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Full Discussion: C multiplication with GCC
Top Forums Programming C multiplication with GCC Post 302544652 by Corona688 on Thursday 4th of August 2011 12:33:38 PM
Old 08-04-2011
How many bits is your architecture?

---------- Post updated at 10:25 AM ---------- Previous update was at 10:06 AM ----------

Researching this a bit, gcc asm doesn't have MUL/MULL, differentiating between the two based on their source operands. Or rather, it does have MULL, but if you use it it's VERY picky about which types it'll accept. Trying to figure out how to work this...

---------- Post updated at 10:33 AM ---------- Previous update was at 10:25 AM ----------

Here is how you get a 64-bit result from a 32-bit multiply in x86 gcc:
Code:
#include <stdio.h>
#include <limits.h>

int main(void)
{
        unsigned long a=INT_MAX,b=INT_MAX-1;

        printf("a=%lu b=%lu\n", a, b);

        // Need to use the special one-operand mul instruction, which
        // dumps its result in eax:edx.
        asm( "mul %1"
                : "=a"(a), "=d"(b) /* output registers eax, edx */
                : "a"(a), "r"(b) /* input registers eax, any */ );

        printf("a=%lu b=%lu\n", a, b);
}

Hope this helps.
 

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LEARN ABOUT DEBIAN

cpu_capabilities

cpu_capabilities(3alleg4)					  Allegro manual					 cpu_capabilities(3alleg4)

NAME

       cpu_capabilities - Contains the capability flags of the CPU. Allegro game programming library.

SYNOPSIS

       #include <allegro.h>

       extern int cpu_capabilities;

DESCRIPTION

       Contains CPU flags indicating what features are available on the current CPU. The flags can be any combination of these:

	  CPU_ID       - Indicates that the "cpuid" instruction is
			 available. If this is set, then all Allegro CPU
			 variables are 100% reliable, otherwise there
			 may be some mistakes.
	  CPU_FPU      - An FPU is available.
	  CPU_IA64     - Running on Intel 64 bit CPU
	  CPU_AMD64    - Running on AMD 64 bit CPU
	  CPU_MMX      - Intel MMX  instruction set is available.
	  CPU_MMXPLUS  - Intel MMX+ instruction set is available.
	  CPU_SSE      - Intel SSE  instruction set is available.
	  CPU_SSE2     - Intel SSE2 instruction set is available.
	  CPU_SSE3     - Intel SSE3 instruction set is available.
	  CPU_3DNOW    - AMD 3DNow! instruction set is available.
	  CPU_ENH3DNOW - AMD Enhanced 3DNow! instruction set is
			 available.
	  CPU_CMOV     - Pentium Pro "cmov" instruction is available.

       You  can check for multiple features by OR-ing the flags together.  For example, to check if the CPU has an FPU and MMX instructions avail-
       able, you'd do:

	  if ((cpu_capabilities & (CPU_FPU | CPU_MMX)) ==
	      (CPU_FPU | CPU_MMX)) {
	     printf("CPU has both an FPU and MMX instructions!
");
	  }

       You can read this variable after you have called check_cpu() (which is automatically called by allegro_init()).

SEE ALSO

       check_cpu(3alleg4), cpu_vendor(3alleg4), cpu_family(3alleg4), cpu_model(3alleg4), cpu_capabilities(3alleg4), allegro_init(3alleg4)

Allegro 							   version 4.4.2					 cpu_capabilities(3alleg4)
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