Splain me 32 bit vs 64 bit

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This topic contains 5 replies, has 3 voices, and was last updated by  HotDawg 2 years, 7 months ago.

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    Rev 2

    I mean, if I install Windows 7 64 bit does everything take twice as much space as 64-bit? Do I need twice the memory and twice the disc space? Why would I want it anyway?



    One good reason for going with 64-bit is the amount of memory the OS will be able to see. I think a 32-bit operating system isn’t able to really use a full 4 GB of RAM — the 64-bit version lets you use probably all the memory you would be able to afford.

    So, if you really want twice the memory — maybe you can do it with 64 bit while the 32 bit wouldn’t let you.

    There might be some considerations when you are running a lot of 32 bit applications under a 64 bit OS. Maybe someone else can comment on that — but I seem to remember early on that was a bad combination. Something about having to switch the CPU mode back and forth. Certainly all 64 bit processing should be faster.


    Rev 2

    Well, the question is this: If every location and every hunk of data is now going to be 64 bits wide, doesn’t that effectively cut your memory in half? Doesn’t that mean that six gigs in 64-bit means that you only have the three gigs that you had in XP?



    But, that 6 gigs of memory is 6 gigabytes. A byte is 8 bits — and is what most computers use to store a character. When we talk of 32 bit or 64 bit, that would be word length.

    I guess with a 64 bit system, the number of words in memory would be half of what it would be in 32 bit system. But the number of bytes of storage is still the same.

    In a 64 bit system, where the addressing and data busses are that wide, then you can move twice as much information at a time, cutting the number of machine cycles in half. And, a 64 bit processor has more instructions (that can be more powerful) and that makes programs more efficient.

    IIRC, computers of the IBM 360 era were mostly 32 bit. The desktop computers of today are big jump ahead of what the commercial mainframes of yesterday were.




    Wikipedia mentions that the advantage of 64 bit systems is mostly seen when working with large data sets in applications such as digital video, scientific computing, and large databases.

    I’d like to point out that 32 bit systems are not really limited to 4GB of RAM. There is a feature called physical address extension which allows them to use up to 64GB of RAM, albeit with some overhead. I’m still using a 32 bit operating system and I’ve got 6GB of RAM. If I ever get around to re-installing, I’ll switch to 64 bit.

    If every location and every hunk of data is now going to be 64 bits wide, doesn’t that effectively cut your memory in half?

    Not necessarily. If you have a large chunk of data to store, the 64 bit architecture will simply use half as many 64 bit locations to store it. If the data set doesn’t divide evenly into 64 bits though, the wasted padding will obviously be larger than with a 32 bit system, but I don’t think this is a big deal in practice. Also, I think the 64 bit term applies only to the processor’s register size. I think RAM is usually addressed as individual bits, not as 64 bit chunks.

    Disk space is addressed in much larger chunks (see filesystem block size). 4096 bytes is typical, so there’s no waste with a 64 bit system, and it will much faster reading the data since it needs half the number of clock cycles to do so.

    I don’t think the programs themselves will be take up more space. That was an issue in the RISC vs CISC debate, but Intel’s dominance has rendered it moot.



    Thanks for that link, Christian.

    Lots of very good information in it.

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