Basic Memory Hardware Settings

Since processes reside in memory while running, to run an instruction, we need to fetch it from memory first, which makes memory a critical part of the operating system.

The CPU can only access its registers and main memory. It can't access the hard drive or I/O devices directly without going through memory.

A CPU accesses its registers very quickly, usually within 1 clock tick, while memory accesses take longer time, usually a few clock ticks. One solution to decrease access time is cache, which is located closer to the CPU and therefore is accessed faster than memory.

A process 'sees' its own memory in a limited way: it is not able to access memory of other processes or OS-specific memory (the chunk of memory controlled by the kernel.)