Difference between physical/logical/virtual memory address

Question

I am a little confused about the terms physical/logical/virtual addresses in an Operating System(I use Linux- open SUSE)

Here is what I understand:

Answer 1

I am referring to below answer base on intel x86 CPU

Difference Between Logical to Virtual Address

Whenever your program is under execution CPU generates logical address for instructions which contains (16 bit Segment Selector and 32 bit offset ).Basically Virtual(Linear address) is generated using logical address fields.

Segment selector is 16 bit field out of which first 13bit is index (Which is a pointer to the segment descriptor resides in GDT,described below) , 1 bit TI field ( TI = 1, Refer LDT , TI=0 Refer GDT )

Now Segment Selector OR say segment identifier refers to Code Segment OR Data Segment OR Stack Segment etc. Linux contains one GDT/LDT (Global/Local Descriptor Table) Which contains 8 byte descriptor of each segments and holds the base (virtual) address of the segment.

So for for each logical address, virtual address is calculated using below steps.

1) Examines the TI field of the Segment Selector to determine which Descriptor Table stores the Segment Descriptor. This field indicates that the Descriptor is either in the GDT (in which case the segmentation unit gets the base linear address of the GDT from the gdtr register) or in the active LDT (in which case the segmentation unit gets the base linear address of that LDT from the ldtr register).

2) Computes the address of the Segment Descriptor from the index field of the Segment Selector. The index field is multiplied by 8 (the size of a Segment Descriptor), and the result is added to the content of the gdtr or ldtr register.

3) Adds the offset of the logical address to the Base field of the Segment Descriptor, thus obtaining the linear(Virtual) address.

Now it is the job of Pagging unit to translate physical address from virtual address.

Refer : Understanding the linux Kernel , Chapter 2 Memory Addressing