Re: 32 bit processors / 64 bit processors

1. The size of the processor's internal address bus (virtual address
space) is what qualifies it as a 32-bit / 64-bit processor.
Whether or not the OS changes the processor mode to 64-bit(IA-32e/Long
etc) (and thereby has access to the 64-bit virtual address space,
extended register set etc) is what qualifies it as a 32-bit / 64-bit
OS.

AFAIK, no x86 processor truly supports the complete 64-bit address space (yet) -
a. The virtual address space is 64 bits, but still limited by the
canonical addressing restriction ( no access to the complete 64-bit
virtual address space)
b. The physical address is not 64 bits (no system uses such a large
amount of physical memory, so there's no point providing such a large
number of address pins out to the memory controller / DRAM)

2. sizeof(int) would be depend on the compiler (which in turn depends
on the OS it is compiling for, which in turn depends on what processor
it is running on). (Though personally I haven't seen sizeof(int) being
anything other than 4 bytes .. but I've seen sizeof(long int) being 64
bits for x86-64 gcc, and 32 bits for i386 gcc)

3. If you consider x86 architecture, since they're always backward
compatible, any code you generated for an earlier processor should
theoretically run on a newer processor (when the processor in a
specific mode). Hence, a 64-bit processor can run a 32-bit OS / 64-bit
OS. A 64-bit OS can run programs compiled for 64-bit /32-bit mode(by
reverting to a 32-bit mode on the fly). This might not apply to other
architectures though.

C

On Tue, Oct 20, 2009 at 11:02 PM, 益牙 <[email protected]> wrote:
>
> On Oct 21, 2009, at 1:39 PM, Siddu wrote:
>
>>
>>
>> On Wed, Oct 21, 2009 at 8:32 AM, Rick Brown <[email protected]>
>> wrote:
>> Hi,
>>
>> Firstly, I'm trying to understand what exactly characterizes a
>> procesor or an operating system as 32bit / 64 bit. I've read that it
>> means the "native word size" of a machine. But what exactly is that?
>> Register size? Address bus size? Anything else?
>>
>>
>> When they say its 32/64 bit machine its the capability of the system or
>> processor to process so many bits at once !
>
> I had an understanding of this being how many bits you have to address your
> RAM.
>
>>
>>
>> Secondly, I'm trying to understand what exactly does a sizeof(int)
>> depend on when we say it is "platform specific". Is it dependent of
>> compiler being 32bit / 64 bit? Or the OS being 32bit / 64 bit? Or the
>> processor being 32 / 64 bit?
>>
>> its dependent on processor being 32/64 bit
>
> Just did a comparison test but it tells me otherwise. But I'm not sure if
> the test case if correct, can someone improve it?
> On a 64bit CentOS:
> [root@yyan ~]# more test.c
> #include <stdio.h>
>
> int main ()
> {
>    printf ("%d\n", sizeof(int));
>    return 0;
> }
> [root@yyan ~]# gcc test.c
> [root@yyan ~]# ./a.out
> 4
> [root@yyan ~]# uname -a
> Linux yyan.pmlab.com 2.6.18-128.el5 #1 SMP Wed Jan 21 10:41:14 EST 2009
> x86_64 x86_64 x86_64 GNU/Linux
> [root@yyan ~]#
>
> On my Leopard: (Hopefully it's a 32bit OS as it shows)
> simonmac:coding yansimon$ cat test.c
> #include <stdio.h>
>
> int main ()
> {
>    printf ("%d\n", sizeof(int));
>    return 0;
> }
> simonmac:coding yansimon$ gcc test.c
> simonmac:coding yansimon$ ./a.out
> 4
> simonmac:coding yansimon$ uname -a
> Darwin simonmac.local 9.8.0 Darwin Kernel Version 9.8.0: Wed Jul 15 16:55:01
> PDT 2009; root:xnu-1228.15.4~1/RELEASE_I386 i386
> simonmac:coding yansimon$
>
>
>>
>> Lastly, How are "sizes" of compiler / OS / processor inter related?
>> Here is my understanding. A 32 bit processor can only be running a 32
>> bit OS.
>>
>> No ... 32 bit OS can run anything <= 32 bit
>>
>> A 64 bit processor may run a 32 or a 64 bit OS.
>>
>> yes thats right !
>>
>> A 32 bit
>> compiler genrates code only to be run on a 32 bit OS; ditto for 64
>> bit.
>>
>> Not sure about this let others have their say !
>> Is this right?
>>
>>
>> Please correct me if i am wrong !
>> TIA,
>>
>> Rick
>>
>> --
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>>
>>
>>
>>
>> --
>> Regards,
>> ~Sid~
>> I have never met a man so ignorant that i couldn't learn something from
>> him
>>
>
> --
> 益牙
>
> http://www.google.com/profiles/simonyanix
>
> --
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