Slot 2 Mapping Mapped Code

Missing Flywheel Bug

This game contains a bug: the bottom flywheel in Gimmick Mountain Zone, Act 1, occasionally seems to stop working. More accurately, the "active" part of the flywheel (the yellow object in the centre) completely disappears (the grey rotating disk is simply part of the level background).

Bottom flywheel presentBottom flywheel missing
(MEKA savestate)(MEKA savestate)

Investigating this bug reveals that the presence of the yellow object depends on the byte at address $d700 being equal to $28.

The byte has the value $28 at the start of the level, but while the player is at the top of the room containing the flywheel, it cycles through the values $28->$fe->$ff->$00->$28. If the player falls back to the bottom of the room, the cycle is interrupted - if this happens while the byte contains $fe, the value $28 will not be restored and the yellow object will be missing.

ROM Version Differences

The main differences between the v0 and v1 ROMs are:

  1. Internally, v0 is Ver1.00 1992/09/05 and v1 is Ver2.20 1992/12/09
  2. The very top of Sky High Zone, Act 2, is passable in v0 but solid in v1. The solid "ceiling" in v1 makes it easier to control the hang glider and thus easier to reach the Chaos Emerald.
  3. Both versions suffer from the "missing flywheel" bug. In v0, when this occurs there is no alternative route; the player is forced to wait until the level timer expires. v1 contains a workaround for this bug: a spring is added to the floor allowing the player to bypass the flywheel and continue the level.
    Bottom flywheel missing [v0]Bottom flywheel missing [v1]
    (MEKA savestate)(MEKA savestate)
  4. v1 contains a fix for another (minor) bug in v0: if the player's invincibility item expires shortly after spinning the Bonus Panel, the level's background music may continue to play as the score is tallied.

There are a number of other minor differences between the ROMs.


Researched by Paul Baker

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