Game Genie

Overview

The Game Genie is a cheat cartridge made for the Game Gear. It sits between the cartridge and the system and offers the user the ability to apply ROM patches.

See also the Pro Action Replay and X Terminator.

User interface

The device has three modes:

• Off: it does nothing, simply passing data to and from the cartridge.
• On: it applies the currently active ROM patches. This mode is selected with a switch.
• Code entry: where the user may input cheat codes which tell the device what patches to apply. This mode is entered when the system starts.

Code format

The device makes use of an obfuscated code format, with either two or three groups of three digits.

 DDA-AAA-RRR

The first two digits give the value to be written. This is not obfuscated.

The next four digits are the address (in the Z80 memory map) to replace with the given value. To decode it:

1. Move the last digit to the start
2. Invert its bits

In C code (assuming the Game Genie code has been parsed to a 36-bit integer variable code):

Do the inverse to encode.

The final three digits are optional, and are often missing from home-made codes. They represent a "cloak" value and a "reference" value. The "cloak" apparently changes the device's behaviour to make it avoid detection by software, but seems always to have the same value (8) in known codes. The "reference" value is a number which specifies the original value at the specified offset. This allows a reference to a mapped page to be used without the device changing the value when another page is mapped to the same slot - but only if the other page has a different value at that offset.

To decode:

1. XOR the first and second numbers with each other. This gives the "cloak" value - which seems to always be 8.
2. Take the first and third digits, and form an 8-bit number.
3. Rotate this number right by 2 bits.
4. XOR it with 0xba to get the "reference" value.

In C code (again assuming the Game Genie code has been parsed to a 36-bit integer variable code):

Do the inverse to encode.

How it works

The device presumably checks the current address lines with the ones specified in the internally stored codes and, when they match, checks the value stored in ROM at that address and patches it if appropriate.

When the code entry button is pressed, it presumably presents its internal ROM after triggering a jump to the right address, which then resets the console's state and displays the code entry screen.

All codes were originally produced by the manufacturer, who probably did it by dumping and disassembling ROMs. It has no "training mode" (it would be hard to make such a thing for ROM hacks).

Additional information

Credits:

Game Genie built by

Richard Aplin

Code by

Jon the Programmer

Managed by

Paul Ranson

Hardware overview

The Game Genie has two glop-top ICs, one is a ROM containing the Z80 program and the other is an ASIC that implements the cheat functionality. The front pushbutton breaks out of game execution to run the Game Genie software, and the back pushbutton enables or disables codes (and the green status LED) once codes have been entered.

Jumper JP1 is normally shorted to +5V to enable display of the Galoob logo on the Game Genie title screen. If the wire link is cut, the Galoob logo is absent.

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