Modifying existing games

[D]

First of all great that there is a special Development Forum here.
I hope I do not insult you guys.

OK for example.
I love Shinobi for the Sega Master System.

How can I modify the game rom.
When I open the rom i see this: ÆÍÔÍ ø
It's programming code.

Is there a way to discipher this.

What I'd like to achieve is for instance. have two ninja's walk at you instead of one.

Are there any tools to achive this?
Any tips.

If this works I can start with other games as well and make harder (more challenging) games.

[ForteMP3]

This would be more along the lines of Rom Hacking, honestly. And it's not easy to do if you want to make a change that dramatic...

[shiftace]

Yeah romhacking is difficult. If you're lucky, there will be a nice, concise segment somewhere that specifies stage layouts. If you can find and understand that, then you can start changing things around within the allocated space. You'll have to get real comfortable with a hex editor and/or disassembler to get anywhere, though.

There's at least one crazy Japanese guy who's been hacking at Recca, he had some patch up for download that seemed to let you do Score Attack and Zanki Attack (a hidden mode) on any stage in the game. And he converted the score display to hex, because a limit of 9,999,999 is inadequate for hard mode. He had one other screenshot that showed the big green boss from the ending firing a pattern from the brown & grey one in the boss rush (#6).

[raiden]

Yeah romhacking is difficult.

not necessarily, depending on what you aim for. Look at the way some of these cheat cartridges work: encountering an unknown game, you can let them compare game states and look for changes between them. If, for example, you see a number 03 when the game starts, and after dying that byte changes to 02, chances are this is where lives are stored.

Of course, if you are talking about serious romhacking, implying new program code that wasn´t there before, things get much more complicated. Not only would you have to learn assembly code of the main processor the game is running on, you would also have to deal with the specific hardware, the way graphic chips are adressed, data is stored and so on.

[shiftace]

Just for the sake of quibbling, the ROM never changes, so comparing game states isn't romhacking. I agree, though, that some changes may be very easy to make.

The Guardian Legend ROM stores game text in plain ASCII alphabet.

[SheSaidDutch]

There's at least one crazy Japanese guy who's been hacking at Recca, he had some patch up for download that seemed to let you do Score Attack and Zanki Attack (a hidden mode) on any stage in the game. And he converted the score display to hex, because a limit of 9,999,999 is inadequate for hard mode. He had one other screenshot that showed the big green boss from the ending firing a pattern from the brown & grey one in the boss rush (#6).

Have you got the link by any chance?

[shiftace]

Well, the guy's homepapge is http://www.kitanaka.net/~tiff/, and if you look at URLs in your browser status bar you can find a couple pages about Recca. All sorts of weird stuff there, looks like a disassembler in Perl, the scoring in hex patch, and a whole bunch of ROM notes (all in Japanese, of course). His more "interesting" IPS patches are no longer posted, but you might locate them by creative use of a particular search engine.

Also, FWIW, I ran a couple of those pages through Babelfish a long time ago, and the hex patch makes material changes to the scoring system. Specifically, the game normally gives you 5 points/frame if you hold on to a fully charged bomb, but with the hex patch applied it gives you 8. This would be important if anyone wanted to have some sort of scoring contest.