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

Answer 2

My answer is true for Intel CPUs running on a modern Linux system, and I am speaking about user-level processes, not kernel code. Still, I think it'll give you some insight enough to think about the other possibilities

Address Types

Regarding question 3:

I have come across discussion that virtual and logical addresses/address space are the same. Is it true?

As far as I know they are the same, at least in modern OS's running on top of Intel processors.

Let me try to define two notions before I explain more:

• Physical Address: The address of where something is physically located in the RAM chip.
• Logical/Virtual Address: The address that your program uses to reach its things. It's typically converted to a physical address later by a hardware chip (mostly, not even the CPU is aware really of this conversion).

Virtual/Logical Address

The virtual address is well, a virtual address, the OS along with a hardware circuit called the MMU (Memory Management Unit) delude your program that it's running alone in the system, it's got the whole address space(having 32-bits system means your program will think it has 4 GBs of RAM; roughly speaking).

Obviously, if you have more than one program running at the time (you always do, GUI, Init process, Shell, clock app, calendar, whatever), this won't work.

What will happen is that the OS will put most of your program memory in the hard disk, the parts it uses the most will be present in the RAM, but hey, that doesn't mean they'll have the address you and your program know.

Example: Your process might have a variable named (counter) that's given the virtual address 0xff (imaginably...) and another variable named (oftenNotUsed) that's given the virtual address (0xaa).

If you read the assembly of your compiled code after all linking's happened, you'll be accessing them using those addresses but well, the (oftenNotUsed) variable won't be really there in RAM at 0xaa, it'll be in the hard disk because the process is not using it.

Moreover, the variable (counter) probably won't be physically at (0xff), it'll be somewhere else in RAM, when your CPU tries to fetch what's in 0xff, the MMU and a part of the OS, will do a mapping and get that variable from where it's really available in the RAM, the CPU won't even notice it wasn't in 0xff.

Now what happens if your program asks for the (oftenNotUsed) variable? The MMU+OS will notice this 'miss' and will fetch it for the CPU from the Harddisk into RAM then hand it over to the CPU as if it were in the address (0xaa); this fetching means some data that was present in RAM will be sent back to the Harddisk.

Now imagine this running for every process in your system. Every process thinks they have 4GB of RAMs, no one actually have that but everything works because everyone has some parts of their program available physically in the RAM but most of the program resides in the HardDisk. Don't confuse this part of the program memory being put in HD with the program data you can access through file operations.

Summary

Virtual address: The address you use in your programs, the address that your CPU use to fetch data, is not real and gets translated via MMU to some physical address; everyone has one and its size depends on your system(Linux running 32-bit has 4GB address space)

Physical address: The address you'll never reach if you're running on top of an OS. It's where your data, regardless of its virtual address, resides in RAM. This will change if your data is sent back and forth to the hard disk to accommodate more space for other processes.

All of what I have mentioned above, although it's a simplified version of the whole concept, is what's called the memory management part of the the computer system.

Consequences of this system

• Processes cannot access each other memory, everyone has their separate virtual addresses and every process gets a different translation to different areas even though sometimes you may look and find that two processes try to access the same virtual address.
• This system works well as a caching system, you typically don't use the whole 4GB you have available, so why waste that? let others share it and let them use it too; when your process needs more, the OS will fetch your data from the HD and replace other process' data, at an expense of course.

Answer 3

Normally every address issued (for x86 architecture) is a logical address which is translated to a linear address via the segment tables. After the translation into linear address, it is then translated to physical address via page table.
A nice article explaining the same in depth:
http://duartes.org/gustavo/blog/post/memory-translation-and-segmentation/