Cat mem vs. cat ram0
(Just a disclaimer: this is being done on a VERY old PC with no precious files on it, just as a way to learn the in depth workings of linux; even if something gets irreparably messed up it's no biggy)
Why does "cat /dev/mem" work whilst "cat /dev/ram0" does not? Are they not both reading what is currently stored in memory? (albeit the second one is reading only a section of the memory?)
/dev/ram0 is a ram disk and is unmounted after start on most systems.
initrd(4): boot loader initialized RAM disk - Linux man page
What about the other "ram" in /dev/ then (ram1 through ram9); I don't see them mentioned in that man page; or are they performing similar functions?
Yes, you configure how many ram disks exist when you compile a new kernel.
I think the maximum is 16.