Thread: Physical and Logical Memory.

  1. #1
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    Aug 2009

    Physical and Logical Memory.

    I do understand that logical address space/memory is generated by the CPU for the program and physical memory is the one on-to which the logical address space is mapped and which is on the physical memory. Plus, physical memory is not directly accessible to the user.

    Now, my question is (I don't know whether this makes sense or not):

    1- How is the logical address space generated by the CPU? And is this randomly generated?

    2- Why do we need a "relocation register" to map it on the physical memory? Why can't it be directly mapped on the physical memory?


  2. #2
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  3. #3
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    Aug 2008
    1. Depends on the architecture.
    X86 family of processors use combination of segmentation and paging, although most operating systems today don't really use the "advantages" of segmentation.
    For example, Linux use only 5 segments (kernel code and data, user code and data plus one dummy segment), each 4 gigs large and overlap.

    So in x86 arch. logical address is actually called far pointer consisting of segment selector(16 bit) and offset(32bit).
    You can choose segments through one of the segment registers: CS, DS, SS, ES, GS and FS. Through them, in protected mode, you actually give an index to GDT(Global description table) where information about each segment are stored (start address, size, premissions....).
    After it goes "through" the GDT to pickup it's base address, an offset is added to form a linear address. Then, that linear address is translated through the paging mechanism to form a physical address.
    *Also note, when you say physical address, it is an array of bits that will be presented on processor's pins which are connected to address bus. They don't have to necessarily address a system memory(RAM).

    In operating system environment all that mechanism is well hidden from you.
    Operating system allocates a huge block of logical address space for a process...say 4GB...and that addr. space is divided in pages, each fixed size(4kB on x86). Then memory manager (part of the OS kernel) translates only allocated pages to physical address.
    So logical address is "not" generated by the CPU, but software, operating system.

    2. Not really sure what "relocation register" is...

  4. #4
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    Nov 2010
    The relocation register is used by the MMU to map logical addresses to physical addresses. The relocation register has a base physical memory address which is added to the base logical address of the process, that is it maps the logical address to a physical address. The reason for this is that you don't want the process to know their true memory addresses, this could pose a security risk to other processes.

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