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Chris Frantz edited this page Jan 31, 2017 · 2 revisions

Understanding World Numbers

In Zelda2, overworld map tranfer table can place link into sideview areas which are in different "worlds" (e.g. palaces and towns).

Destination World ROM Bank
Western Hyrule Side-Scroll Areas 0 1
Death Mountain Side-Scroll Areas 1 1
Eastern Hyrule Side-Scroll Areas 0 2
Maze IslandSide-Scroll Areas 1 2
Western Hyrule Towns 4 3
Eastern Hyrule Towns 10 3
Palace 1 12 4
Palace 2 12 4
Palace 5 14 4
Palace 3 16 4
Palace 4 17 4
Palace 6 18 4
Palace 7 22 5

At first glance, this list of words seems haphazard, however there is structure when the world number is examined as a bitfield.

The meaning of the world number

Let n be the normalized world number and o be the overworld number (where 0 is West Hyrule, 1 is Death Mountain/Maze Island, and 2 is East Hyrule). The destination world in the tranfer table can be thought of as a byte arranged as nnnnnnoo.

Destination Normalized World Overworld
Western Hyrule Side-Scroll Areas 0 0
Death Mountain Side-Scroll Areas 0 1
Eastern Hyrule Side-Scroll Areas 0 2
Maze IslandSide-Scroll Areas 0 1 *
Western Hyrule Towns 1 0
Eastern Hyrule Towns 2 2
Palace 1 3 0
Palace 2 3 0
Palace 5 3 2
Palace 3 4 0
Palace 4 4 1 *
Palace 6 4 2
Palace 7 5 2

* In the FDS version of the game, Death Mountain and Maze Island shared a bank. In the US version of the game, they are in separate banks, but they are the exact same maps. There is a special case in the code to handle switching to Death Moutain and Maze Island.

Converting World number to Bank Number

In order to convert from the world number to a bank number, the game first converts the world number to the Overworld Number and Normalized World Number and stores these values at $706 and $707 respectively. It then calls a subroutine at $cd40 (in bank 7) to determine the ROM bank number.

cd40: ac0707        LDY $0707    # Normalized world number into Y
cd43: b9b7c4        LDA $c4b7,Y  # Load bank number from table
                                 # cb47: 01 03 03 04 04 05 00 00 00 ...

cd46: c901          CMP #$01     # Bank 1 represents overworlds
cd48: d00f          BNE $0f      # if not overworld, then done
                                 # Determine if we really want bank 1 or 2

cd4a: ac0607        LDY $0706    # Get the overworld index
cd4d: f00a          BEQ $0a      # If index==0 then WestHy (bank1).  done.
cd4f: 88            DEY          # Decrement index
cd50: d005          BNE $05      # If index!=0 then EastHy (bank2).
cd52: ac0a07        LDY $070a    # index was 1, so its either DM or MZ.
                                 # Use the previous overworld ($070a) to
                                 # determin if bank 1 or bank 2.

cd55: f002          BEQ $02      # If Y==0, then bank 1.
cd57: a902          LDA #$02     # Load bank 2
cd59: 8d6907        STA $0769    # Store desired bank number

TL;DR

The world number is a bitfield which combines the overworld number and the normalized world number. Apart from special handling for Death Moutain and Maze island, the ROM bank to load is stored in a table in bank 7 at $cb47.

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