Code Explaination Please (Listing Drives)

Index > Windows > Code Explaination Please (Listing Drives)

Author

Thread

Nameless

Joined: 30 Apr 2010
Posts: 95

Nameless 30 Apr 2010, 05:45

i just started FASM, and i put up this code from random places, it works, but i dunno how it works Surprised

i was wondering if anyone can explain what each line does, i know it looks for #0 char and replace/skip it, but how?

Oh, also, how can i declare a long string?
i use 'Buf rb 429496', but its not good i guess

here is the code:

Code:

format PE GUI 4.0
include 'win32ax.inc'

.data
 Drive             db ?, ':\ ', 0
 ldstrings     db 128 dup(?)
 TheStr        db 128 dup (?)
 
start:
 invoke GetLogicalDriveStrings, 128, ldstrings
 mov  ebx, ldstrings
 
 enum_drives:
    mov     dl, byte[ebx]
       mov     [Drive],dl
  invoke  lstrcat, TheStr, Drive
      jz      next_drive
 
 next_drive:
 add     ebx,4
       cmp     byte[ebx], 0
        jne     enum_drives
 jmp     endcall

 endcall:
        invoke  MessageBox, 0, TheStr, "Drives :", 0

.end start

cheers, thanks alot

30 Apr 2010, 05:45

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 30 Apr 2010, 08:47

Nameless,

GetLogicalDriveStrings() returns in lpBuffer something like

Code:

db "A:\", 0
db "C:\", 0
db "D:\", 0
db 0

enum_drives: loop concatenates those strings into TheStr;
MessageBox() shows them.

Then we get access violation (MessageBox() returns IDOK==1, 00 00 is decoded by CPU as add [eax], al and byte at address 1 is inaccessible).

30 Apr 2010, 08:47

Nameless

Joined: 30 Apr 2010
Posts: 95

Nameless 30 Apr 2010, 08:58

im very sorry to disappoint u but.....
im a total n00b XD

can u tell me how it did this? like what does ' mov dl, byte[ebx]' do?
and 'mov [Drive],dl' ? and the rest of those?

thats my problem, im a delphi coder btw, and i know some C, so anything after invoke is ok with me
the rest is the problem

thanks

30 Apr 2010, 08:58

ass0

Joined: 31 Dec 2008
Posts: 518
Location: ( . Y . )

ass0 30 Apr 2010, 09:31

http://board.flatassembler.net/topic.php?t=2530
http://board.flatassembler.net/topic.php?t=2527

_________________

Nombre: Aquiles Castro.
Location2: about:robots

30 Apr 2010, 09:31

Nameless

Joined: 30 Apr 2010
Posts: 95

Nameless 30 Apr 2010, 10:56

which part should i look for? keywords plz?

30 Apr 2010, 10:56

baldr

Joined: 19 Mar 2008
Posts: 1651

baldr 30 Apr 2010, 13:31

Nameless,

Google: assembly OR assembler tutorial. Look for those which are focused on 32-bit programming and Windows.

30 Apr 2010, 13:31

Nameless

Joined: 30 Apr 2010
Posts: 95

Nameless 30 Apr 2010, 14:55

ok, i messed with it a bit more
when i changed ebx to ecx it showed nothing, just froze
switched back to ebx it showed all of them

y did this happen?
arent they same type "General Purpose Registers" ? and same size "32bits"?

30 Apr 2010, 14:55

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20459
Location: In your JS exploiting you and your system

revolution 30 Apr 2010, 15:12

invoke lstrcat won't preserve eax, ecx, edx or the flags.

30 Apr 2010, 15:12

Nameless

Joined: 30 Apr 2010
Posts: 95

Nameless 30 Apr 2010, 16:04

now a smarter question
how should i know what function preserves what and doesnt preserver what?

30 Apr 2010, 16:04

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20459
Location: In your JS exploiting you and your system

revolution 30 Apr 2010, 16:22

Windows API uses Standard Call convention. All functions (with only two special exceptions) use stdcall.

30 Apr 2010, 16:22

Nameless

Joined: 30 Apr 2010
Posts: 95

Nameless 30 Apr 2010, 18:13

i mean how should i know that lstrcat wont use ebx and will use the others?

30 Apr 2010, 18:13

bitshifter

Joined: 04 Dec 2007
Posts: 796
Location: Massachusetts, USA

bitshifter 30 Apr 2010, 19:03

It called ABI. (application binary interface)
Register useage is a small part of this convention.

30 Apr 2010, 19:03

< Last Thread | Next Thread >

Forum Rules:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum