I've seen a lot of information on CDTV ROM switchers, but it is still unclear to me what is the actual purpose of it. As I understand it allows some compatibility with A500 games, but this disables CD and other system specific features. There are also mods which offer 3 KS chips option, but why?
I've seen a lot of information on CDTV ROM switchers, but it is still unclear to me what is the actual purpose of it.
The CDTV is essentially an Amiga 500 sans keyboard and floppy but with CD and other additions. It uses an of the shelf Kickstart 1.3 ROM plus an extension ROM providing CD access, audio player and alike. But development didn't end with Kickstart 1.3 in 1988(?) but continued, so there is 2.0 in 1990 which as well works with the CDTV hardware. Newer, post 1990, Amiga applications may need functions provided with 2.0. An while simply exchanging the ROMs is a possible way, it disables the CDTV specific add-ons mentioned above. So the ability of switching freely forth and back seems like a good idea, doesn't it?
As I understand it allows some compatibility with A500 games, but this disables CD and other system specific features.
These are only disabled as long as there are other ROMs than the 1.3 version switched in. That's why it's switchable, so the 2.0 Amiga (which it essentially is) can be turned back into a 1.3 CDTV by the flip of a switch.
The reason lies within the extension CDTV ROMs. They hook their services into the 1.3 ROM system. Cool way to keep a 100% original system ... except, it wasn't done in an upward compatible way, so they simply don't work with a later (2.0) ROM.
There are also mods which offer 3 KS chips option, but why?
I assume you're referring to an option to have three Kickstart ROMs on the board? Well, 1.3 and 2.0, although being the most common, aren't the only ones. So a third ROM simply offers to hold another version. Although that's usually more relevant to later Amiga models, not so much the CDTV.
Another use, especially if the third is made to hold two more standard 8 bit EPROMs, is to add a custom patched version - like from today's developers.
The CDTV CD drive is non-standard. The interface is not IDE or SCSI, it requires a special driver that was built in to the CDTV ROM. The ROM consists of a standard Kickstart 1.3 image plus the additional CD-ROM support.
Unfortunately the CD-ROM support only works with Kickstart 1.3 and without reverse-engineering it there was no way to fix it for later versions, so it had to be disabled.
Additionally the CD-ROM support uses some memory for the driver and filesystem. Disabling the CD-ROM frees up some memory. The CDTV shipped with 1MB of RAM so for compatibility with software designed for an A500 expanded to 1MB it was sometimes helpful.