The Linear Address Space

Discussion in 'Computer Information' started by Cameron Gibbs, Apr 27, 2004.

  1. Hey people.

    I have been working through the IA-32 Software Developer's Manuals. I have
    read pretty much all of it in some detail (skipping mostly just tables and
    lists that can be referenced as needed). Still there is something very basic
    in my understanding that is lacking.

    There is a Physical Address Space of 4GBytes (2^32 bytes) which basically
    identifies all of the binary numbers that can be used as addresses for
    assembly language programming.
    With the 36-bit address extensions it is possible to increase this to
    64Gbytes.
    The problem is that in a computer there is Hard Drive storage, RAM storage
    and a heap of other devices and the like that all require byte addresses.
    Say for example you had a 180GByte HDD in your system, even with the 36 bit
    address extension mechanisms which bring the Physical Address Space up to
    64GBytes, there are still just not enough Logical Addresses to reference the
    whole of the addresses on a system.

    Also with Hard Drive partitions and with every program having to be put in a
    particular spot on the Hard Drive, it is a very rare thing that one would
    write a program that is to be put at address zero.
    From all this I have come to think that the Linear Address Space is simply
    an abstraction or a virtualization of the true state of all of the addresses
    within a computer so what I need to know is how are these abstractions that
    are used in a program "mapped" to the real addresses of the computer?
    How does this work?

    Also, I think that paging has something to do with RAM management and the
    "checking in" and "checking out" of data to and from RAM but frankly I can
    neither confirm nor deny this and hence I just cannot figure out what is
    actually going on with paging.
    Am I on the right track with paging? - Please correct!

    Might the mapping to real addresses have something to do with the Page
    Directory Base Register (PDBR) which is Control Register 3 (CR3)?

    All help appreciated.

    Cameron.

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    Cameron Gibbs, Apr 27, 2004
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