How to use floating point numbers in x86-64 assembly?

Question

I'm struggling with some x86-64 assembly, and floating point numbers are giving me a headache. For instance, when I run this code :

section .data
     omega:  dq 2.0
     omega2: dq 3.0

section .text
global func

func: push rbp
      mov rgp, rsp

      FINIT

      FLD qword [omega]
      FLD qword [omega2]
      FADD st0, st0

      mov rsp, rbp
      pop rbp
      ret

This function is called from a C code like that : printf("%Lf \n", func() ); Unfortunately the result is some bizarre number... I tried adding two integers using FIADD, and it worked fine. I dug through a ton of material already, but maybe someone here can point me to decent FPU tutorial, or share her/his experience and wisdom :)

Wrapping up the essentials:

• language: x86-64 assembler
• assembler: nasm v. 2.09.04 installed from repositories
• compiler (for C): gcc v. 4.5.2 (installed with Ubuntu)
• OS: Ubuntu 11.04 64bit on Oracle VM
• Host OS: Windows 7 SP1 64bit
• Processor: Intel i5 - 2430M 64bit (checked twice :D )
• Problem: FPU can't add two numbers :(

Just in case : in the end i hope to use FSINCOS and other fancy FPU instructions, but seeing as even simple addition fails...

Thanks all in advance!

Answer 1

Ok, so in the end it turned out that my problems were related to the fact, that FPU registers are organized as a stack, and i didn't pay enough attention to it which resulted in garbage and unwanted leftovers. Switching from "standard" to "pop" versions of instructions helped!

Anyway - thanks to all who bothered to read, it is much appreciated! :)

If anyone is interested - I was doing a class assignment where I had to compute motion of one planet from the view of another (geocentric model) in assembly. The final result translated to processing can be viewed here.

Answer 2

The L qualifier to printf %f makes that argument a long double (80-bit float), which is not the assembly data type. Remove the L and it will default to a double (64-bit float) which is what you are computing.

Also, it should be

  FADD st1, st0

to add the two numbers together. Otherwise it will double the second value.