SADX/SA1 Hacking/Modding

Oh, ok. I think it came from seeing a petition asking for Heroes and Shadow on Steam, and I forgot there were releases before this (I'm really not a PC person, I prefer consoles). Thanks for the info though!

 

Use the return to previous post icon in the quote below for context. This is related to the "checksum" inside the encrypted DLC data that was preventing us from creating valid downloads for SA1.

PkR messaged me on Discord this morning to ask if I ever looked at this more. I had not. Given that Ghidra supports SH4 now, I decided to give it another go.

In short, 0x2C in the Payload is the checksum and now we can calculate it. The values I got in testing match for the existing DLC that I tested it with. We should have everything we need to create custom/modified DLC that works on actual Dreamcast hardware. I have the modified one that I mentioned previously in this thread re-signed so the game will accept it, I just haven't had a chance to test that yet.

In SA1 US Version 1.005 here is the relevant part of the function responsible for decrypting and loading DLC quests from the VMS:

Code (Text):

1. FUN_8c0357a0(int param_1)
2. {
3.   char cVar1;
4.   uint *vmsDataLocal;
5.   int *piVar3;
6.   uint uVar7, uVar9, uVar12;
7.   int iVar5, iVar8, iVar11;
8.   char *pcVar10;
9.   int *piVar13;
10.  
11.   (*(code *)PTR_FUN_8c035880)(0);
12.   iVar5 = (*(code *)PTR_FUN_8c035884)();
13.   vmsDataLocal = VMS_data;
14.   cVar2 = *(char *)(iVar5 + 10);
15.   DecryptVMS(VMS_data,param_1);
16.   iVar5 = VMS_fileheader;
17.   uVar9 = 0;
18.   iVar8 = 0;
19.   pcVar10 = VMSdata_checksumStart;
20.   if (0 < param_1 + -0x40) {
21.     do {
22.       iVar8 = iVar8 + 1;
23.       cVar1 = *pcVar10;
24.       pcVar10 = pcVar10 + 1;
25.       uVar9 = uVar9 + (int)cVar1;
26.     } while (iVar8 < param_1 + -0x40);
27.   }
28.   if (~uVar9 == *VMS_Checksum) {
29.     //File is valid, continue loading....
30.  

After DecryptVMS() is called, it does a very simple checksum and compares the result to the value at 0x2C in the payload.

Here is a C# implementation of the CRC. It's literally adding up all of the bytes (as chars) and inverting the result.

Code (Text):

1.  
2.        private uint SonicAdvCrc(ref byte[] buf, int start, int end)
3.         {
4.             uint result = 0;
5.             for(int i = start; i < end; i++)
6.             {
7.                 int notByte = buf[i];
8.                 if(notByte >= 128)
9.                 {
10.                     notByte -= 256;
11.                 }
12.                 result = (uint)(result + notByte);
13.             }
14.             return ~result;
15.         }
16.  

The signed portion begins at 0x40 inside the decrypted payload, which is after where that "checksum" is stored so you don't need to worry about pretending it's 0. So you would just call this from 0x40 to the end of the file, but if you're in the actual VMS file it's 0x2C0 from the beginning.

PkR has informed me that they plan to use this information in the future for some kind of tool. If there's interest I can release a stand-alone EXE that calculates the checksum & re-encrypts. Otherwise it sounds like that will be taken care of by more advanced tools later. Just wanted to bump this with the details so it's finally documented how it works.

 

I made this as a proof of concept, but hey let's have a real event like they did back in '99!

 

It's beyond awesome that you managed to make something like this. Great job!

 

Cool. I haven't had a chance to check but is this a different mod from the DLC one or will I get this if I update from the mod loader?

 

The first sentence on the webpage states it's for the dreamcast release.

 

Well there I go making a fool of myself!

 

There are multiple videos up now, but here's one I recorded from actual hardware:


There's a bonus at the end where I show the website running on DC hardware and the download failing to work. Very few websites work now, due to Dreamcast only supporting 128-bit SSL for HTTPS websites.

I wanted to have a video posted yesterday, but the Dreamconn that I ordered back in June (Dreamcast Wavebird basically) finally arrived yesterday. I tried to use the PC software for it to send the file to my VMU directly to the controller over bluetooth, but instead it corrupted my entire VMU multiple times. I ended up using DreamExplorer running on the DC itself to restore a backup directly to my VMU and installing the DLC from there. In the end it worked, and it was neat to try out some new DLC. Great job.

 

Here's a Dreamcast-friendly link to the DLC file that has been tested on hardware: http://sonic1.dreampipe.net/dlc/TIKAL.VMI
If you use DreamPi to connect your Dreamcast to the Internet, you should also be able to download it from the revived SA1 homepage (Thanks to Xiden from DreamPipe for putting it on the page).

 

Now THIS is a Sonic Adventure level mod. Amazing. And with custom objects and a custom boss no less. Like this just looks like an all around great Sonic level.

I haven't seen a Sonic Adventure level mod like this since Dude's Mushroom Hill.

Here is this excellent Hill Top mod for Sonic Adventure. It's a brand new built from the ground up level with custom objects, enemies, and even a custom boss at the end which correct me if I'm wrong but I believe that's a first in SADX modding.

This just looks fun to play.

OL EDIT: Merged 2 video posts together, removed extraneous video, kept text of both posts.

 

Oh whoops, my bad. Mixed up my SA1 modders lol. But ya probably a good point, honestly was just lazy and didn't want to find it

 

Looks awesome! If more stages could be recreated that way...

 

Wait what? People are actually capable of making well designed sonic stages that aren't mile long linear hallways? Impossible...

On a serious note, now someone needs to flesh this out into a full sonic 2 mod :p

 

Sorry if this might be the wrong thread, or if these questions might be obvious, but looking around I couldn't find anything about this specifically.

Recently played through the game again, and to spice things up, started messing around with the character select mod. Together with the physics swap mod and the extra moves mod, it's made for a really fun experience, which makes it all the more frustrating when, for instance, I can play all of Sonic's Final Egg as Amy, but not finish the stage since it ends in a line of rings over a chasm; or go through Windy Valley as Knuckles and be stuck in the tornado section because of the number pads.

So I'm wondering-

1- Is there any mod that changes Sonic level layouts so they can be playable for others? Adding a couple platforms in that section in Final Egg, or replacing number pads with springs, stuff like that.
2- Additionally, is there any mod that makes it so characters other than Sonic and Tails can use the number pads?
3- Finally, is there any mod that makes it so attack effects on things are standardised? For instance, Gamma can't break the objects in Knuckles' Speed Highway that often hide Emerald Shards, and Amy can't beat a Gamma level since the bosses don't register her attacks, only registering Gamma's bullets.

Mostly hoping there's solutions to all this or, failing that, an explanation so I can go "hmm I see" and know why these things would be so difficult to change when it seems to me so much other stuff has been modded already.

 

The randomizer mod does a lot of what you're asking, as it has to accommodate all the layouts for all the characters. It also changes some object behavior to work with other characters, but I don't think the number pads work for all characters, although in those cases there are work arounds.

 

I remember SonicFreak94 posted videos of successfully using order-independent transparency with DX11, but I didn’t see anything official released. Was that rolled into the Dreamcast conversion mod? Transparency issues were the major reason DX was so broken, so it would be awesome if OIT were available similar to what the Dreamcast hardware supported.