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Linux nasm square root algorithm poblem

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

joghurt

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.

SaumZ

ASKER

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.

joghurt

"jnc", sorry.

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joghurt

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SaumZ

ASKER

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.