The memory system

L0 : registers
L1 : on-chip L1 cache (SRAM)
L2 : off-chip L2 cache (SRAM)
L3 : main memory (DRAM)
L4 : local secondary storage (local disk)
L5 : remote secondary storage (distributed file systems, Web servers)

from top to botton, Larger,slower, and cheaper storage devices
register file is a cache for the L1 cache, which is a cache for the L2 cache, which is a cache for the main memory, whici is a cache for the disk.

The Operating System Manages The Hardware

Processes

A process is the operating system’s abstraction for a running program.