diff --git a/harness/udpstream/vidstream.c b/harness/udpstream/vidstream.c
index c1ecae5..0fb24a9 100644
--- a/harness/udpstream/vidstream.c
+++ b/harness/udpstream/vidstream.c
@@ -342,6 +342,33 @@ extern void tile_blit_();	/* vram_tile.asm: PIXBLT B,XY, 32x32 tile   */
 extern void vid_dint_();	/* vram_b.asm: DINT                          */
 extern void vid_eint_();	/* vram_b.asm: EINT                          */
 
+/*
+ * VID_OWN_CPU -- mask interrupts once at startup and never re-enable them.
+ *
+ * This is spec 8.9's "the resident OS may die", and it is also a *rate* fix,
+ * not just a robustness one.  With the OS's timer still running, this loop is
+ * preempted and the machine accepts ~15 frames/s whatever the payload size: a
+ * 5-byte one-tile delta measured the same as a 140-tile one (195 frames in
+ * 13.4 s, MAME at 100% speed).  That is scheduling, not decode.  With IE held
+ * clear nothing preempts the loop and the LANCE is polled as fast as the chip
+ * delivers.
+ *
+ * Consequences, stated because they are real: the OS's drivers stop being
+ * serviced, so its console and network stack are gone for good -- the payload
+ * owns the machine either way once it takes the LANCE.  The harness reaches
+ * the payload through the debugger, not through the OS, so the replay modes
+ * are unaffected.
+ *
+ * Set VID_OWN_CPU=0 to build the old behaviour (bursts of EINT around each
+ * transfer) for a comparison run.
+ */
+#ifndef VID_OWN_CPU
+#define VID_OWN_CPU 1
+#endif
+#if VID_OWN_CPU
+#define vid_eint_()	((void)0)	/* masked for good; see main() */
+#endif
+
 /*
  * The firmware's free-space counter is a 32-bit count IN BITS at
  * 0x1E8BA0 (dwt/notes/HEAP.md).  POOL_FREE reports bytes, because the
@@ -1777,7 +1804,11 @@ int main()
 
 	vid_dint_();
 	lance_ok = lance_start(ptr);
+#if VID_OWN_CPU
+	vid_dint_();		/* once, for good: nothing may preempt this loop */
+#else
 	vid_eint_();
+#endif
 	*(unsigned int *)ST_BUF = status_buf;
 
 	for (;;) {