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:

1. Physical Address- When the processor is in system mode, the address used by the processor is physical address.

2. Logical Address- When the processor is in user mode, the address used is the logical address. these are anyways mapped to some physical address by adding a base register with the offset value.It in a way provides a sort of memory protection.

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

Any help is deeply appreciated.

Answer 1

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 2

Physical Address- When the processor is in system mode, the address used by the processor is physical address.

Not necessarily true. It depends on the particular CPU. On x86 CPUs, once you've enabled page translation, all code ceases to operate with physical addresses or addresses trivially convertible into physical addresses (except, SMM, AFAIK, but that's not important here).

Logical Address- When the processor is in user mode, the address used is the logical address. these are anyways mapped to some physical address by adding a base register with the offset value.

Logical addresses do not necessarily apply to the user mode exclusively. On x86 CPUs they exist in the kernel mode as well.

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

It depends on the particular CPU. x86 CPUs can be configured in such a way that segments aren't used explicitly. They are used implicitly and their bases are always 0 (except for thread-local-storage segments). What remains when you drop the segment selector from a logical address is a 32-bit (or 64-bit) offset whose value coincides with the 32-bit (or 64-bit) virtual address. In this simplified set-up you may consider the two to be the same or that logical addresses don't exist. It's not true, but for most practical purposes, good enough of an approximation.