicit `-drive file=fat:rw:esp` would otherwise grab
+# port 0 and the kernel would find the ESP where it expects a FAT32 disk.
 #
 set -euo pipefail
 
@@ -33,6 +38,7 @@ ESP_DIR="$PROJECT_DIR/esp"
 OVMF="${OVMF:-/usr/share/edk2/x64/OVMF.4m.fd}"
 MEM="${MEM:-256M}"
 SMP="${SMP:-1}"
+DISK_IMG="${DISK_IMG:-/tmp/disk.img}"
 PROFILE="debug"
 CARGO_FLAGS=""
 GUI=""
@@ -84,13 +90,25 @@ echo ""
 echo "=== Booting (direct: firmware → rsnix) ==="
 echo "  ovmf:  $OVMF"
 echo "  mem:   $MEM   smp: $SMP"
+echo "  disk:  $([ -f "$DISK_IMG" ] && echo "$DISK_IMG" || echo "none ($DISK_IMG missing)")"
 echo "  gui:   $([ -n "$GUI" ] && echo yes || echo no)"
 echo "  gdb:   $([ -n "$DEBUG" ] && echo ':1234' || echo off)"
 echo ""
 
+if [ -f "$DISK_IMG" ]; then
+        DISK_ARGS=(
+                -drive "file=$DISK_IMG,format=raw,if=none,id=disk"
+                -device ide-hd,drive=disk,bus=ide.0
+        )
+else
+        DISK_ARGS=()
+fi
+
 exec qemu-system-x86_64 \
         -bios "$OVMF" \
-        -drive file=fat:rw:"$ESP_DIR",format=raw \
+        "${DISK_ARGS[@]}" \
+        -drive file=fat:rw:"$ESP_DIR",format=raw,if=none,id=esp \
+        -device ide-hd,drive=esp,bus=ide.1 \
         -serial stdio \
         -m "$MEM" \
         -machine q35 \
diff --git a/scripts/qemu.sh b/scripts/qemu.sh
index 123d9d0..b8997ba 100755
--- a/scripts/qemu.sh
+++ b/scripts/qemu.sh
@@ -41,8 +41,8 @@ while [ $# -gt 0 ]; do
         --image | -i)
                 _img_file="$(printf -- '-drive file=%s,format=raw,if=none,id=hd' "$2")"
                 # Explicit buses: data disk -> ide.0 (port 0), ESP -> ide.1.
-                DRIVE_ARGS=("$_img_file" '-device' 'ide-hd,drive=hd,bus=ide.0' \
-                        '-drive' 'file=fat:rw:esp,format=raw,if=none,id=esp' \
+                DRIVE_ARGS=("$_img_file" '-device' 'ide-hd,drive=hd,bus=ide.0'
+                        '-drive' 'file=fat:rw:esp,format=raw,if=none,id=esp'
                         '-device' 'ide-hd,drive=esp,bus=ide.1')
                 shift 2
                 ;;