The document discusses processor and memory hierarchy. It defines memory hierarchy as the organization of memory systems in a computer according to speed and size. Faster but smaller memory like registers and cache are closer to the processor, while larger but slower memory like main memory, disks, and tapes are further from the processor. Previously, computers were designed without memory hierarchy, which caused performance issues due to the large speed differences between processor and main memory. To improve performance, computer systems adopted a memory hierarchy model that organizes different types of memory based on their access times and capacities.