In my operating systems class we were shown a picture depicting a hierarchy of memory starting from most expensive and fastest at the top and least expensive and slowest at the bottom. At the very top was registers and underneath it was cache. The professor said that the best place to run programs is in cache. I was wondering why programs can't be run in registers? Also, how can a program load itself into cache? Isn't the cache something that's controlled by the CPU and works automatically without software control?