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32 or 64 bit processor info in C

Hello,

Is there a way in C to get information at runtime if a processor is 32
or 64 bit?

Cheers,

Broeisi

Apr 10 '07 #1
168 7326
In article <11************ **********@b75g 2000hsg.googleg roups.com>,
broeisi <br*******@gmai l.comwrote:
>Is there a way in C to get information at runtime if a processor is 32
or 64 bit?
No, not in standard C.

First you'd have to define exactly what it means for a processor
to be 32 or 64 bit, which is something that has at least 4 different
correct (but contradictory) answers.
--
I was very young in those days, but I was also rather dim.
-- Christopher Priest
Apr 10 '07 #2


On 10 apr, 21:07, rober...@ibd.nr c-cnrc.gc.ca (Walter Roberson) wrote:
In article <1176230815.876 762.147...@b75g 2000hsg.googleg roups.com>,

broeisi <broeis...@gmai l.comwrote:
Is there a way in C to get information at runtime if a processor is 32
or 64 bit?

No, not in standard C.

First you'd have to define exactly what it means for a processor
to be 32 or 64 bit, which is something that has at least 4 different
correct (but contradictory) answers.
--
I was very young in those days, but I was also rather dim.
-- Christopher Priest

Thank you very much for you answer Christopher.
But how does an OS like linux or windows know that it's installed on a
computer with a 32 or 64 bit processor?
Apr 10 '07 #3
In article <11************ **********@b75g 2000hsg.googleg roups.com>,
broeisi <br*******@gmai l.comwrote:
>On 10 apr, 21:07, rober...@ibd.nr c-cnrc.gc.ca (Walter Roberson) wrote:
>In article <1176230815.876 762.147...@b75g 2000hsg.googleg roups.com>,
>broeisi <broeis...@gmai l.comwrote:
>Is there a way in C to get information at runtime if a processor is 32
or 64 bit?
>No, not in standard C.
>First you'd have to define exactly what it means for a processor
to be 32 or 64 bit,
>But how does an OS like linux or windows know that it's installed on a
computer with a 32 or 64 bit processor?
The OS is implementation, and so is allowed to do things that
C leaves undefined or unspecified.

When an OS is compiled, it already has certain hardware assumptions
built into it -- assumptions such as what the machine language of
the processor looks like. OSes may have access to processor status
registers that C does not define; the status registers may even
require special instructions to access. Since the OS knows what
kind of processor (generally) it is running on, it knows how to
interpret the status registers to determine processor capabilities
(such as whether there is hardware audio support instructions.)

Some people define "32 or 64 bit processor" according to the
instructions that are supported. That's not the best of definitions,
though, because you also need to know things like how many data
bits may be transfered at a time on the databus: a particular
processor model might support several different databus widths
and hide all the details from the users. A processor might
have an instruction to multiply two 32 bit numbers and produce
a 64 bit number, but it might transfer those 64 bits to memory
16 bits at a time. Do you define "32 or 64 bit" by the instruction
set, or do you define it by what the hardware actually does?
Note: generally, the processor reads some cpu pins hardwired on the
motherboard in order to figure out bus widths.

The C language itself provides no mechanisms to access hardware
directly, and provides almost no restrictions on how the
hardware operates. The C langauge standard provide user facilities
to write -portable- code, and leaves the details of the portable
facilties to the implementation. If you have something that
depends on whether the processor is 32 or 64 bit (whatever that
might mean), then you have something that is almost certainly
not portable C.
--
Programming is what happens while you're busy making other plans.
Apr 10 '07 #4
On 10 Apr, 20:23, "broeisi" <broeis...@gmai l.comwrote:

<snip>
But how does an OS like linux or windows know that it's installed on a
computer with a 32 or 64 bit processor?
Normally you tell this sort of information when you compile it. So
you'll compile different binaries for 32 bit and 64 bit.

This is an operating system, though. *Usually*, a well-written app
doesn't really need to know whether it's running on 32 or 64 bit.

Hope that helps,
Doug

Apr 10 '07 #5

"broeisi" <br*******@gmai l.comwrote in message
news:11******** **************@ b75g2000hsg.goo glegroups.com.. .
Hello,

Is there a way in C to get information at runtime if a processor is 32
or 64 bit?

Cheers,
int is the natural integer size for the machine. CHAR_BIT gives the number
of bits in a byte.
So printf("%d-bit processor\n", sizeof(int) * CHAR_BIT);
should tell you whether you are dealing with a processor that handles 64-bit
values nicely or an inferior machine.
--
Free games and programming goodies.
http://www.personal.leeds.ac.uk/~bgy1mm

Apr 10 '07 #6
broeisi wrote, On 10/04/07 19:46:
Hello,

Is there a way in C to get information at runtime if a processor is 32
or 64 bit?
Is that really what you want to know? Depending on what your real
problem is a combination of the information in limits.h and the result
of the sizeof operator should help.
--
Flash Gordon
Apr 10 '07 #7
"broeisi" <br*******@gmai l.comha scritto nel messaggio
news:11******** **************@ b75g2000hsg.goo glegroups.com.. .
Hello,

Is there a way in C to get information at runtime if a processor is 32
or 64 bit?
Nothing guarantees it to always work, but the standard says "A ''plain'' int
object has the natural size suggested by the architecture of the execution
environment", so CHAR_BIT*sizeof (int) is likely to do that (and when it
isn't, it is very likely to be more useful than the actual processor bits in
a C program).
Apr 10 '07 #8


On 10 apr, 21:07, rober...@ibd.nr c-cnrc.gc.ca (Walter Roberson) wrote:
In article <1176230815.876 762.147...@b75g 2000hsg.googleg roups.com>,

broeisi <broeis...@gmai l.comwrote:
Is there a way in C to get information at runtime if a processor is 32
or 64 bit?

No, not in standard C.

First you'd have to define exactly what it means for a processor
to be 32 or 64 bit, which is something that has at least 4 different
correct (but contradictory) answers.
--
I was very young in those days, but I was also rather dim.
-- Christopher Priest

Thank you very much for you answer Christopher.
But how does an OS like linux or windows know that it's installed on a
computer with a 32 or 64 bit processor?
Apr 10 '07 #9
On 10 apr, 21:30, Flash Gordon <s...@flash-gordon.me.ukwro te:
broeisi wrote, On 10/04/07 19:46:
Hello,
Is there a way in C to get information at runtime if a processor is 32
or 64 bit?

Is that really what you want to know? Depending on what your real
problem is a combination of the information in limits.h and the result
of the sizeof operator should help.
--
Flash Gordon

Flash Gordon,

Yes, that's really what I want to know.
Just trying to learn C by writing lots of silly little programs that
make sense to me..

I think that the answer given by Malcolm is a good one.

Thanks you all for your answers....

Cheers,

Broeisi

Apr 10 '07 #10

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