How logical address converted into physical address

An address generated by the CPU is commonly referred to as a logical address, whereas an address, is 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. We use the logical address and virtual address interchangeably in this text. The set of all logical addresses generated by a program is logical address space. The set of all physical addresses corresponding to these logical addresses is physical address space. Thus, in the execution-time address-binding scheme, the logical and physical address spaces differ.

The run-time mapping from virtual to physical addresses is done by a hardware device called the memory-management unit (MMU). For the time being, we illustrate this mapping with a simple MMU scheme that is a generalization of the base-register scheme. The base register is now called a relocation register.

Only when it is used as a memory address (in an indirect load or store, perhaps) is it relocated relative to the base register. The user program deals with logical addresses. The memory-mapping hardware converts logical addresses into physical addresses. The final location of a referenced memory address is not determined until the reference is made.

We now have two different types of addresses: logical addresses (in the range 0 to max) and physical addresses (in the range R + 0 to R + max for a base value R). The user program generates only logical addresses and thinks that the process runs in locations 0 to max. However, these logical addresses must be mapped to physical addresses before they are used. The concept of a logical address space that is bound to separate physical address space is central to proper memory management.

Segmentation

• Segmentation is a memory-management scheme that supports this programmer's view of memory. A logical address space is a collection
  of segments.

• Each segment has a name and a length. The addresses specify both the segment name and the offset within the segment.

  

• The programmer, therefore, specifies each address by two quantities: a segment name and an offset. For simplicity of implementation, segments are numbered and are referred to by a segment number, rather than by a segment name.

• Thus, a logical address consists of two tuples: <segment-number, offset>. Normally, when a program is compiled, the compiler automatically constructs segments reflecting the input program.

A-C compiler might create separate segments for the following:

• The code

• Global variables

• The heap, from which memory is allocated

• The stacks used by each thread

• The standard C library

  Libraries that are linked in during compile time might be assigned separate segments. The loader would take all these segments and assign them segment numbers.