[source]Simulate lstrlen

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krackwar

Joined: 24 May 2008
Posts: 13
Location: Chile

krackwar 02 Sep 2008, 04:15

Code:

;This function simulate the lstrlen of windows and return in ecx
proc len,string
mov ebx,[string]
mov ecx,0h
.bucle:
inc ecx
cmp byte[ebx+ecx],0
JNE  .bucle
ret
endp

Last edited by krackwar on 03 Sep 2008, 18:46; edited 1 time in total

02 Sep 2008, 04:15

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 02 Sep 2008, 04:40

Code:

;This function simulate the lstrlen of windows and return in ecx
proc len, pString
  mov ecx, [pString]
  jmp .checkNull

; You may want "align 16" here

.loop:
  inc ecx

.checkNull:
  cmp byte [ecx], 0
  jne .loop

  sub ecx, [pString]
  ret
endp

In your code you can safely remove "JE .termino" without changing procedure semantics. Note that your code has a bug, consider a string like " string db 0, 'GARBAGE', 0", your procedure will incorrectly report much more than zero. In some cases it could crash too because if there is no zero in the path before reaching a non-allocated memory region you'll get a page fault.

02 Sep 2008, 04:40

Garthower

Joined: 21 Apr 2006
Posts: 158
Location: Ukraine

Garthower 02 Sep 2008, 08:44

Here a fast x64 version of strlen. Without its special problems it's possible to alter easily on 32 bits.

Code:

proc StrLenA
                    ;ANSI PChar string in rcx. Length returns in rax
                    ;str    equ rcx
     mov rax,rcx
 lea rdx,[rax+7]
     mov r9,0101010101010101h
    mov r10,8080808080808080h
   mov r8,[rax]
        add rax,8
   mov rcx,r8
  sub rcx,r9
  not r8
      and rcx,r8
  and rcx,r10
 jnz .J1
.L1:
 mov r8,[rax]
        add rax,8
   mov rcx,r8
  sub rcx,r9
  not r8
      and rcx,r8
  and rcx,r10
 jz .L1
.J1:
  test ecx,80808080h
  jnz .L2

 shr rcx,32
  add rax,4
.L2:
       test rcx,8080h
      jnz .L3

 shr rcx,16
  add rax,2
.L3:
       shl cl,1
    sbb rax,rdx

     ret
endp

02 Sep 2008, 08:44

asmcoder

Joined: 02 Jun 2008
Posts: 784

asmcoder 02 Sep 2008, 11:49

[content deleted]

Last edited by asmcoder on 14 Aug 2009, 14:56; edited 3 times in total

02 Sep 2008, 11:49

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 02 Sep 2008, 12:59

asmcoder, your code can't work even for a zero length string, it will crash with any input, check it. Additionally I don't understand why do you expect overflow.

BTW, this specific topic was discussed before, if someone founds the link please post it.

02 Sep 2008, 12:59

edfed

Joined: 20 Feb 2006
Posts: 4353
Location: Now

edfed 02 Sep 2008, 17:48

http://board.flatassembler.net/topic.php?t=2628 ???
is it possible to limit the number of topics.

make the fusion of many identical topics in only one.

not about make it very fast, "all topics shall be compressed before november" for example.
No, just: when we see this kind of remake, combine it with the previous one, and then, readers will see a lot of things for each subject.

do it sometimes, not everydays, but do it...

os construction can be limited to ~ 30 sub topics.
only one page can show us the vast choice.

Quote:

os construction:
boot
2nd boot
kernel
shell
files systems
fat12
file formats
memory manager
video manager
mouse & keyboard
dma
L1&2 caches
TSS
/TSS
...
..
.

or maybe, if i want it i can do it???
but no, if i need it it means i cannot do it : ( .

02 Sep 2008, 17:48

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