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 Implem

Answer 1

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 2

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.

Answer 3

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.