What is “=qm” in extended assembler

Question

I was looking through an Intel provided reference implementation of RDRAND instruction. The page is Intel Digital Random Number Generator (DRNG) Software Implementation Guide , and the code came from Intel Digital Random Number Generator software code examples .

The following is the relevant portion from Intel. It reads a random value and places it in val , and it sets the carry flag on success.

char rc;
unsigned int val;

__asm__ volatile(
    "rdrand %0 ; setc %1"
    : "=r" (val), "=qm" (rc)
);

// 1 = success, 0 = underflow
if(rc) {
    // use val
    ...
}

Soory to have to ask. I don't think it was covered in GNU Extended Assembler , and searching for "=qm" is producing spurious hits.

What does the "=qm" mean in the extended assembler?

Answer 1

What you're looking at is an inline assembler constraint . The GCC documentation is at 6.47.3.1 Simple Constraints and 6.47.3.4 Constraints for Particular Machines under x86 family section. This one ( =qm ) combines three flags which indicate:

• = : The operand is write-only - its previous value is not relevant.
• q : The operand must be in register a , b , c , or d (it cannot be in esi , for instance).
• m : The operand may be placed in memory.

Answer 2

qm probably means 1 byte 8 bit mem =qm will be valid constraint for storing 1 byte result See what setc wants

http://web.itu.edu.tr/~aydineb/index_files/instr/setc.html

reg8 and mem8

as we know only eax , ebx edx ecx .. a,b,c,d registers that q refer can be used cause they can accessed with low byte al dl cl ...With combining qm we are getting mem8 . m meant memory. Thats what I meant

Answer 3

Wow that stumped me at first but I searched around a bit and found out that it is a reference to the model of the processor this peice of code is meant for.

Spicically I read that it is for the i7 Quadcore.

Is that where you got this code from?

It is a simple value indicator for a variable syntax.