To answer your first question, (as far as I understand it): 
- Cache is a type of memory that only stores the frequently used information. A useful way for me to think about it, is that when I clear cache in my web browser, things speed up a bit. 
- As memory goes, RAM is bigger but slower. It's the main memory, so sometimes it's easier to rely on cache to do a job quickly, than to get the RAM involved. 
(can someone fact check this please? This is how I understand it, but I'm not 100% sure) 
- Register is a very small type of memory which is built directly into the processor - so this type of memory is dedicated to fulfilling the needs of the processor, not the system as a whole. 
             
            
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For the rest please refer to the lecture on 05.10.2020