Difference between logical addresses and physical addresses

Question

I am reading Operating Systems Concept and I am on the 8th chapter  However I could use some clarification  or reassurance that my understanding is correct    Logical Addresses  Logical addresses are generated by the CPU  according to the book  What exactly does this mean   In an execute-generated address system    I assume when code is compiled for a program  the program has no idea where the code will be loaded in memory  All the compiler does is set up a general sketch of the program layout and how the image should be laid out  but doesn t assign any real addresses to it  When the program is executed the CPU takes this layout image that the compiler made and hands out some addresses  logical ones  to the ones generated from the code    Physical Addresses  The physical addresses are not generated until after the CPU generates some set of logical addresses  consisting of a base address and an offset   The logical addresses go through the MMU or another device and somewhere along the line the logical addresses are mapped to physical RAM addresses    What then is the actual difference  I can see one benefit  Using logical addresses gives more freedom to the applications  If the physical addresses were hard coded  then the program success would depend heavily on the physical computer machine  available RAM addresses etc    Doesn t the use of logical addresses converted to physical address impose two steps instead of a one to one  and therefore more over head    Where then do the logical addresses reside after generation  They may exist in a register on the CPU while the CPU is servicing a process  but before and after  where do they go  I understand this is implementation dependent  I assume they may be stored in some special register space or buffer on the CPU such as a TLB  correct  If not  then the table may exist in the actual RAM itself  and the CPU only holds a pointer address to the base address of the table in RAM  correct    It seems holding the addresses in RAM is counter productive to the purpose of logical memory addresses  I can only assume my understanding is incorrect

User · Answer

I found a article about Logical vs Physical Address in Operating System, which clearly explains about this.

Logical Address is generated by CPU while a program is running. The logical address is virtual address as it does not exist physically therefore it is also known as Virtual Address. This address is used as a reference to access the physical memory location by CPU. The term Logical Address Space is used for the set of all logical addresses generated by a programs perspective. The hardware device called Memory-Management Unit is used for mapping logical address to its corresponding physical address.

Physical Address identifies a physical location of required data in a memory. The user never directly deals with the physical address but can access by its corresponding logical address. The user program generates the logical address and thinks that the program is running in this logical address but the program needs physical memory for its execution therefore the logical address must be mapped to the physical address bu MMU before they are used. The term Physical Address Space is used for all physical addresses corresponding to the logical addresses in a Logical address space.

Source: www.geeksforgeeks.org

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A logical address is the address at which an item  memory cell  storage element  network host  appears to reside from the perspective of an executing application program

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An address generated by the CPU is commonly referred to as a logical address  The set of all logical addresses generated by a program is known as logical address space  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 physical address  The set of all physical addresses corresponding to the logical addresses is known as physical address space   The compile-time and load-time address-binding methods generate identical logical and physical addresses  However  in the execution-time address-binding scheme  the logical and physical-address spaces differ  The user program never sees the physical addresses  The program creates a pointer to a logical address  say 346  stores it in memory  manipulate it  compares it to other logical addresses- all as the number 346   Only when a logical address is used as memory address  it is relocated relative to the base relocation register  The memory-mapping hardware device called the memory- management unit MMU  converts logical addresses into physical addresses  Logical addresses range from 0 to max  User program that generates logical address thinks that the process runs in locations 0 to max   Logical addresses must be mapped to physical addresses before they are used  Physical addresses range from  R 0  to  R   max  for a base relocation register value R  Example    Mapping from logical to physical addresses using memory management unit  MMU  and relocation base register  The value in relocation base register is added to every logical address generated by a user process  at the time it is sent to memory  to generate corresponding physical address  In the above figure  base  relocation value is 14000  then an attempt by the user to access the location 346 is mapped to 14346

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Logical address - Logical address generated by the CPU   when we are give the problem to the computer then our computer pass the problem to the processor through logical address   which we are not seen this address called logical address    Physical address  - when our processor create process and solve our problem then we store data in secondary memory through address called physical address

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Logical Vs Physical Address space  An address generated by the CPU is commonly refereed as Logical Address whereas the address seen by the memory unit that is one loaded into the memory address register of the memory is commonly refereed as the Physical Address The compile time and load time address binding generates the identical logical and physical addresses However  the execution time address binding scheme results in differing logical and physical addresses   The set of all logical addresses generated by a program is known as Logical Address Space whereas the set of all physical addresses corresponding to these logical addresses is Physical Address Space Now  the run time mapping from virtual address to physical address is done by a hardware device known as Memory Management Unit Here in the case of mapping the base register is known as relocation register The value in the relocation register is added to the address generated by a user process at the time it is sent to memory Let s understand this situation with the help of example If the base register contains the value 1000 then an attempt by the user to address location 0 is dynamically relocated to location 1000 an access to location 346 is mapped to location 1346   The user program never sees the real physical address space it always deals with the Logical addresses As we have two different type of addresses Logical address in the range  0 to max  and Physical addresses in the range R to R max  where R is the value of relocation register The user generates only logical addresses and thinks that the process runs in location to 0 to max As it is clear from the above text that user program supplies only logical addresses these logical addresses must be mapped to physical address before they are used

User · Answer

logical address is address relative to program  It tells how much memory a particular process will take  not tell what will the exact location of the process and this exact location will we generated by using some mapping  and is known as physical address

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This answer is by no means exhaustive but it may explain it enough to make things click   In virtual memory systems  there is a disconnect between logical and physical addresses   An application can be given a virtual address space of  let s say  4G  This is its usable memory and it s free to use it as it sees fit  It s a nice contiguous block of memory  from the point of view of the application    However  it is not the only application running  and the OS has to mediate between them all  Underneath that nice contiguous model  there is a lot of mapping going on to convert logical to physical addresses   With this mapping  the OS and hardware  I ll just call these the lower layers from here on in  is free to put the application pages anywhere it wants  either in physical memory or swapped out to secondary storage    When the application tries to access memory at logical address 50  the lower levels can translate that to a physical address using translation tables  And  if it tries to access logical memory that s been swapped out to disk  a page fault is raised and the lower levels can bring the relevant data back into memory  at whatever physical address it wants   In the bad old days when physical addresses were all you had  code had to be relocatable  or fixed up on load  since it could load anywhere  With virtual memory  that code  and data  can be at logical memory location 50 in a dozen different processes at the same time - it s actual physical address will be different however   It can even be shared so that one physical copy exists in the address space of many processes at once  This is the crux of shared code  so we don t use more physical memory than we need  and shared memory to allow easy inter-process communication    It is  of course  less efficient than a pure physical-address environment but the CPU manufacturers try to make it as insanely efficient as possible  since it s used heavily  The advantages far outweigh the disadvantages

User · Answer

A logical address is a reference to memory location independent of the current assignment of data to memory  A physical address or absolute address is an actual location in main memory   It is in chapter 7 2 of Stallings

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To the best of my memory  a physical address is an explicit  set in stone address in memory  while a logical address consists of a base pointer and offset   The reason is as you have basically specified   It allows for not only the segmentation of programs and processes into threads and data  but also for the dynamic loading of such programs  and the allowance for at least pseudo-parallelism  without any actual interlacing of instructions in memory needing to take place

[memory-management] Difference between logical addresses, and physical addresses?

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