First, you need to physically expand the rom.
Duplicate the first 128k after the NES header, then change the header to indicate that the PRG size is now $10 instead of $08.

Then make these changes to allow the game to use the expansion:


Paste this code into FF64 (3FF74)
	A9 00     LDA #$00
	8D FE 7F  STA $7FFE
	8D FF 7F  STA $7FFF
	4C 70 FF  JMP $FF70

Modify reset vector at FFFC (4000C)
 replace this:
  70 FF
 with this:
  64 FF


Paste this code into FF4C (3FF5C)
	4A        LSR
	4A        LSR
	48        PHA
	29 08     AND #$08
	8D FE 7F  STA $7FFE
	68        PLA
	29 07     AND #$07
	60        RTS

Go to CBA6 (3CBB6)
 Replce this:
  4A        LSR
  4A        LSR
  29 07     AND #$07
 With this:
  20 4C FF  JSR $FF4C
  EA        NOP

Paste this code into FF58 (3FF68)
	8D 69 07  STA $0769 
	48        PHA
	AD FE 7F  LDA $7FFE 
	8D FF 7F  STA $7FFF 
	68        PLA
	60        RTS

Go to CD59 (3CD69)
 Replace this:
  8D 69 07  STA $0769
 with this:
  20 58 FF  JSR $FF58

Replace the bankswitching code at FFC5 (3FFD5) with this:
	A9 00     LDA #$00
	F0 06     BEQ $FFCF
	AD 69 07  LDA $0769
	0D FF 7F  ORA $7FFF
	8D 00 E0  STA $E000
	4A        LSR
	8D 00 E0  STA $E000
	4A        LSR
	8D 00 E0  STA $E000
	4A        LSR
	8D 00 E0  STA $E000
	4A        LSR
	8D 00 E0  STA $E000
	60        RTS

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Okay, here's an ASM hack that allows you to expand Zelda 2.
Read the text file, zelda2_expand.txt.

Also read the overworld locations table format text file, zelda2_overwold.txt, so you know what the hell I mean when I say "W" byte.

The ASM hack allows you to specify that an exit to an overworld/town/palace is on the second 128k.
You do that by setting bit $20 of the "W" byte.
LIMITATIONS:
When you leave a palace or town located on the second 128k, you will go back to an overworld on the second 128k, even if the teleporter that took you there was on the first 128k.
You also need to specify the $20 bit for ALL towns and palaces you want to place on the second 128k, otherwise they will switch back to the first 128k.
When you continue or restore from a save, you start on that half of the rom.

Example!

An example of using the expanded rom:
First expand the rom and apply the asm hacks
Then copy 4000 bytes at 8010 [file address] to 24010.

Let's change that forest with the 50 point P bag to another overworld.

Hack these bytes:
4631: AA
46AF: 00
46EE: 20

After doing all the hacking, that forest now takes you to a messed-up version of the second overworld (because we copied that rom bank from 8010 to 24010).

Possible hack ideas: (use your imagination!)
Double the width of the overworld maps by having caves link the two halves together.
Make huge palaces, now that you have twice as much room as before.
Have a light world/dark world. Note that palaces share RAM areas to store their state, so you could put the key in one world, and the locked door in the other.
Have a Palace that you enter in such a way that is hard to exit from, then when you clear it, you emerge on the second world.

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Posted by Koolboyman

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Zelda: Parallel worlds

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