Mason363 · Oct 16, 2026 · Oct 14, 2026 · Oct 14, 2026 · Oct 14, 2026 · Oct 14, 2026
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
@@ -53,6 +53,7 @@ jobs:
           python3 tests/test_small_ram.py build/NumPlay.nwa
           make -C games/numblocks host test
           make -C games/championisland host test
+          make -C games/undertale host test
 
       - uses: actions/upload-artifact@v4
         with:

diff --git a/Makefile b/Makefile
@@ -24,7 +24,7 @@ RAM_LIMIT = 148928
 # custom builds of Epsilon 25.2's source give apps less: 133652 bytes; every game fits there too, in NumPlay and on
 # its own (their big buffers are on the stack, 32 KB apart from that RAM, or in the RAM the software has left)
 CUSTOM_RAM = 133652
-BIG_APPS = Celeste HollowKnight ChampionIsland FinalEarth2 NumPowder NumBlocks NumDrive
+BIG_APPS = Celeste HollowKnight Undertale ChampionIsland FinalEarth2 NumPowder NumBlocks NumDrive
 # the calculator's space for installed apps (0x90200000 to 0x903F0000 in its flash)
 APP_SPACE = 2031616
 # Upsilon, the N0110 and N0115's custom software, gives .nwa apps less RAM: NumPlay-Upsilon.nwa has
@@ -205,6 +205,7 @@ APP_numvisuals = games/numvisuals/output/numvisuals.nwa:NumVisuals.nwa
 APP_celeste = games/celeste/output/celeste.nwa:Celeste.nwa
 APP_championisland = games/championisland/output/championisland.nwa:ChampionIsland.nwa
 APP_hollowknight = games/hollowknight/output/hollowknight.nwa:HollowKnight.nwa
+APP_undertale = games/undertale/output/undertale.nwa:Undertale.nwa
 APP_finalearth2 = games/finalearth2/output/finalearth2.nwa:FinalEarth2.nwa
 APP_numpowder = games/numpowder/output/numpowder.nwa:NumPowder.nwa
 
@@ -220,6 +221,7 @@ apps: | $(B)
 	$(MAKE) -C games/celeste build NWLINK="$(NWLINK)"
 	$(MAKE) -C games/championisland build NWLINK="$(NWLINK)"
 	$(MAKE) -C games/hollowknight build NWLINK="$(NWLINK)"
+	$(MAKE) -C games/undertale build NWLINK="$(NWLINK)"
 	$(MAKE) -C games/finalearth2 build NWLINK="$(NWLINK)"
 	$(MAKE) -C games/numpowder build NWLINK="$(NWLINK)"
 	cd games/tetris/tetris && NWLINK="$(NWLINK)" $(CARGO) build --release --quiet
@@ -235,6 +237,7 @@ apps: | $(B)
 	cp $(word 1,$(subst :, ,$(APP_celeste))) $(B)/apps/$(word 2,$(subst :, ,$(APP_celeste)))
 	cp $(word 1,$(subst :, ,$(APP_championisland))) $(B)/apps/$(word 2,$(subst :, ,$(APP_championisland)))
 	cp $(word 1,$(subst :, ,$(APP_hollowknight))) $(B)/apps/$(word 2,$(subst :, ,$(APP_hollowknight)))
+	cp $(word 1,$(subst :, ,$(APP_undertale))) $(B)/apps/$(word 2,$(subst :, ,$(APP_undertale)))
 	cp $(word 1,$(subst :, ,$(APP_finalearth2))) $(B)/apps/$(word 2,$(subst :, ,$(APP_finalearth2)))
 	cp $(word 1,$(subst :, ,$(APP_numpowder))) $(B)/apps/$(word 2,$(subst :, ,$(APP_numpowder)))
 	arm-none-eabi-strip --strip-unneeded $(B)/apps/Tetris.nwa
@@ -291,6 +294,7 @@ clean:
 	-$(MAKE) -C games/numvisuals clean
 	-$(MAKE) -C games/championisland clean
 	-$(MAKE) -C games/hollowknight clean
+	-$(MAKE) -C games/undertale clean
 	-$(MAKE) -C games/finalearth2 clean
 	-$(MAKE) -C games/numpowder clean
 	-cd games/tetris/tetris && $(CARGO) clean

diff --git a/README.md b/README.md
@@ -79,7 +79,10 @@ These are too big to fit next to NumPlay, so each one is its own app. Install it
   <tr>
     <td align="center"><a href="games/celeste/README.md"><img src="docs/media/thumb_celeste.png" width="200" alt="Celeste"><br><b>Celeste</b></a><br><sub>The whole climb, with every B-side and C-side</sub><br><a href="https://github.com/Mason363/NumPlay/releases/latest/download/Celeste.nwa">Download</a></td>
     <td align="center"><a href="games/championisland/README.md"><img src="docs/media/thumb_championisland.png" width="200" alt="Champion Island"><br><b>Champion Island</b></a><br><sub>The whole island and its seven sports</sub><br><a href="https://github.com/Mason363/NumPlay/releases/latest/download/ChampionIsland.nwa">Download</a></td>
-    <td align="center"><a href="#roadmap"><b>More on the way</b></a><br><sub>Undertale, The Binding of Isaac, Plants vs. Zombies, Doom and more: see the roadmap</sub></td>
+    <td align="center"><a href="games/undertale/README.md"><img src="docs/media/thumb_undertale.png" width="200" alt="Undertale"><br><b>Undertale</b></a><br><sub>The whole Underground, from the golden flowers to the barrier and the neutral ending</sub><br><a href="https://github.com/Mason363/NumPlay/releases/latest/download/Undertale.nwa">Download</a></td>
+  </tr>
+  <tr>
+    <td align="center"><a href="#roadmap"><b>More on the way</b></a><br><sub>The Binding of Isaac, Plants vs. Zombies, Doom and more: see the roadmap</sub></td>
   </tr>
 </table>
 
@@ -171,7 +174,7 @@ Filmed on my own calculator, no emulator.
 - [x] The Final Earth 2
 - [x] The Powder Toy (NumPowder)
 - [x] Block Blast
-- [ ] Undertale
+- [x] Undertale: the whole Underground, from the Ruins to New Home and the neutral ending
 - [ ] The Binding of Isaac
 - [ ] Plants vs. Zombies
 - [ ] Doom
@@ -199,6 +202,7 @@ NumPlay is a fan project, made out of love for these games and for the NumWorks
 - **Minecraft** by Mojang inspired NumBlocks, which follows Minecraft 1.8.8 exactly: it makes its worlds the way Minecraft 1.8.8 does and uses its blocks, items, recipes, textures, font and screens.
 - **Celeste** inspired NumPlay's Celeste, which uses the game's own pictures, maps, texts and fonts. Not affiliated with Maddy Makes Games.
 - **Hollow Knight** inspired NumPlay's Hollow Knight, which uses the game's own pictures, animations, maps, texts and fonts. Not affiliated with Team Cherry.
+- **Undertale** by Toby Fox inspired NumPlay's Undertale, which uses the game's maps, pictures and texts from the [Undertale Wiki](https://undertale.fandom.com) and the game's free demo, and the **Pixel Operator** fonts by Jayvee Enaguas (public domain). Not affiliated with Toby Fox.
 - **[The Powder Toy](https://powdertoy.co.uk)** inspired NumPowder, which ports its materials' behaviour, its font and its icons from [its source code](https://github.com/The-Powder-Toy/The-Powder-Toy) (GNU GPL v3.0), by Stanislaw K Skowronek, Simon Robertshaw and its contributors. Not affiliated with them.
 - **[The Final Earth 2](https://store.steampowered.com/app/1180130/The_Final_Earth_2/)** by [Florian van Strien](https://florianvanstrien.nl) inspired NumPlay's The Final Earth 2, which uses the game's own pictures, fonts and texts, with additional sprites by LukXD99. Not affiliated with him.
 - **Buckshot Roulette** by Mike Klubnika inspired NumPlay's Buckshot Roulette, whose scenes are rendered from the game through the Open Buckshot Roulette project (1503Dev). Fonts: Fake Receipt by Ray Larabie and Dot Matrix by Dionaea.
@@ -215,7 +219,7 @@ NumPlay is a fan project, made out of love for these games and for the NumWorks
 
 NumPlay is licensed under the [GNU General Public License v3.0](LICENSE). Copyright (c) 2026 Mason Chen. Code and fonts from others keep their own licenses: the `UNLICENSE` files in the games' folders and the [LICENSES](LICENSES) folder. The names, artwork and music of the games NumPlay is inspired by belong to their makers and are not covered by it.
 
-<sub>Unofficial fan games, not affiliated with NumWorks, RobTop Games (Geometry Dash), Hipster Whale (Crossy Road), the makers of Drive Mad, LocalThunk or Playstack (Balatro), Mike Klubnika or Critical Reflex (Buckshot Roulette), Hungry Studio (Block Blast!), Mojang or Microsoft (Minecraft), Maddy Makes Games (Celeste), Team Cherry (Hollow Knight), Florian van Strien (The Final Earth 2), The Powder Toy's developers (The Powder Toy), MateoConLechuga (Portal Returns), Valve (Portal), .GEARS (Flappy Bird), Bandai Namco (Pac-Man), Hasbro (Connect Four), Google (Snake, Solitaire, Block Breaker, Doodle Champion Island Games), STUDIO4°C or Microsoft (Minesweeper). Tetris is a trademark of The Tetris Company, Minecraft of Mojang Synergies AB, Pac-Man of Bandai Namco and Connect Four of Hasbro.</sub>
+<sub>Unofficial fan games, not affiliated with NumWorks, RobTop Games (Geometry Dash), Hipster Whale (Crossy Road), the makers of Drive Mad, LocalThunk or Playstack (Balatro), Mike Klubnika or Critical Reflex (Buckshot Roulette), Hungry Studio (Block Blast!), Mojang or Microsoft (Minecraft), Maddy Makes Games (Celeste), Team Cherry (Hollow Knight), Toby Fox (Undertale), Florian van Strien (The Final Earth 2), The Powder Toy's developers (The Powder Toy), MateoConLechuga (Portal Returns), Valve (Portal), .GEARS (Flappy Bird), Bandai Namco (Pac-Man), Hasbro (Connect Four), Google (Snake, Solitaire, Block Breaker, Doodle Champion Island Games), STUDIO4°C or Microsoft (Minesweeper). Tetris is a trademark of The Tetris Company, Minecraft of Mojang Synergies AB, Pac-Man of Bandai Namco and Connect Four of Hasbro.</sub>
 
 ## Gallery
 
@@ -262,5 +266,6 @@ NumPlay is licensed under the [GNU General Public License v3.0](LICENSE). Copyri
   </tr>
   <tr>
     <td align="center"><img src="docs/media/gallery/championisland.gif" width="100%" alt="Champion Island: climbing, artistic swimming and the marathon"><br><a href="games/championisland/README.md"><b>Champion Island</b></a></td>
+    <td align="center"><img src="docs/media/gallery/undertale.gif" width="100%" alt="Undertale: Flowey's friendliness pellets, Toriel's tile puzzle and her fire"><br><a href="games/undertale/README.md"><b>Undertale</b></a></td>
   </tr>
 </table>
diff --git a/docs/media/gallery/undertale.gif b/docs/media/gallery/undertale.gif
diff --git a/docs/media/thumb_undertale.png b/docs/media/thumb_undertale.png
diff --git a/docs/media/undertale.gif b/docs/media/undertale.gif
diff --git a/docs/play.md b/docs/play.md
@@ -254,6 +254,24 @@ Every sport shows its rules and keys before it starts. Your progress is also cop
 
 <br clear="right">
 
+## Undertale
+
+<img src="media/undertale.gif" width="320" align="right" alt="Undertale: Flowey's friendliness pellets, Toriel's tile puzzle, Napstablook's tears and Toriel's fire">
+
+Undertale comes as its own app, `Undertale.nwa`: it is too big to share the calculator with NumPlay.
+
+| Key | What it does |
+| --- | --- |
+| **Arrows** (or 4, 6, 8, 2) | Walk, move your SOUL, choose |
+| **OK**, **EXE** or 5 | Talk, read, confirm (Z in the game) |
+| **Back** or **⌫** | Cancel, show the whole text at once (X in the game) |
+| **toolbox**, **shift** or **var** | The menu: ITEM, STAT, CELL (C in the game) |
+| **Home** | Keep the moment and quit |
+
+Name the fallen human, then walk and talk your way through the Ruins, Snowdin, Waterfall, Hotland, the MTT Resort, the CORE and New Home. In a battle, FIGHT attacks (press **OK** when the bar crosses the middle), ACT checks a monster or does something to it, ITEM eats, and MERCY spares a monster that no longer wants to fight, or flees. Then move your SOUL to dodge; when it turns blue, it falls, and **Up** jumps; when it turns green, it stays put, and the arrows turn its shield to block. On ice you slide until something stops you. In Snowdin Town, walk into the shop to buy, and into the inn to rest. In Waterfall, walk into a bridge seed to push it into the water, where it blooms into a flower you can walk on; the bell calls the seeds back. A green bullet heals you. Walk up to a star and press **OK** to SAVE. Home keeps the moment you were at, and the next start is back there. Your save is also copied into `undertale_saves.py`, so installing the app again doesn't erase it. The Tem Shop in Temmie Village also buys what you carry. In Hotland, steam vents throw you over the lava, orange lasers hit you if you stand still and blue ones if you move, and in a shooting puzzle the arrows push the dark blocks and Z shoots. On the second floor, press the switches as the top conveyor belt carries you past them, and in Mettaton's news report, touch each bomb and defuse it while it's in the DEFUSE ZONE. When Muffet turns your SOUL purple, it keeps to three strings: **Up** and **Down** hop between them. In the tile puzzle, red tiles are walls, yellow ones shock you back, orange makes you smell of oranges (and blue water's piranhas bite then), and purple soap slides you on and smells of lemons. When your SOUL turns yellow, **OK** shoots. At the MTT Resort, the desk rents you a room and the hotel's doors want room service. On the CORE's laser bridge, walk while the power is off and freeze when it comes back on. Mettaton EX's show is won on its ratings: Boast, Pose, a Heel Turn and his brand's food raise them. In New Home, find both keys to unchain the stairs. Asgore breaks MERCY, so you'll have to fight him. Against Omega Flowey, dodge, and reach the ACT button that comes down among a SOUL's attack; once all six SOULs are on your side, reach FIGHT. The game ends at the neutral ending.
+
+<br clear="right">
+
 ## Portal Returns
 
 <img src="media/portal.gif" width="320" align="right" alt="Portal Returns: falling through a portal and flying across a test chamber">

diff --git a/games/undertale/.gitignore b/games/undertale/.gitignore
@@ -0,0 +1,2 @@
+output/
+build/
diff --git a/games/undertale/Makefile b/games/undertale/Makefile
@@ -0,0 +1,79 @@
+# Undertale on the NumWorks calculator.
+#   make            output/undertale.nwa
+#   make check      link it like the calculator does (output/undertale.bin)
+#   make host       build/play: the game on a computer, for tests and screenshots
+#   make test       the game played on a computer (tests/check.py, with the sanitizers)
+#   make data       src/data.bin and src/data.h from the public material tools/fetch.py gathers
+#                   (REF=its folder, Python + Pillow + NumPy)
+Q ?= @
+# the ARM toolchain on the PATH, or in TOOLCHAIN=<its bin folder>
+TOOLCHAIN ?=
+BIN = $(if $(TOOLCHAIN),$(TOOLCHAIN)/)
+CC = $(BIN)arm-none-eabi-gcc
+NWLINK ?= npx --yes -- nwlink@1.0.0
+STRIP ?= $(BIN)arm-none-eabi-strip --strip-unneeded
+SIZE = $(BIN)arm-none-eabi-size
+BUILD_DIR = output
+REF ?=
+
+# the drawing runs every frame at -O2; the rest is small at -Os
+HOT = src/gfx.c src/bullets.c
+COLD = $(filter-out $(HOT) src/plat_host.c src/plat_eadk.c,$(wildcard src/*.c)) src/lzma/LzmaDec.c
+COLD_OBJ = $(patsubst src/%.c,$(BUILD_DIR)/cold/%.o,$(COLD))
+HEADERS = $(wildcard src/*.h) ../common/epsilon_app.h ../common/epsilon_files.h
+
+CFLAGS = -std=gnu11 $(shell $(NWLINK) eadk-cflags-device) -DDATA_BIN='"src/data.bin"'
+CFLAGS += -O2 -Wall -Wextra -Wno-unused-parameter -fno-math-errno -fsingle-precision-constant
+CFLAGS += -fno-tree-loop-distribute-patterns -flto -fno-fat-lto-objects -fwhole-program -fvisibility=internal
+CFLAGS += -ffunction-sections -fdata-sections $(EXTRA)
+LDFLAGS = -Wl,--relocatable -nostartfiles --specs=nano.specs
+LDFLAGS += -Wl,-e,main -Wl,-u,eadk_app_name -Wl,-u,eadk_app_icon -Wl,-u,eadk_api_level
+LDFLAGS += -Wl,--gc-sections -flinker-output=nolto-rel
+
+.PHONY: build check clean host test data
+build: $(BUILD_DIR)/undertale.nwa
+
+# apps get 133652 bytes of RAM on custom builds of Epsilon 25.2 (148928 or more on official ones)
+check: $(BUILD_DIR)/undertale.nwa
+	$(Q) $(NWLINK) nwa-bin --ram-length 133652 $< $(BUILD_DIR)/undertale.bin
+	@echo "BIN     $(BUILD_DIR)/undertale.bin: $$(wc -c < $(BUILD_DIR)/undertale.bin) bytes"
+
+$(BUILD_DIR)/data.o: src/data.S src/data.bin | $(BUILD_DIR)
+	$(Q) $(CC) -c -DDATA_BIN='"src/data.bin"' $< -o $@
+
+$(BUILD_DIR)/cold/%.o: src/%.c $(HEADERS)
+	@mkdir -p $(@D)
+	$(Q) $(CC) $(CFLAGS) -Os -c $< -o $@
+
+$(BUILD_DIR)/undertale.nwa: $(HOT) src/plat_eadk.c $(COLD_OBJ) $(HEADERS) $(BUILD_DIR)/icon.o $(BUILD_DIR)/data.o
+	@echo "LD      $@"
+	$(Q) $(CC) $(CFLAGS) $(LDFLAGS) $(HOT) src/plat_eadk.c $(COLD_OBJ) $(BUILD_DIR)/data.o $(BUILD_DIR)/icon.o -lm -o $@
+	$(Q) $(STRIP) $@
+	$(Q) $(SIZE) $@
+
+$(BUILD_DIR)/icon.o: src/icon.png | $(BUILD_DIR)
+	@echo "ICON    $<"
+	$(Q) $(NWLINK) png-icon-o $< $@
+
+$(BUILD_DIR):
+	$(Q) mkdir -p $@
+
+# on a computer: tests/play.c plays it with scripted keys and saves screenshots
+HOST_FLAGS = -std=gnu11 -O1 -g -Wall -Wextra -Wno-unused-parameter -Isrc -DHOST -DDATA_BIN='"src/data.bin"'
+HOST_SAN = -fsanitize=address,undefined -fno-sanitize-recover=undefined
+HOST_SRC = $(filter-out src/plat_eadk.c src/main.c,$(wildcard src/*.c)) src/lzma/LzmaDec.c
+host: build/play
+build/play: $(HOST_SRC) tests/play.c src/data.S src/data.bin $(HEADERS)
+	$(Q) mkdir -p $(dir $@)
+	$(Q) cc $(HOST_FLAGS) $(HOST_SAN) $(HOST_SRC) tests/play.c src/data.S -lm -o $@
+	@echo "HOST    $@"
+
+test: build/play
+	python3 tests/check.py build/play
+
+data:
+	@test -n "$(REF)" || { echo "make data REF=<the folder tools/fetch.py filled>"; exit 1; }
+	python3 -I tools/pack.py $(REF)
+
+clean:
+	$(Q) rm -rf $(BUILD_DIR) build