I think there are a couple obstacles with this. The big one is that the part of the BIOS that handles CD access is on the Sub CPU, which has no access to any hardware on the cartridge slot. Games have too much low-level control over the comm registers and the main CPU to get away with doing a remote procedure call of sorts to the main CPU for SD card access.KanedaFr wrote:Since Bourricot made me ask if the hack I planned was the right one, I thought about the BIOS hack.
We already talked about it on this thread but it was about modding the BIOS, hack nterface, etc....
What about the all-in-a-cart way ?
since we know (look at Everdrive) it's possible to load a bios from the cart.
would it be possible to get a cart with BIOS, SD reader (using /TIME) and audio player (SR/SL) included ?
It would be awesome : no hardware to mod, only a plug and play hack!
I'm not enough aware of the way data is loaded from MCD which could be a problem (DMA?)
Any feeback on the do-ability of this idea ?
The other issue is indeed DMA. In part for performance reasons and in part because at least in some games, the hardware isn't completely abstracted by the BIOS. I was never an expert on the CD hardware and it's been a long time since I looked at it, but I believe while CDC access is completely abstracted behind the BIOS, the same is not true for the CDD.
I think the best options are either to go with the existing route of having a device that replaces the hardware on the other side of the ribbon cable or to replace the Sega CD completely with an FPGA hanging off the expansion slot. The first option has to deal with the annoyance of 3 different drive designs (Model 1, Model 2 Sony, Model 2 Funai), but is probably the least amount of work overall. The second gives some additional flexibility for enhancements.