Printing a Float number on the screen

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White-spirit

Joined: 26 Mar 2008
Posts: 27

White-spirit 08 Apr 2008, 13:41

Hello, i've writted a little code in Asm for my OS pour display a Float number on the screen, but it displays for example 28.0710391998291015625 instead of 28.07104 Confused

, here's my code :

Code:

format elf

public ftol
public print_float
extrn print
extrn putchar
extrn itoa

half dd 0.5
i dd ?
tmp dd ?
e dd ?

ftol:
fld [half]
fld dword [esp+4]
fsub st0, st1
fld dword [esp+4]
faddp st2, st0
fistp [i]
mov eax, [i]
fistp [i]
add eax, [i]
sar eax, 1
ret

print_float2:
push ebp
mov ebp, esp
mov ebx, dword [esp+8]
mov [tmp], ebx
push [tmp]
call ftol
add esp, 4
mov [e], eax
push 0
push 0
push 10
push [e]
call itoa
add esp, 16
push eax
call print
add esp, 4
push '.'
call putchar
add esp, 4
fld [tmp]
fisub [e]
fst [tmp]
.do:
push 10
fimul dword [esp]
fst [tmp]
push [tmp]
call ftol
add esp, 4
mov [e], eax
push 0
push 0
push 10
push [e]
call itoa
add esp, 16
push eax
call print
add esp, 4
fisub [e]
fst [tmp]
ftst
fstsw ax
fwait
sahf
jnz .do
leave
ret

Thanks for help Smile

08 Apr 2008, 13:41

AlexP

Joined: 14 Nov 2007
Posts: 561
Location: Out the window. Yes, that one.

AlexP 08 Apr 2008, 21:12

Use a rounding function, I've recently used one in a high-level language course in school but I'm afraid I cannot provide it to you.

08 Apr 2008, 21:12

sakeniwefu

Joined: 23 Mar 2008
Posts: 29

sakeniwefu 09 Apr 2008, 00:37

Wait I realized I was stupid. Learn to do it in float.
OK. adding +-(5/10^precision+1) and reading it up to precision should round it arithmetically, but it doesn't look optimal. I just cannot find any web or source that will explain how to round floats without libraries or by hand.
Or just round down.

Last edited by sakeniwefu on 09 Apr 2008, 01:11; edited 3 times in total

09 Apr 2008, 00:37

edfed

Joined: 20 Feb 2006
Posts: 4314
Location: Now

edfed 09 Apr 2008, 00:42

you can also try to convert from the binary image of the floating point value with the needed decimals.
decode it and dispaly it without any FP instructions. it will free the FPU that is supposed to calculate other things at the same time.

and better, it will be able to convert any FP in any format with only the general regisers.

09 Apr 2008, 00:42

AlexP

Joined: 14 Nov 2007
Posts: 561
Location: Out the window. Yes, that one.

AlexP 09 Apr 2008, 12:55

Sakeniwefu found my idea Smile

09 Apr 2008, 12:55

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