Blog:Frogger (Genesis)

CRV (talk) 15:47, 25 April 2013 (CEST) [permalink] [comments]

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The last licensed game for the Sega Genesis in North America. What better way to send off one of the all-time great consoles than with one of the all-time great arcade games?

Okay, so the Genesis didn't exactly go out with a bang here, but it's an exact replica of the arcade version, right? Well, not exactly. I talked to programmer Scott Marshall a while back, and he shared some information about the development.

Matching the Genesis Frogger to the arcade version was a lot of work and a long story, but I really enjoyed it. Here are a few details:

I started off with an arcade emulator of Frogger downloaded from the Internet, with images of the original ROM data. I ran the ROM images through a disassembler, printed them out, studied them, dumped them in hex and created a hex editor, and made videotapes of myself playing this emulator.

Although the Genesis has a complete Z80 system, it seemed impractical to port the arcade's Z80 code to run on the Genesis Z80 because of the specific screen-mapped display functions it used.

I did, however, locate the music and graphics data in the arcade image ROMs. The graphics were converted with a utility I wrote. The music sequence data was loaded directly into the Genesis version, and I wrote a sequencer in Genesis 68000 code to play it.

The game logic was controlled by a new program I wrote in 68000 assembly language. I was able to match the action by studying the video tapes and duplicating the positions, speeds, and sequences of all the sprites. Sometimes I was able to match motions of objects and rows by carefully timing them with a stop watch. Some functions were divined by studying the disassembled Z80 code from the arcade game. The sound of the frog jumping was duplicated by recording on tape the arcade sound, slowing the tape to half speed, figuring out the musical notes, then locating the sound chip register settings that played those notes.

I only had enough skill to play the first five or 10 levels of arcade Frogger, so I had to invent the patterns of the higher levels.

Marshall used the Genecyst emulator to develop the game. He also mentioned a backdoor, but I couldn't get it to work: at the player select screen, hold down C and press right nine times.

So it's not arcade perfect. So no new features were added. But if you want Frogger action on your Genesis/Mega Drive, this is it. At least the music wasn't butchered. (The GBA version on Konami Arcade Advance comes to mind.)

Comments

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andlabs said ...

<comment date="2013-04-25T15:28:15Z" name="andlabs" url="http://segaretro.org/" signature="Andlabs (talk)"> His description of the backdoor is incomplete. Here's the full one; the function that handles it is at $15AA in the ROM. 1) On the Select Number of Players screen, hold C. 2) Press right 9 times. 3) With C held, press A to add one and B to subtract one from a number that will only appear at the top of the screen once you press A or B once. Your range is from 01 to 0B (hex). This number tells you both which level you start at. There appears to be some unused code at $1622 that plays a sound (I guess when you press A or B?). </comment>

andlabs said ...

<comment date="2013-04-25T15:31:00Z" name="andlabs" url="http://segaretro.org/" signature="Andlabs (talk)"> Oops; that unused code just sets the counter to 0B, nothing too special :/ </comment>

CRV said ...

<comment date="2013-04-26T12:42:19Z" name="CRV" url="http://gdri.smspower.org/" signature="CRV (talk)"> There are at least two versions of Frogger. My cartridge copy has an additional Konami copyright and no second score display (that eventually goes away in the video above). </comment> <endFeed />