Basic Lynx ROM hacking questions

[JonteP83]

Hi all,

 

So I'm into simple ROM hacking projects for my own amusement and education and have so far focused on Master System and NES. I use the ConTEXT editor and WLA-DX compiler together with the MAME debugger.

 

Now, I intend to use the same setup for Lynx. After reading up a bit, I understand that the ROM is slightly differently organized from what I'm used to. Basically, it seems that the encrypted loader is the main hurdle to handle. What is the best way to cope with this? First, I was thinking of decrypting the loader, updating the checksum info and encrypt again. But I can't seem to find tools for doing that. Maybe there's an easier solution?

 

/Jonas

[sage]

Maybe there's an easier solution?

 

 

Yes. there is.

This question has been answered before in more detail, search forum.

 

In short, it depends on the ROM you have. Depending on that, there might be no checksumming at all. Canm be checked easilý by changing the last byte of the rom.

 

if there is checksumming, you could reencrypt the loader after patching, which is easy but annoying. Its easier to just replace the loader by a version with disabled checksum.

 

Now it depends, if you have an "old" Rom type or a "new" ROM type, this defines where the "directory" is located. The hacked loader is for the smaller ones, if you have the larger loader you have to copy over the first two diretory entries to a new location. aprt from that youhave to know the ROM layout, as the loader expects to know it. thus you should check the rom size. Means the real ROM size. There are a lot of 128k ROMs which are distributed as 256k images.

 

if you have this information available you just replace the first four-houndred something bytes by the hacked loader.

 

for the byte content, please have a look in the following source code:

http://lynxdev.atari.org/lynxdir_1.7.tgz

 

PS: you could use the very small mini loader, too but then it will skip the title picture.

(http://lynxdev.atari.org/blog/o2lyx_hack.xhtml)

 

For more hacking, you can read:

http://lynxdev.atari.org/blog/chips.xhtml

 

(now with fixed link typos)

Edited October 17, 2015 by sage

[JonteP83]

Thank you for your reply and some very useful links! The easy but annoying way is somehow tempting. But it's good to know that there is the option of replacing with a hacked loader.

 

I'll be sure to read through the forum more carefully!

[sage]

can be easy as

 

dd if=hackedrom_or_header.img of=encryptedrom.img bs=1 count=410 conv=notrunc

 

if its one of the newer roms and the blocksize is the same. just make sure its a rom image, not a lnx file. then you should add skip/seek=64

 

But you want to do complicated thing (ROM hacking), thus you shouldnt expect one-liners.

[sage]

You will anyway have a lot of fun if the ROM files are packed like in lemmings, chips challenge, simis, alpine games etc...

 

but spoil the fun and tell us what you want to archive.