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 things start to happen when the process is running. Now,
when any memory reference is generated by the process, it is translated
by the processor in the following manner:
Each memory reference generated by the process is a virtual address;
the hardware in turn adds the contents of the base register to this address
and the result is a physical address that can be issued to the memory
system.