flat assembler - Float to string

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manfred

Joined: 28 Feb 2009
Posts: 43
Location: Racibórz, Poland

manfred 04 Mar 2009, 17:04

Hello!

I wrote function, which prints double precision number to stdout with using FPU (w/o FPU is probably too hard for me now), but it gives strange results like that:

Quote:

manfred@manfred-desktop:/media/Projekty/asm/foo$ ./foo
10000.0
manfred@manfred-desktop:/media/Projekty/asm/foo$ ./foo
1.0
manfred@manfred-desktop:/media/Projekty/asm/foo$ ./foo
10000.0
manfred@manfred-desktop:/media/Projekty/asm/foo$ ./foo
1.0
manfred@manfred-desktop:/media/Projekty/asm/foo$ ./foo
1.0
manfred@manfred-desktop:/media/Projekty/asm/foo$ ./foo
1.0

program code (but I think there is no bugs):

Code:

format ELF executable
entry _start
segment readable executable

include './manfred.inc'

_start:
  
  stdcall printDouble, wynik
  exit 0 ;macro
  
segment readable writable
  wynik dq 10000.0

And, the probably most important thing - function:

Code:

printDouble:
  push ebp
  mov ebp, esp
  sub esp, 5112
  digits equ ebp - 5112
  fpustate equ ebp - 112
  ten equ ebp - 4
  temp equ ebp - 2
  pushf
  push esi
  push ebx
  fsave [fpustate]
  
  xor esi, esi
  mov word [ten], 10
  fnstcw [temp]
  or word [temp], (0Ch shl 8) ;rounding: cut
  fldcw [temp]
  
  mov eax, [ebp + 8]
  fld qword [eax] ;fpu registers: number
  
  ftst
  fstsw ax
  and ah, 01000101b
  jz .printDouble_ok
  cmp ah, 01000101b
  jnz .printDouble_check
  jmp .printDouble_error
  
  .printDouble_zero:
    push "0.0"
    mov eax, 4
    mov ebx, 1
    mov ecx, esp
    mov edx, 3
    int 80h
    jmp .printDouble_end
  
  .printDouble_check:
    cmp ah, 01000000b
    jz .printDouble_zero
    cmp ah, 1
    jnz .printDouble_error
    fchs
    push "-"
    call putc
  
  .printDouble_ok:
    fild word [ten] ;10, number
    fld st1 ;number, 10, number
    frndint ;integer part, 10, number
    fsub st2, st0 ;integer, 10, fractional
    align 16
    .printDouble_div10:
      fxch st1 ;10, integer, fractional
      fld st1 ;integer, 10, integer, fractional
      fprem ;integer mod 10, 10, integer, fractional
      fistp word [digits + esi] ;10, integer, fractional
      fxch st1 ;integer, 10, fractional
      fdiv st0, st1
      frndint ;integer(integer / 10), 10, fractional
      inc esi
      ftst
      fstsw ax
      sahf
      jnz .printDouble_div10
    fstp st0 ;10, fractional
    fxch st1 ;fractional, 10
    .printDouble_print_i:
      test esi, esi
      jz .printDouble_print_i_end
      dec esi
      mov al, [digits + esi]
      or al, "0"
      push eax
      call putc
      inc ch
      cmp ch, 80
      jbe .printDouble_print_i      
    .printDouble_print_i_end:
      push "."
      call putc
      xor ch, ch
    .printDouble_print_f:
      fmul st0, st1 ;fractional*10, 10
      fist word [temp]
      fstsw ax
      and al, 18h
      jnz .printDouble_error
      mov al, [temp]
      or al, "0"
      push eax
      call putc
      inc ch
      fild word [temp]
      fsubp st1, st0
      ftst 
      fstsw ax
      sahf
      jz .printDouble_end
      cmp ch, 80
      jae .printDouble_end
      jmp .printDouble_print_f
    
  .printDouble_error:
    push "r"
    push "Erro"
    mov eax, 4
    mov ebx, 1
    mov ecx, esp
    mov edx, 5
    int 80h
  
  .printDouble_end:
  frstor [fpustate]
  pop ebx
  pop esi
  popf  
  restore temp
  restore ten
  restore fpustate
  restore digits  
  mov esp, ebp
  pop ebp
  ret 4

So, where is that bug? I can't see any bug...

Thanks in advance,
Manfred Steinhauer

EDIT: For 6.9 it prints 6.9 or 6.9000000000000003552713678800500929355621337890625 (what is correct), for 666.999 - 6.9 or 666.999000000000023646862246096134185791015625, 321.69 - 3.6 or 321.68999999999999772626324556767940521240234375. So, if result is wrong, it prints first digit of integer (integral?) part, and first digit of fractional part. What the f**k?

_________________
Sorry for my English...

04 Mar 2009, 17:04

comrade

Joined: 16 Jun 2003
Posts: 1150
Location: Russian Federation

comrade 04 Mar 2009, 20:04

Some decimal numbers cannot be presented exactly in binary floating-point. See here. The Windows Calculator has special logic to correct for that.

04 Mar 2009, 20:04

manfred

Joined: 28 Feb 2009
Posts: 43
Location: Racibórz, Poland

manfred 04 Mar 2009, 21:23

You probably misunderstood me. I said this function randomly prints correct value, or only first digits of integer and fractional part for same number in same executable. About 35% of program runs produces good results (eg. 666.999000...) and 65% gives only that two digits (6.9 in example with 666.999)...

04 Mar 2009, 21:23

comrade

Joined: 16 Jun 2003
Posts: 1150
Location: Russian Federation

comrade 04 Mar 2009, 22:16

Ah, yes, I misread.

Can you factor out the int 80h syscall into putchar or something like that, so I can test this on Windows? I see you already have a putc, so what are those other int 80h for?

04 Mar 2009, 22:16

manfred

Joined: 28 Feb 2009
Posts: 43
Location: Racibórz, Poland

manfred 04 Mar 2009, 22:26

Int 80h with eax = 4 (sys_write) and ebx = 1 (stdout) is write_to_stdout(ecx = pointer to data, edx = length). I've implemented my own print function, but I can't use it here (in final code I will replace they by print calls, if I discover how can I pass pointer to string built on stack).

04 Mar 2009, 22:26

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20299
Location: In your JS exploiting you and your system

revolution 04 Mar 2009, 23:01

Try inserting an finit at the start. Maybe the precision is not set by the OS before starting your app.

04 Mar 2009, 23:01

comrade

Joined: 16 Jun 2003
Posts: 1150
Location: Russian Federation

comrade 04 Mar 2009, 23:22

Shouldn't you be cleaning up the stack after you call putc/int80h?

I refactored your program for Windows, seems to work fine:

Code:

;##########################################################################
; printDouble
; 04.03.2008
;##########################################################################
format PE console 4.0
entry start
;##########################################################################
_TITLE          equ  "printDouble"
_NAME           equ  "printDouble"
_VERSION        equ  "0.0"
_VERSIONTEXT    equ  _VERSION
;##########################################################################
include "%include%/win32am.inc"
include "%include%/equates/kernel32.inc"
include "%include%/equates/user32.inc"
include "%include%/macro/if.inc"
include "%include%/macro/macros.inc"
OFFSET equ
;##########################################################################
;##########################################################################
;##########################################################################
section ".code" code readable executable
;##########################################################################
start:
;##########################################################################
;##########################################################################
        push    ebx esi edi

        stdcall [GetStdHandle],STD_OUTPUT_HANDLE
        mov     [hStdOut],eax

        stdcall printDouble,OFFSET wynik

        pop     edi esi ebx
        stdcall [ExitProcess],0
;##########################################################################
printDouble:
  push ebp 
  mov ebp, esp 
  sub esp, 5112 
  digits equ ebp - 5112 
  fpustate equ ebp - 112 
  ten equ ebp - 4 
  temp equ ebp - 2 
  pushf 
  push esi 
  push ebx 
  fsave [fpustate] 
   
  xor esi, esi 
  mov word [ten], 10 
  fnstcw [temp] 
  or word [temp], (0Ch shl 8) ;rounding: cut 
  fldcw [temp] 
   
  mov eax, [ebp + 8] 
  fld qword [eax] ;fpu registers: number 
   
  ftst 
  fstsw ax 
  and ah, 01000101b 
  jz .printDouble_ok 
  cmp ah, 01000101b 
  jnz .printDouble_check 
  jmp .printDouble_error 
   
  .printDouble_zero: 
    push "0.0" 
    mov eax, 4 
    mov ebx, 1 
    mov ecx, esp 
    mov edx, 3 
    call int80h
    add esp,4
    jmp .printDouble_end 
   
  .printDouble_check: 
    cmp ah, 01000000b 
    jz .printDouble_zero 
    cmp ah, 1 
    jnz .printDouble_error 
    fchs 
    push "-" 
    call putc 
   
  .printDouble_ok: 
    fild word [ten] ;10, number 
    fld st1 ;number, 10, number 
    frndint ;integer part, 10, number 
    fsub st2, st0 ;integer, 10, fractional 
    align 16 
    .printDouble_div10: 
      fxch st1 ;10, integer, fractional 
      fld st1 ;integer, 10, integer, fractional 
      fprem ;integer mod 10, 10, integer, fractional 
      fistp word [digits + esi] ;10, integer, fractional 
      fxch st1 ;integer, 10, fractional 
      fdiv st0, st1 
      frndint ;integer(integer / 10), 10, fractional 
      inc esi 
      ftst 
      fstsw ax 
      sahf 
      jnz .printDouble_div10 
    fstp st0 ;10, fractional 
    fxch st1 ;fractional, 10 
    .printDouble_print_i: 
      test esi, esi 
      jz .printDouble_print_i_end 
      dec esi 
      mov al, [digits + esi] 
      or al, "0" 
      push eax 
      call putc 
      inc ch 
      cmp ch, 80 
      jbe .printDouble_print_i       
    .printDouble_print_i_end: 
      push "." 
      call putc 
      xor ch, ch 
    .printDouble_print_f: 
      fmul st0, st1 ;fractional*10, 10 
      fist word [temp] 
      fstsw ax 
      and al, 18h 
      jnz .printDouble_error 
      mov al, [temp] 
      or al, "0" 
      push eax 
      call putc 
      inc ch 
      fild word [temp] 
      fsubp st1, st0 
      ftst  
      fstsw ax 
      sahf 
      jz .printDouble_end 
      cmp ch, 80 
      jae .printDouble_end 
      jmp .printDouble_print_f 
     
  .printDouble_error: 
    push "r" 
    push "Erro" 
    mov eax, 4 
    mov ebx, 1 
    mov ecx, esp 
    mov edx, 5 
    call int80h
    add esp,8
   
  .printDouble_end: 
  frstor [fpustate] 
  pop ebx 
  pop esi 
  popf   
  restore temp 
  restore ten 
  restore fpustate 
  restore digits   
  mov esp, ebp 
  pop ebp 
  ret 4
;##########################################################################
putc:
        lea     eax,[esp+4]
        push    eax ecx edx
        stdcall [WriteConsole],[hStdOut],eax,1,esp,0
        pop     edx ecx eax
        retn    4
;##########################################################################
int80h:
        cmp     eax,4   ; sys_write
        jne     .end
        cmp     ebx,1   ; stdout
        jne     .end
        push    eax ecx edx
        stdcall [WriteConsole],[hStdOut],ecx,edx,esp,0
        pop     edx ecx eax
.end:
        retn
;##########################################################################
;##########################################################################
;##########################################################################
;##########################################################################
section ".data" data readable writeable
    data import
        library kernel32,"kernel32.dll",user32,"user32.dll"
        include "%include%/api/kernel32.inc"
        include "%include%/api/user32.inc"
    end data
;##########################################################################
;##########################################################################
        ; initialized data
        wynik           dq      699.0
;##########################################################################
        ; uninitialized data
        hStdOut         dd      ?
        argc            dd      ?
        argv            rd      16
;##########################################################################
;##########################################################################
;##########################################################################

04 Mar 2009, 23:22

manfred

Joined: 28 Feb 2009
Posts: 43
Location: Racibórz, Poland

manfred 05 Mar 2009, 06:52

No. Putc is my function and it's calling convention is stdcall. After syscalls I must only destroy string (ouch, I'm not doing it! base register, source index and flags content is destroyed!).

EDIT: Aarrgh! I forgot one fact - functions do not preserve ecx, but I used ch as counter in printing loops. Argh.
Good code:

Code:

_printFloatingPoint:
  push ebp
  mov ebp, esp
  sub esp, 5112
  digits equ ebp - 5112
  fpustate equ ebp - 112
  ten equ ebp - 4
  temp equ ebp - 2
  pushf
  push esi
  push ebx
  fsave [fpustate]
  
  xor esi, esi
  mov word [ten], 10
  finit
  fnstcw [temp]
  or word [temp], (0Ch shl 8) ;rounding: cut
  fldcw [temp]
  
  cmp dword [ebp + 12], 64
  ja ._printFloatingPoint_ld80
  jb ._printFloatingPoint_ld32
  
  mov eax, [ebp + 8]
  fld qword [eax] ;fpu registers: number
  jmp ._printFloatingPoint_start
  
  ._printFloatingPoint_ld80:
  mov eax, [ebp + 8]
  fld tword [eax] ;fpu registers: number
  jmp ._printFloatingPoint_start
  
  ._printFloatingPoint_ld32:
  mov eax, [ebp + 8]
  fld dword [eax] ;fpu registers: number
  
  ._printFloatingPoint_start:
  ftst
  fstsw ax
  and ah, 01000101b
  jz ._printFloatingPoint_ok
  cmp ah, 01000101b
  jnz ._printFloatingPoint_check
  jmp ._printFloatingPoint_error
  
  ._printFloatingPoint_zero:
    push "0.0"
    push 3
    lea eax, [esp + 4]
    push eax
    call print
    add esp, 4
    jmp ._printFloatingPoint_end
  
  ._printFloatingPoint_check:
    cmp ah, 01000000b
    jz ._printFloatingPoint_zero
    cmp ah, 1
    jnz ._printFloatingPoint_error
    fchs
    push "-"
    call putc
  
  ._printFloatingPoint_ok:
    fild word [ten] ;10, number
    fld st1 ;number, 10, number
    frndint ;integer part, 10, number
    fsub st2, st0 ;integer, 10, fractional
    align 16
    ._printFloatingPoint_div10:
      fxch st1 ;10, integer, fractional
      fld st1 ;integer, 10, integer, fractional
      fprem ;integer mod 10, 10, integer, fractional
      fistp word [digits + esi] ;10, integer, fractional
      fxch st1 ;integer, 10, fractional
      fdiv st0, st1
      frndint ;integer(integer / 10), 10, fractional
      inc esi
      ftst
      fstsw ax
      sahf
      jnz ._printFloatingPoint_div10
    fstp st0 ;10, fractional
    fxch st1 ;fractional, 10
    xor bh, bh
    ._printFloatingPoint_print_i:
      test esi, esi
      jz ._printFloatingPoint_print_i_end
      dec esi
      mov al, [digits + esi]
      or al, "0"
      push eax
      call putc
      inc bh
      cmp bh, 80
      jbe ._printFloatingPoint_print_i      
    ._printFloatingPoint_print_i_end:
      push "."
      call putc
      xor bh, bh
    ._printFloatingPoint_print_f:
      fmul st0, st1 ;fractional*10, 10
      fist word [temp]
      fstsw ax
      and al, 18h
      jnz ._printFloatingPoint_error
      mov al, [temp]
      or al, "0"
      push eax
      call putc
      inc bh
      fild word [temp]
      fsubp st1, st0
      ftst 
      fstsw ax
      sahf
      jz ._printFloatingPoint_end
      cmp bh, 21
      jae ._printFloatingPoint_end
      jmp ._printFloatingPoint_print_f
    
  ._printFloatingPoint_error:
    push "r"
    push "Erro"
    push 5
    lea eax, [esp + 4]
    push eax
    call print
    add esp, 8
  
  ._printFloatingPoint_end:
  frstor [fpustate]
  pop ebx
  pop esi
  popf  
  restore temp
  restore ten
  restore fpustate
  restore digits  
  mov esp, ebp
  pop ebp
  ret 8

_________________
Sorry for my English...

05 Mar 2009, 06:52

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