Comments by Charles Keepax (Top 4 by date)

Your variables r0-r4 are effectively arrays of 4 16-bit numbers; a 16-bit number can only be 0 to 65536 or -32768 to 32767 since you are using signed variables. 6,000,000 is too large to store in a 16-bit number, thus you will need to extract it into a 32-bit number. The following code (assuming int is 32-bit on your system) does this with the first number however it is almost certainly a poor way to use the MMX extensions, but the best way to use them would depend rather heavily on context.

unsigned short B[4]={2000,2000,2000,2000};
unsigned short C[4]={3000,3000,3000,3000};
unsigned short D[4]={0,0,0,0};
unsigned short E[4]={0,0,0,0};
__m64*b =(__m64*)B;
__m64*c =(__m64*)C;
__m64*d =(__m64*)D;
__m64*e =(__m64*)E;

*d =_mm_mullo_pi16(*b,*c);
*e =_mm_mulhi_pi16(*b,*c);

int temp = E[0];
temp = (temp<<16) | D[0];