Want to win a PS4? Go Premium and enter to win our High-Tech Treats giveaway. Enter to Win

x
?
Solved

Linux nasm square root algorithm poblem

Posted on 2004-04-07
5
Medium Priority
?
544 Views
Last Modified: 2008-02-01
I am programming with the FPU under linux and the NASM assembler and linking using gcc

Here is the algorithm I am implementing:

take in x from the keyboard

 Initialize x_new = x
 do
     x_old = x_new;
     x_new = (x_old + x / x_old) / 2;
   while |x_old - x_new|/x_new > e


I am having the problem with my compare and my conditional jump.
I have tested the values of the algorthm throughout and the values are correct to what I worked out on paper.
;------------------------------------------------------------------------------
extern  printf
extern  scanf
 
global main
 
segment .data
prompt: db "Please enter a number: ",0
format: db "%lf", 0
output: db "%lf",10, 0
 
epsilon dq  0.0001
x2:     dq  2.0
 
segment .bss
x:      resq 1
x_new:  resq 1
x_old:   resq 1
x_temp:  resq 1
segment .text
main:
        pusha
 
        push prompt
        call printf
        add esp, 4
 
        push x
        push format
        call scanf
        add esp, 8
 
        fld qword [x]               ;fstack: x
        fstp qword [x_new]      ;fstack:
                                          ;x_new = x
.L1:    finit
        fld qword [x_new]       ;fstack: x_new
        fstp qword [x_old]       ;fstack:
                                         ;x_old = x_new
        finit                            ;clear the stack
                                         ;now lets make x_new = (x_old + (x/x_old))/2
        fld qword [x]              ;fstack: x
        fdivr qword [x_old]     ;fstack: x/x_old
        fadd qword [x_old]      ;fstack:(x_old + (x/x_old))
        fdiv qword [x2]           ;fstack:(x_old + (x/x_old))/2
        fstp qword [x_new]      ;fstack:
 
        finit                            ;clear the stack
                                          ;now lets make |x_old - x_new|/x_new
        fld qword [x_old]         ;fstack: x_old
        fsub qword [x_new]     ;fstack: x_old - x_new
        fabs                            ;fstack: |x_old - x_new|
        fdiv qword [x_new]      ;fstack: |x_old - x_new|/x_new
 
        fld qword [epsilon]     ;fstack: epsilon, |x_old - x_new|/x_new
 ;PROBLEM IS WITH THE COMPARE AND JUMP HERE
        fcomi st1                       ;compare ST0 with ST1
        jl .L1
 
        finit
        push dword [x_new + 4]   ;outputting x_new after loop
        push dword [x_new]
        push output
        call printf
        add esp, 12

        popa
        mov eax, 0
        ret
0
Comment
Question by:SaumZ
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 3
  • 2
5 Comments
 
LVL 6

Expert Comment

by:joghurt
ID: 10784584
I believe you have a reason for not using FSQRT instead.

Well, by definition "jl" jumps if "SF <> OF".
However, fcomi sets the following flags: ZF, PF, CF.
So change jl to jc. Voila.
0
 

Author Comment

by:SaumZ
ID: 10784909
I tried the jump with jc, and my code is still only doing one iteration. and outputting for example:

Please enter a number: 20
10.500000

This number is correct, but only after one iteration.
this is the output that is at the bottom. of my code, where it outputs x_new

I also tested the number that I have from |x_old - x_new|/x_new
by putting it in a temp variable and outputting it and comparing to what I calculated using pencil and paper, and that is correct also.

Maybe the problem lies somewhere else? I am not sure at this point.

The reason for not just using fsqrt is that I am just learning the fpu programming, and I am trying to get myself used to programming in it by using different instructions with it.  

So, I may have logic errors someplace else in my code, but to me, it all looks correct.
0
 
LVL 6

Expert Comment

by:joghurt
ID: 10786189
"jnc", sorry.
0
 
LVL 6

Accepted Solution

by:
joghurt earned 500 total points
ID: 10786259
Well, another approach. There were too many finit's even for the first sight so I've tried to reorganize the things a bit.

.L1:
fld         qword ptr [x_new]
fstp        qword ptr [x_old]
fld         qword ptr [x]
fdiv        qword ptr [x_old]
fadd        qword ptr [x_old]
fdiv        qword ptr [x2]
fstp        qword ptr [x_new]
fld         qword ptr [x_old]
fsub        qword ptr [x_new]
fabs
fdiv        qword ptr [x_new]
fcomp       qword ptr [epsilon]
fnstsw      ax  
test        ah,41h
je          .L1

I haven't used fcomi because it won't run on older machines. (And because my development environment couldn't compile P6 instructions. :-)
But you can give it a try.
0
 

Author Comment

by:SaumZ
ID: 10810023
My whole problem in the task was a fdivr in place of a fdiv in my origional code...
I will give you the points, thank you for your help.
0

Featured Post

Free Tool: ZipGrep

ZipGrep is a utility that can list and search zip (.war, .ear, .jar, etc) archives for text patterns, without the need to extract the archive's contents.

One of a set of tools we're offering as a way to say thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

An overview of cyber security, cyber crime, and personal protection against hackers. Includes a brief summary of the Equifax breach and why everyone should be aware of it. Other subjects include: how cyber security has failed to advance with technol…
As much as Microsoft wants to kill off PST file support, just as they tried to do with public folders, there are still times when it is useful or downright necessary to export Exchange mailboxes to PST files. Thankfully, it is still possible to e…
In this video, Percona Director of Solution Engineering Jon Tobin discusses the function and features of Percona Server for MongoDB. How Percona can help Percona can help you determine if Percona Server for MongoDB is the right solution for …
Please read the paragraph below before following the instructions in the video — there are important caveats in the paragraph that I did not mention in the video. If your PaperPort 12 or PaperPort 14 is failing to start, or crashing, or hanging, …
Suggested Courses

610 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question