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Proto:Super Mario World 2: Yoshi's Island

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This page details one or more prototype versions of Super Mario World 2: Yoshi's Island.

Several prototypes of Super Mario World 2: Yoshi's Island were leaked alongside source code on July 24, 2020. The same leak also included an unreleased game called Super Donkey, which featured various elements that ended up later being used in Yoshi's Island.

To do:
There are 4 files in the gigaleak, labeled ys_rom_0, ys_rom_1, ys_rom_2, and ys_rom_3, located in both the Ver0\sfc folder as well as in Ver2\sfc. Combining these files will yield unique, functioning ROMs, but it is undetermined if they predate ys_main_db.


Dated December 6, 1994 and rather different from the final game, including the presence of Super Mario World music and sound effects as placeholders.
Dated December 19, 1994. Largely the same as ys_romX_0, but there are some new added elements, such as the final's music and sound effects.

Other Builds

A few other builds with minor differences were also present in the leak. Most of these require changes to the internal ROM header in order to be emulated correctly. To fix a header, set the byte at 0x7FBD to 05, and set the bytes starting at 0x7FD5 to 20 15 0B 00 00. Build dates can sometimes be found at 0x7FA0 in the HH-MM-DD-hh-mm format.


Dated December 19, 1994. Mostly the same as ys_rom_0_D.bin but with a few minor code differences, including a slight change to how the aiming cursor moves when you first begin to throw an egg: Specifically, when charging up a throw, the cursor stays fixed in place instead of starting at a low angle and moving upward.


Dated June 26, 1995. A near-final build with a few minor unknown code differences. In addition, the final game's debug features are enabled by default.

This build has no internal ROM header at all and requires a fix.


Identical to the final build, but with some additional debugging features accessible through Controller 2 during gameplay:

  • Pressing A/B/X/Y/Select toggles graphic layers on/off.
  • Pressing L toggles the background color gradient on/off.
  • When the background gradient is disabled, pressing R cycles through a few solid background colors.

The first time you start a level, all graphic layers are disabled by default for some reason, so you'll need to use this feature to actually make the game playable.


Same as ys_rom_16_0.bin, except with the final game's debug features also enabled. Requires a header fix.


Combining the 4 512kb bin files and patching the header yields working ROMs. This applies to both the ones found in Ver0\sfc, as well as Ver2\sfc. Differences are present when compared to other ROMs in hex editors. The ROM from Ver0 is also dated June 26, 1995, strangely, whereas the one from Ver2 has a date of Oct 2, 1995.


Included in the leak are ROMs that appear to contain editors allowing for map and level creation and play testing. In order to run these, an emulator with SFX-DOS support is required, such as this branch of bsnes-plus. The mouse should be configured to be in controller Slot 2. Most of these tools require a header fix, as described above. Set the byte at 0x7FBD to 07 (1Mbit of Expansion RAM) instead of 05 to fix graphical corruptions.

To do:
Add more details about the functionality, usage, and differences between the various editor ROMs. In addition, investigate and compare graphics, sprites, etc.


Upon booting this level editor, there are two choices: ENMY DATA SET and BG EDIT. No header fix is required. The first allows for placement of enemies on the map, and the second choice allows for editing of map tiles.

Ystool-Main Menu.png
Ystool-ENMY DATA SET.png
Ystool-BG EDIT.png


Dated December 23, 1994. Seems to be pretty close to ystool.hex in functionality.


Mostly the same as yst_rom.bin and ystool.hex and but with a few minor differences. Setting the byte at 0x7FBD to 03 causes a 3rd option, "UNDO BACK-UP", to appear.

Yst rom 0 - Main Menu.png


Combining the 4 ysm_rom*.bin files (0-3) produces a bootable ROM. This ROM appears to be an editor for the main overworld mode 7 map. This tool allows you to place objects on and around the island, save/load maps, as well as manually walking Yoshi around the island.


  • Set Mouse to Controller Port 2.
  • Click on MAP A thru H to change the map. Only MAPs A thru D seem to have working graphics.
  • Click on UNIT NO. to browse thru the objects (left click a digit to increase it from 0-F, or right click to go backwards), then click the object in the black area to select it, then place it on the map.
  • Click the MOVE button to change modes to move placed objects, CLR to delete.
  • Click the MAP button to walk Yoshi around the Island using the DPad. To get back to the editor, press the right and left mouse button at the same time.
  • Click P.SET to enter MAP POINT DATA SET MODE. This allows one to set the locations of each World on the map, indicated by yellow star sprites.
  • Click MENU to display a list of all World points that have been placed on the maps. From here, click DATA to bring up a settings menu. Clicking MAP in this menu will enter PLAYER MOVE DATA SET MODE, allowing one to define the route of the Worlds. Click TEST to show Yoshi walk to each world, similar to how its done in the final game, when beating a world and progressing to the next.
  • Click FILE to save, load, and manage .mpd and .pdt files on the floppy disk.
Ysm rom-Editor View.png
MENU view FILE view
Ysm rom-Menu View.png
Ysm rom 0-FILE menu.png


Mostly the same as ysm_rom*.bin, but now displays baby Mario upon booting, as opposed to the brown suit man. There are more objects available to place than in ysm_rom*.bin, starting at UNIT NO. 6B, depicted below.

ysm_rom_0.bin ysm_rom*.bin
Ysm rom 0 - Editor View.png
Ysm rom - Editor View.png