Question

I’m trying to understand this code:

inline SInt32 smul32by16(SInt32 i32, SInt16 i16)
{
    register SInt32 r;
    asm volatile("smulwb %0, %1, %2" : "=r"(r) : "r"(i32), "r"(i16));
    return r;
}

Does anybody know what this assembly instruction does?

Update:
P.S. I use objective C. and I should understand some code from assembly. That’s why it’s difficult for me to understand this code.

Answer 1

It does signed 32 bit by signed 16 bit multiplication and returns the top 32 bit of 48 bit result. The b specifies to use the bottom 16bit of the third operand.

So, translating it into pseudo code:

int_48 temp;
temp = i32*i16;
result = temp >> 16;