Metroid Database.com

                         METROID MAP DATA FORMAT v1.1

                    The death of the "Secret Worlds" myth

                           by Kent Hansen (SnowBro)
                             <kentmhan@online.no>


        Most fans of this timeless NES classic have probably heard
        of the so-called "Secret Worlds" of Metroid. For a long time,
        I maintained a site concerning these strange areas. It was my
        hope that they weren't just a glitch, but actually put there
        on purpose, as an added bonus to those who had played the game
        to death (and there are a lot of us). So I spent many hours
        creating maps and writing detailed descriptions on how to
        access every one of these parts of the game. Recently, however,
        I've come to the (sad) conclusion that the areas are simply bugs 
        in the game engine. To understand what really happens when you
        access one of these areas, one has to know about the format of
        the level data.
                Each of the main parts of Metroid: Brinstar, Norfair,
        Tourian, Kraid's Hideout and Ridley's Hideout, have an AREA MAP.
        (Actually, they are all part of ONE large map.) Each location in
        this map contains a value which holds the room number to display
        at that location in the area. OK, let's take a concrete example.
        Here's what the map data for the first horizontal corridor in
        the game (where you start out) looks like:

        08 17 09 14 13
            |  |
            |  |
            |  +------ This is where you get beamed down.
            +--------- This is where the Maru Mari (crystal ball) is.

TIP:    I suggest downloading my Metroid level editor (see link at the
        end of this file), MetEdit, to play around with this. It displays
        the room number of each map location, so you can see for yourself
        what I mean, if the info above is not clear. Also, it has a map
        viewer, which lets you view the entire game map.

        The actual ROOM DATA (the objects and enemies in the rooms) for
        these map locations is defined elsewhere in the ROM. (I won't get
        into the format of that data, since it's quite complex and you
        don't really need to know it, unless you're planning to make your
        own level editor for Metroid.)
                What does this mean, then? Well, it means that the game
        can use the same rooms more than once; it simply has to reference
        the appropriate room number in the map data. For example, the
        following sequence of values make up a long horizontal corridor:

        14 14 14 14 14

TIP:    MetEdit has the ability to change which room number is contained
        in each map location, so I suggest you change the room numbers in
        the start area of the game, save the changes and run the ROM in
        an emulator to see the full effect.

        Next follows an explanation of how the game code uses the map
        data when you're playing the game.
               At all times, when you're playing Metroid, there is a
        value in NES memory which holds the offset (pointer) into the
        area map. When you go one room left, the pointer is decreased
        by one. When you go one room right, the pointer is increased
        by one. When you go one room up, the pointer is decreased by
        32 (32 is the width of the map). When you go one room down,
        the pointer is increased by 32. This way the game code can
        effectively fetch the room number from the current area map
        position and set up the room you've entered. For a graphical
        representation of what I just explained, check out the map
        in the upper left corner of Zelda; it's very similar to how
        the Metroid area maps look like in memory.

        OK, now for the "hidden area" part. You probably know about the
        so-called "wall-door" trick; the method that must be employed in
        order to access these areas. (Actually, you don't have to anymore,
        because there are Game Genie codes which enable you fall right
        through the floor and jump through walls - see end of this
        document.) The game programmers never intended this to be
        possible.
                So what happens when you go up or down, when you're not
        really supposed to be able to? Well, the map pointer will be
        decreased/increased as usual. But the data it points to will
        NO LONGER BE VALID, because the data it accesses isn't part of
        the area's map! This won't crash the game though; it will
        continue to treat the data as valid. So there you have your
        "secret world": a mix of invalid map locations containing
        who-knows-what.
                But wait: the map data accessed in such cases, is
        actually technically valid; it IS part of the game map. The
        problem is that the data belongs to a different area. You've
        probably heard of the "huge secret world" above Ridley's
        Hideout. Well, what really happens when you enter that area,
        is that you're accessing the map data from Norfair, since that
        is the data contained in memory directly above the map for
        Ridley's Hideout. I know that this may sound strange, but to
        make things clearer, let me explain what happens when you take
        an elevator from one part of the game to the other.
                The elevators are actually just excuses for the game
        code to bank-switch. As you probably know, the NES has a pretty
        limited amount of memory; only 32K of program ROM space. So
        when you take an elevator, the game uses this opportunity to
        switch to another ROM bank, containing the music, graphics
        data (since Metroid has no VROM), pointer tables, room data
        and other things specific to that area of the game. This
        explains why the "secret world" above Ridley's Hideout uses
        map data from Norfair, yet the rooms look just like they are
        part of Ridley's Hideout: the game code hasn't bank-switched.
        It will use the map data from Norfair, but the ROOM DATA and
        graphics from Ridley's Hideout!
        
TIP:    MetEdit doesn't require you to go via the elevators to change
        the graphics and room data; every time you enter a new room,
        the editor checks which area that position in the map belongs
        to, and sets the pointers to the data accordingly. This makes
        it easy to test what I just explained above:
                From the elevator in Ridley's Hideout, go down one
        room, then go to the far right, until you reach the top of a
        long vertical shaft. Above this point, one of the "secret
        worlds" is located. But by going up a couple of times you see
        that, WOW, this isn't a hidden zone; it's Norfair! Press
        spacebar to enter the map viewer, then you can see where
        you're actually at. Does it look like a secret world to you?

        And that's it. This document has probably crushed the hopeful
        dreams of quite a few avid Metroid gamers, but on the other
        hand; at least you know the truth now.      

        Send questions, complaints or suggestions to kentmhan@online.no.

        Download MetEdit from: http://www.classicgaming.com/mdb

        Some interesting Game Genie codes for Metroid:

        SUPEAI - descend through floor a little each time you shoot up.
        TTXTTT - walk left through walls.
        EEXTTY - jump through ceilings.
        KAPGEI - jump in mid-air.
        SSAEAI - travel freely through walls when you're a ball.
        SSXAEI - get stuck in walls (it CAN be useful).
        KUSTIN - standby if you get stuck in a wall.

        Thanks to Dave Stewart & Jeffrey Toyes for these codes.

        (c) Kent Hansen 1998

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