Dynamic (Hardware-based) Relocation

To gain some understanding of hardware-based address translation, we’ll first discuss its first incarnation. Introduced in the first time-sharing machines of the late 1950’s is a simple idea referred to as base and bounds; the technique is also referred to as dynamic relocation; we’ll use both terms interchangeably.

Specifically, we’ll need two hardware registers within each CPU: one is called the base register, and the other the bounds (sometimes called a limit register). This base-and-bounds pair is going to allow us to place the address space anywhere we’d like in physical memory, and do so while ensuring that the process can only access its own address space.

In this setup, each program is written and compiled as if it is loaded at address zero. However, when a program starts running, the OS decides where in physical memory it should be loaded and sets the base register to that value. In the example above, the OS decides to load the process at physical address 32 KB and thus sets the base register to this value.

Interesting ...