What is at the CS2 area?

[RockinB]

In the SCU manual, there is 1MByte of the CS2 region mapped on the A-BUS. The original CS2 region is 32MByte large...

Does anyone know if that region can be used and what physical memory or device it's actually belonging to? The SCU memory map does not list this space as access prohibited.

I'm asking because the SH2 can access the CS2 region in little-endian mode, which is interesting for emulators. Why should SEGA design the SCU to have unused features?

 

[antime]

The SH2's CSn and the A-bus CSn regions are not connected, they just happen to have similar names. Compare the SH2 memory map and the Saturn memory map.

When you consider the function of the SH2 bus state controller you'll see why the Saturn's memory map is laid out like it is. The SH2 CS1 region is completely used by the A-bus CS0 region, which means whatever's plugged into the cartridge slot can have different access methods from the rest of the hardware. Similarly the SH2 CS3 region is used only for workram-H, which is SDRAM and must thus be accessed differently.

 

[RockinB]

Wow, I'm starting to get excited.....that means that I can access the VDP1, VDP2 and Sound (and SCU) RAM (and register) region in little endian mode!!!!

Will have to take care of little endian when writing registers, this looks like a brilliant example for using the SGL replacement which I could easily modify to write registers correctly.

Now just some benchmarks have to determine how much slower these RAM regions are and if the speed advantage if LSB access makes it worth it.