How to Fight a Vanleadr

Introduction

The role-playing game Phantasy Star II for Sega Genesis has over 100 kinds of monsters. Many of them look just like other monsters with their colors changed. This method of designing monsters is called a palette swap. It saves time and trouble for the monster designer.

For example, the monsters FIRE ANT, ARMORANT, and PINCHANT look alike, except that the FIRE ANT is red, the ARMORANT is green, and the PINCHANT is blue:

In one set of robot monsters, the VAN, VANLEADR, and AEROTANK, the second monster VANLEADR does not appear in the game. Here is how you can fight one anyway.

Hacking the ROM Image

To encounter a VANLEADR you must modify the game program. If you use the game's original cartridge, you can use a Game Genie to effectively alter the game's memory. If you use a Genesis/Megadrive emulator to play the game, you can also alter the game's memory image (ROM, read-only memory) with a file editor.

Genesis ROM images may have either of two formats: SMD (Super Magic Drive, a patent copying device) format and BIN (binary) format. The SMD groups a sector's even-numbered bytes and the odd-numbered bytes separately, which is not convenient for editing. Instead, use an image in BIN format, in which the memory bytes appear in their normal sequence.

The part of the ROM image to be modified is the table of attack parties.

In the following text, hexadecimal values are prefixed with a dollarsign ($).

The Attack Party Table

The table of attack parties (monster groups) starts at byte $2372a of the ROM. Each group takes up 5 bytes:

Address	Byte 1	Byte 2	Byte 3	Byte 4	Byte 5
$2372a	$01	$03	$00	$00	$03
$2372f	$02	$03	$00	$00	$03
$23734	$03	$02	$00	$00	$02
$23739	$04	$03	$00	$00	$03
$2373e	$05	$04	$00	$00	$04
…

Byte 1 specifies the kind of monster, byte 2 tells how many of them there are, byte 3 specifies a second kind of monster if any, byte 4 tells how many of those there are, and byte 5 tells the total number of monsters, for convenience in displaying them on the screen. For example, in the first row shown above, $01 is the code for a FIRE ANT, so the group consists of 3 FIRE ANTs and no other monsters.

In this entry,

Address	Byte 1	Byte 2	Byte 3	Byte 4	Byte 5
$2374d	$08	$01	$13	$02	$03

monster $08 is a VORTEX and monster $13 is a FROGGY. This group has 1 VORTEX and 2 FROGGYs, making 3 monsters in all.

Modifying an Attack Party

To change the monsters in an attack party, just use a binary file editor to change the appropriate bytes. For example, this is the attack party of 3 FIRE ANTs, which is found near the city of Paseo:

$2372a

$01

$03

$00

$03

The Table of Monsters shows that the code for VANLEADR is $59. Change three bytes of this record as shown below, and you replace 3 FIRE ANTs with 1 VANLEADR:

$2372a

$59

$01

$00

$01

Now if you run your emulator on the modified ROM image and have Rolf and Nei walk around the territory surrounding Paseo, you will soon encounter a VANLEADR. Of course, you may not survive the encounter.

Using a Game Genie

You can effectively modify the game's ROM with a Game Genie, or use a Game Genie code on an emulator that allows it. A single Game Genie code modifies two successive bytes of memory, so one code will suffice to replace 3 FIRE ANTs with 1 VANLEADR as described above, leaving the 5th byte of the record unchanged. That code is AE5T—FWVL. (Descriptions of how Genesis Game Genie codes are constructed are available elsewhere.)

One of my correspondents could not get this code to work. He may have been using a slightly different version of the ROM image.

Game Genie is a registered trademark of Lewis Galoob Toys, Inc.

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Col. George Sicherman [ HOME | MAIL ]