SEGA CD Mode 1 Player

There's been a lot of talk since the release of Sonic 1 with Redbook Audio, mainly people asking for code. I can see why they might not want to post their code since it's probably hacked into Sonic 1 pretty bad. However, there still exists a need to show how to use Mode 1. To that end, I did a small demo - this is a tiny CD player done using Mode 1 of the SEGA CD, and includes full source. From this, you should get a good idea of how Mode 1 works. If not, ask questions in the thread here and I'll answer them. THIS is the place for Mode 1 questions, not the Sonic1Mode1 thread.

The project was compiled with my gcc toolchain, but it wouldn't take much work to convert the code to whatever you normally use. The player goes beyond what you would use in a game, but it's an example.

Controls:
A - play from current track to end of disc
B - play current track once
C - stop playing
X - show current track info
Y - play current track repeatedly
Z - pause/resume
START - check disc

Notes: The TOC is invalid until you check the disc. The CDBIOS doesn't initialize its TOC until you call DRVINIT (which is what my check disc routine does). So when you first start the player, the status may show wildly wrong disc info (first and last tracks being over 99, e.g.). If the tray/lid is open, it will show that. Insert a disc and press START - you should get proper disc info after the status is done showing "Reading TOC" and goes to "PAUSED". The current track may not be the same as the playing track - you can start a track playing and then change the "current" track to something else. Pressing X to see the track info shows the info for the current track displayed on the screen, not the track playing. The track info consists of the track time and track type; the track type is pretty straightforward - it's either a DATA track (track 1 of mixed mode discs), or a CDDA track for audio tracks. The track time shows the BCD minutes/seconds/frames/track number as MMSSFFTN. It shows where the track starts, while the track number is the same as the track number you asked for info on, or 00 is an error occured.

While this example is a CD audio player, I could have easily also read data discs, showing sector data. I did the audio player since that's mainly what people wanted - to see how to play CDDA from a game in rom. The Sub-CPU program is really simple - wait for commands from the Main-CPU, then do them. The commands all pertain to handling the CD audio, but they could as easily use the ASIC, use the PCM chip, read data from sectors, etc. When I start doing some ASIC tests, this will be the test bed to start from as it's easier for me to write the test file to SD card and run in Mode 1 than to keep burning CDRs.

I've tested this on a Model 2 Genesis + Model 2 CD and on a CDX. Let me know if you have any trouble on other models, but it SHOULD work on anything.

mode1player.zip

Update - 20111219: Should work WonderMega and X'Eye and might work on LaserActive; also inits drive on startup if a disc is in place from the start.
mode1player2.zip

Update - 20111228: It's been tried on the LaserActive and doesn't work. Still looking into the issue there, but it's currently the only system known not to work.

Update - 20120105: Now works with LaserActive.

Mode1Example-20120104.zip

 

You don't. This is so new, nobody supports it. Maybe with an example out, emulation authors might add it to their next update.

 

Thanks for the info on Kega - good to see at least ONE emulator supports it. Of COURSE it would be Snake's.

EDIT: Just set up 3.63x for linux - my demo works great! Well, now I have an emu I can test stuff like this on besides real hardware. I normally use Gens_GS, but it doesn't seem to support Mode 1.

 

What model/region is it? Need a little more info to go on.

Does it give an error message? How is it not working?

 

I've tested with a US Model 2 and a CDX. If it does everything but play audio, it's finding everything it needs and initializing okay. You did push START before trying to play tracks, right? As I mentioned in the OP, when the CD first starts up, it usually has a bad TOC, and checking the disc reads the TOC so it can actually play the tracks. Also, don't try playing data tracks (track 1 on mixed mode discs) - I'm not sure what it would do.

EDIT: When you try to play a track, it should briefly (sometimes too brief to see) say "NOT READY", then it should say "SEEKING" and then finally say "PLAYING".

 

No problem - did it myself the first time or two.

In any case, I've experimented and found that this is a problem when you start with a disc in the CD. If you start without a disc, open the tray, and insert a disc, it automatically reads the TOC. You only have to do the check the first time if you started with a disc in place. So I can improve the code by making the CD init look if the tray is open or there's no disc. If there IS a disc to start, do the check automatically, and not worry about it when you change the CD.

But this is part of why I posted this app the way it is - it gives us a look inside how the CD operates at a level we don't normally see, and we learn a few things as a result. So thank you for checking this out the way you did. Thanks to anyone else who has tried this, too.

 

We just talked about that - it's from the fact that when you start the CD with a disc in place, the TOC is not valid. You have to do a DRVINIT to make it read the TOC. That's ONLY when you start the CD with a disc in place. Press START to make my code do the DRVINIT. Note that you don't have to do that later if you SWITCH discs as the CD will automatically read the TOC when you switch. It's just not automatic on startup. It's not a bug in my example, it's a "bug" in the CD BIOS. Now, knowing this bug, you can work around it by adding DRVINIT to the startup IF there is a disc already in place. You can't ALWAYS do the DRVINIT because doing so without a disc in place would cause the BIOS to go into the wrong state.

This is example code designed for probing the inner workings of the CD, so there's many safeties not observed that I would add if I were to use this in a game. For example, I'd check if the disc was still in before playing/pausing/etc. I'd do the automatic DRVINIT on startup if the disc were already inserted. I would check that the track is CDDA before playing. Things like that.