Logical Versus Physical Address Space

• An address generated by the CPU is commonly referred to as a logical address, whereas an address seen by the memory unit -that is, the one loaded into the memory-address register of the memory- is commonly referred to as a physical address.
• The compile-time and load-time address-binding methods generate identical logical and physical addresses.
• However the execution-time address-binding scheme results in differing logical and physical addresses. In this case, we usually refer to the logical address as a virtual address.
• The run-time mapping from virtual to physical addresses is done by a hardware device called the memory-management unit (MMU).

  Figure 8.4: Dynamic relocation using a relocation register.

  
• For the time being, we illustrate this mapping with a simple MMU scheme, which is a generalization of the base-register scheme (see Fig. 8.4)).
  • The base register is now called a relocation register.
  • The value in the relocation register is added to every address generated by a user process at the time it is sent to memory
  • For example, if the base is at 14000, then an attempt by the user to address location 0 is dynamically relocated to location 14000; an access to location 346 is mapped to location 14346.
• The user program never sees the real physical addresses. The program can create a pointer to location 346, store it in memory, manipulate it, and compare it with other addresses -all as the number 346.
• The user program deals with logical addresses. The memory-mapping hardware converts logical addresses into physical addresses.
• The concept of a logical address space that is bound to a separate physical address space is central to proper memory management.