Re: what is wrong with this little piece of code?

From: Akyl Tulegenov (pcxast_at_nottingham.ac.uk)
Date: 02/01/04 

Date: Sun, 1 Feb 2004 03:04:21 +0000 (UTC)

Thanks a lot!

It is not an assignment:) I am thinking about writing a set low level
maths routines. Your references are extremely helpful!

Akyl.
On Sun, 1 Feb 2004, Frank Kotler wrote:

> Akyl Tulegenov wrote:
>
> > Dear All!
> > What's wrong with this little piece of code? I am simply trying to print
> > out the value of pi variable, but ain't getting any output!
> >
> > section .text
> > global _start
> > _start:
> > mov eax, 4
> > mov ebx, 1
> > mov ecx, pi
>
> What happened to edx? Sys_write wants the length in edx. If you call it
> with some random value in edx, perhaps zero, you certainly won't get the
> results you expect.
>
> > int 0x80
> > mov eax, 1
> > mov ebx, 0
> > int 0x80
> > section .data
> > pi: dq 1.0
>
> Nasm converts your "dq 1.0" into IEEE<some number I forget> floating
> point notation. Eight bytes, but they appear as pretty much "garbage" -
> 1.0 is 00 00 00 00 00 F0 3F. Mostly not printable characters (3F is
> "?"). You could print these as hex values fairly easily, but if you want
> to see "1.0" out of it (pretty inaccurate value for pi :), you've got
> some processing to do. If you're allowed to call libraries, "printf" or
> "ftoa" are your friends! If you're expected to write your own "ftoa",
> the "fbstp" instruction stores the *integer* part of the float in st(0)
> into a ten-byte buffer - nine bytes of packed BCD (each nibble is a
> decimal digit - still needs to be converted to an ascii character), and
> a "sign byte", 80h if it's negative, else 0. Not too bad to get a
> printable string out of that.
>
> Nice floating-point tutorial by Raymond Filiatreault here:
>
> http://www.masmforum.com/website/tutorials/fptute/index.html
>
> If that's more than you wanted to know, "just call printf". I'm usually
> the last guy who would tell you "just use a library" (okay, next-to-last
> after Betov :), but unless the "assignment" is to write your own "ftoa",
> you don't wanna mess with it.
>
> Best,
> Frank
>
~>
>
>
>