I was wondering, if I have a RAM Disk and put my whole OS onto that RAM disk, then I guess there is no use for the system RAM anymore, since fetching it from there is not faster than fetching it from the system drive. But in all tutorials I found, I always just allocate a fraction of my RAM for a RAM disk and not everything I have and then use the RAM disk as it were a normal hard drive. So my question, is there a way to not have any sytem memory at all and just work directly from the RAM disk?
There are a couple of reasons: 1) the OS has to allocate memory for various data structures: it does that by using RAM, not a RAM disk; 2) access to a RAM disk goes through (at least) the block layer of the OS (which - see point 1 - has to have its data structures allocated in RAM), which makes RAM disk access slower than RAM access.
The OS could bootstrap itself as it does today until a RAM disk is set up and then migrate all allocations to the RAM disk, but there is not much point in that: it complicates the OS and it makes things slower.