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The latest version of this topic can be found at __mul128.

Microsoft Specific**

Multiplies two 64-bit integers passed in as the first two arguments and puts the high 64 bits of the product in the 64-bit integer pointed to by HighProduct and returns the low 64 bits of the product.

__int64 _mul128(   
   __int64 Multiplier,   
   __int64 Multiplicand,   
   __int64 *HighProduct   


[in] Multiplier
The first 64-bit integer to multiply.

[in] Multiplicand
The second 64-bit integer to multiply.

[out] HighProduct
The high 64 bits of the product.

The low 64 bits of the product.


Header file <intrin.h>

// mul128.c  
// processor: x64  
#include <stdio.h>  
#include <intrin.h>  
#pragma intrinsic(_mul128)  
int main()  
    __int64 a = 0x0fffffffffffffffI64;  
    __int64 b = 0xf0000000I64;  
    __int64 c, d;  
    d = _mul128(a, b, &c);  
    printf_s("%#I64x * %#I64x = %#I64x%I64x\n", a, b, c, d);  

0xfffffffffffffff * 0xf0000000 = 0xeffffffffffffff10000000  

Compiler Intrinsics