diff --git a/harness/mame/run.sh b/harness/mame/run.sh
index bdb2f16..88130cd 100755
--- a/harness/mame/run.sh
+++ b/harness/mame/run.sh
@@ -57,7 +57,13 @@ case "$MODE" in
         # IP3 high -> the boot gate at 3F2042D0 jumps to the OS cold-start,
         # skipping all 15 level-1 tests.  Also flips DUART1's IRQ to INT2, which
         # is the routing the OS's DUART driver expects (POST uses INT1).
-        exec env VT1200_SKIP_LEVEL1=1 \
+        #
+        # DUART2 (keyboard/mouse) is modelled as well, and is REQUIRED for the OS:
+        # with it unmodelled the board write-back latch echoes the OS's IWR write
+        # when the OS reads DUART2's ISR (0x06200005), producing a phantom
+        # interrupt that can never clear -- the OS spins forever at the X11 weave.
+        # With it real, the Terminal Manager appears.
+        exec env VT1200_SKIP_LEVEL1=1 VT1200_DUART2=1 \
             "$MAME" vt1200 "${COMMON[@]}" -window "$@"
         ;;