diff --git a/harness/mame/run-vidstream.sh b/harness/mame/run-vidstream.sh
index a5d0b88..87f9bb1 100755
--- a/harness/mame/run-vidstream.sh
+++ b/harness/mame/run-vidstream.sh
@@ -63,8 +63,10 @@
 # memory and compares them itself: the assertion is a relation between two
 # screens, not a rendering to re-derive, and it is why the first negative
 # control's valid first entry matters (a decoder that painted as it decoded
-# would have painted it). Evidence: $LOGDIR/vram-delta-*.bin and
-# delta-status.txt.
+# would have painted it). A last delta changes all 1,280 tiles (3,842 payload
+# bytes: three datagrams), which is the only case where the count, the position
+# list and the value list straddle a fragment boundary; every tile is compared
+# with its new entry. Evidence: $LOGDIR/vram-delta-*.bin and delta-status.txt.
 #
 # The run uses VT1200_SKIP_LEVEL1=1: the payload's heap is only initialised in
 # the OS phase.  In the diagnostics menu the free counter (0x1E8BA0) and the
@@ -132,8 +134,8 @@ fi
 if [ "$MODE" = "delta" ]; then
     rm -f "$LOGDIR/vram-delta-before.bin" "$LOGDIR/vram-delta-after.bin" \
           "$LOGDIR/vram-delta-reject.bin" "$LOGDIR/vram-delta-recover.bin" \
-          "$LOGDIR/dict-delta.bin" "$LOGDIR/indices-delta.bin" \
-          "$LOGDIR/delta-status.txt"
+          "$LOGDIR/vram-delta-large.bin" "$LOGDIR/dict-delta.bin" \
+          "$LOGDIR/indices-delta.bin" "$LOGDIR/delta-status.txt"
 fi
 # Absolute: the Lua runs with MAME's cwd, so a relative default only resolves
 # when the caller happens to be at the repo root.
diff --git a/harness/mame/vidstream.lua b/harness/mame/vidstream.lua
index 68f18f2..9d5fc40 100644
--- a/harness/mame/vidstream.lua
+++ b/harness/mame/vidstream.lua
@@ -178,10 +178,10 @@ elseif MODE == "tiles" and not os.getenv("VID_END") then
 	-- exits, so this is only the catastrophic-stall backstop.
 	DEFAULT_END = AT + 26 * tonumber(os.getenv("VID_TILE_WAIT") or "300") + 120
 elseif MODE == "delta" and not os.getenv("VID_END") then
-	-- Same shape as tiles mode plus the keyframe and the three deltas: one
-	-- ring wait, 23 dictionary chunks and five frames, each bounded by
-	-- VID_DELTA_WAIT.
-	DEFAULT_END = AT + 30 * tonumber(os.getenv("VID_DELTA_WAIT") or "300") + 120
+	-- Same shape as tiles mode plus the keyframe and the five deltas: one
+	-- ring wait, 23 dictionary chunks and six frames, each bounded by
+	-- VID_DELTA_WAIT; the first timeout dumps and exits.
+	DEFAULT_END = AT + 34 * tonumber(os.getenv("VID_DELTA_WAIT") or "300") + 120
 end
 local STOP_AT   = tonumber(os.getenv("VID_END") or tostring(DEFAULT_END))
 local ORIGIN    = tonumber(os.getenv("VID_ORIGIN") or "0x018C0000")
@@ -629,8 +629,9 @@ local STREAM_FILES = { "stream-status.bin", "stream-status.txt" }
 local TILE_FILES = { "vram-tiles.bin", "dict-tiles.bin", "indices-tiles.bin",
 	"tiles-status.txt" }
 local DELTA_FILES = { "vram-delta-before.bin", "vram-delta-after.bin",
-	"vram-delta-reject.bin", "vram-delta-recover.bin", "dict-delta.bin",
-	"indices-delta.bin", "delta-status.txt" }
+	"vram-delta-reject.bin", "vram-delta-recover.bin",
+	"vram-delta-large.bin", "dict-delta.bin", "indices-delta.bin",
+	"delta-status.txt" }
 
 local function clear_dump_files()
 	if MODE == "dict" then
@@ -1998,7 +1999,12 @@ end)()
 --      entries claimed, one carried).  Same assertions -- the list may not run
 --      past the frame;
 --   6. recover: one more valid delta at a different tile, so the refusals left
---      no latch behind; the assertions are the same shape as step 3.
+--      no latch behind; the assertions are the same shape as step 3;
+--   7. large delta: all 1,280 tiles changed.  2 + 3*1,280 = 3,842 payload bytes
+--      = THREE datagrams, so the count, the position list and the value list
+--      each straddle a fragment boundary -- the path a one-tile delta cannot
+--      reach, and the one that would break silently if the fragment walk were
+--      off by a byte.  Every tile is compared with its new entry.
 --
 -- The active area is screen rows 0..1023, bytes 0..159 of each 256-byte row.
 -- Bytes 160..255 are outside every tile and belong to the resident OS, exactly
@@ -2084,6 +2090,7 @@ local delta_prev = nil         -- the screen after the previous accepted frame
 local delta_after = nil
 local delta_reject = nil
 local delta_recover = nil
+local delta_large = nil
 local delta_last_len = 0
 
 local function delta_chunk_frame(off, body)
@@ -2280,6 +2287,9 @@ local function delta_dump(timed_out)
 	if delta_recover then
 		ok = write_file(DUMP_DIR .. "/vram-delta-recover.bin", delta_recover) and ok
 	end
+	if delta_large then
+		ok = write_file(DUMP_DIR .. "/vram-delta-large.bin", delta_large) and ok
+	end
 	local lines = {
 		string.format("RX_OK=%d", prog:read_u32(RX_OK)),
 		string.format("RX_BAD=%d", prog:read_u32(RX_BAD)),
@@ -2574,6 +2584,72 @@ local function delta_step()
 		delta_check(okv, string.format(
 			"the block is dictionary entry %d (first bad byte %d)",
 			(tile_index(RECOVER) + 2) & 0xFF, first))
+		delta_prev = delta_recover
+		-- Large delta: every tile, a new entry for each.  1,280 entries is a
+		-- 2 + 3*1,280 = 3,842-byte payload, delivered as THREE datagrams, so
+		-- the count, the position list and the value list all straddle a
+		-- fragment boundary -- the one path a one-tile delta never reaches.
+		local positions, values = {}, {}
+		for i = 0, TILES_PER_FRAME - 1 do
+			positions[i + 1] = i
+			values[i + 1] = (tile_index(i) + 1) & 0xFF
+		end
+		f, n = delta_send(positions, values, 0x0095)
+		if n <= DRAW_CHUNK then
+			delta_check(false, string.format(
+				"the large delta is more than one datagram (%d bytes)", n))
+			delta_dump(true)
+			return true
+		end
+		if not delta_take(f, n) then
+			delta_check(false, "the ring accepted the large delta")
+			delta_dump(true)
+			return true
+		end
+		delta_phase = 7
+		delta_phase_at = frames
+		return false
+	end
+
+	if delta_phase == 7 then
+		local bad
+		if not delta_settled() then
+			if frames >= delta_phase_at + DELTA_WAIT then
+				delta_check(false, string.format(
+					"the payload consumed the large delta (RX_OK=%d TILES_DROP=%d)",
+					prog:read_u32(RX_OK), prog:read_u32(TILES_DROP)))
+				delta_dump(true)
+				return true
+			end
+			return false
+		end
+		delta_check(delta_last == 3, string.format(
+			"the large delta arrived as 3 datagrams (%d)", delta_last))
+		delta_check(prog:read_u32(RX_OK) == 4,
+			string.format("the large delta reached the screen (RX_OK=%d)",
+				prog:read_u32(RX_OK)))
+		delta_check(prog:read_u32(TILES_DROP) == 2,
+			string.format("the large delta was not dropped (TILES_DROP=%d)",
+				prog:read_u32(TILES_DROP)))
+		delta_check(prog:read_u32(RX_LAST_LEN) == 2 + 3 * TILES_PER_FRAME,
+			string.format("RX_LAST_LEN is the large delta's payload_len (%d, expected %d)",
+				prog:read_u32(RX_LAST_LEN), 2 + 3 * TILES_PER_FRAME))
+		delta_large = read_vram()
+		bad = 0
+		first = -1
+		for i = 0, TILES_PER_FRAME - 1 do
+			okv = block_equals(delta_large, i % TILE_COLS, i // TILE_COLS,
+				(tile_index(i) + 1) & 0xFF)
+			if not okv then
+				bad = bad + 1
+				if first < 0 then
+					first = i
+				end
+			end
+		end
+		delta_check(bad == 0, string.format(
+			"all %d tiles of the large delta match their new entry (%d wrong, first tile %d)",
+			TILES_PER_FRAME, bad, first))
 		delta_check(prog:read_u32(DICT_READY) == 1
 			and prog:read_u32(DICT_BYTES) == TILE_DICT_SIZE
 			and prog:read_u32(DICT_BAD) == 0,