OK, I know it is impossible to do so in the Sega CD because it lacks enough RAM. But in theory, using an Arcade Card, can a simple loading menu be done?
The Sega CD may lack enough RAM to load larger ROMS but there are many that would fit. There are other issues beside just RAM in doing this. The Arcade Card has enough memory to hold any game and this has been done. Look for Super Hucard.
The Super HuCard project uses the Super CD-ROM format and has various games hacked to load into the 256Kbytes of memory. This leaves out many games simply because there isn't enough memory. The Arcade Card has more memory, but it is not mappable like a HuCard ROM and won't work. The Arcade Card's addition 2 megabytes of memory is accessed via a series of ports and not directly by the CPU. Games still use the same 256KB of RAM for loading the game program and executing it. The 2 megabytes of arcade card ram is generally going to be used for storing lots of graphics which will be uploaded to VRAM as needed via DMA through these ports. You could actually have other data stored in the arcade card's RAM to transfer to the CPU but I'm not sure what use that would be since it would require accessing it indirectly. You could use it to speed up loading by loading more data for later in a level ahead of time I suppose. So as pas7680 said, buy an EverDrive. You can't load most ROMs from a CD. The Super HuCard project also isn't perfect even for the games that are included. So just invest in the flash card and enjoy the games.
I own an everdrive already (in fact, I'm a silent reseller of those). I was just carious to know. Thank you all for your explanations, even the mean guy
Interesting. Didn't know that extra memory wasn't mapped. I guess the people who designed the arcade card were more interested in moving data from the CD than letting people run ROMs. Who woulda guessed.
It's a good question to ask, so don't feel bad about it. Most common knowledge about the Arcade Card stops at how much memory it adds. The designers clearly intended it for expanding the graphics capability. The must have felt the 256KB of RAM from the Super CD-ROM format was sufficient for the CPU programs. If you separate graphics into the 2 megabyte added memory then you actually reclaim space from the 256KB for more CPU data. It seemed from the games released to prove itself as being enough memory for the CPU. I wonder what the reason was why they didn't allow for CPU mapping of any additional memory. It could have been cost I suppose. Now nothing is stopping someone from making their own HuCard that contains RAM and accesses the CD-ROM to load ROM files into it. Before the Turbo EverDrive came out it was a possibility someone might make a device like that. But the Turbo ED is a better design and is the standard now.
I think the main reason they didn't make it available as direct memory, is that it's DRAM. It has refresh cycles. Those refresh cycles are small enough to be hidden in between read/write access, and of course the data is prefetched into the port once the vector to the memory has been set. The PCE lacks the /RDY pin on the cart port and only has it in the expansion port: so hucards aren't capable of having devices pause the CPU for something like wait states or refresh cycles. Considering the amount of memory on the card (an extra 2megabytes), I'm sure they were trying to keep the cost down buy avoiding sram. The only option the hucard port has for accessing slower devices, is if the programmer temporarily puts the cpu in 1.79mhz mode - but that still wouldn't be enough of a window for the dram refresh, plus that's waaaay to slow for transferring graphics or accessing game data.
That makes sense. I imagine at the time for 2 megabytes of memory, DRAM was the only viable choice. 2MB of SRAM would have been crazy expensive back then.
