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    SubjectRe: difference b/w 32-bit vs. 64-bit kernel
    On Mon, Sep 13, 1999 at 06:55:57PM +0530, Sandeep Kumar wrote:
    > can anyone elaborately tell the differences b/w
    > 1. 32-bit OS vs. 64-bit OS
    > 2. 32-bit Kernel vs. 64-bit kernel
    > is it necessary to have 64-bit I/O, 64-bit addressing etc. (i.e. all the 64-bit
    > things to be there for an OS to be called 64-bit?).
    Hi there,
    A recent thread that was on the Linux Kernel mailing List might be of help
    On Fri, 10 Sep 1999 11:06:53 +0530, Sachin Tilloo wrote:
    > I have heard about 16/32 bit OS. As far as i know a 32 bit Os means
    > that the instructions that are used to code OS knew only 32 bit
    > operations. Am i right !!!

    I general it means that it has a 64 bit address space. You can always do
    non-32 bit operations on a 32 bit platform: load a single byte is one of
    them (of course given that the CPU can do it).

    > Now if i have say a 32 bit Os and the length of my data bus of PC is
    > 64 bit, can i not run a 64 bit program on my PC.

    The length of the data bus is independent of the CPU. For instance: the
    80386SX processor was a 32 bit processor with a 16 bit databus so it could
    be used with the cheaper 80286 compatible support chips. This was of
    course slower because a single 32 bit load or store had to be divided into
    two 16 bit load/stores.

    > I mean if my compiler and CPU can undertand that instruction where
    > does the OS come in between to prevent it.

    The OS doesn't prevent you to do it. Unless... the OS only saves 32 of the
    64 bits in a register during a context switch. No problem when you run
    a single process using 64 bit registers, but as soon as you use two, they
    will most likely be messed up.

    > I think as far as i am not needing dynamic memory support from OS
    > i can run say a simple program which transfers 64 bits from
    > memory location to my 64 bit register.

    As long as you use only one of those programs.

    Hope this helps


    Syed Khader Vali | The road to wisdom? | | Well it's plain and simple to express:| | Err and err and err again | | but less and less and less. |

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