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AsmGuru62



Joined: 28 Jan 2004
Posts: 1409
Location: Toronto, Canada
AsmGuru62
Can't move memory to insert something without STD. It's either STD or moving memory in small steps - ineffective.
Post 21 Jun 2011, 02:35
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ouadji



Joined: 24 Dec 2008
Posts: 1081
Location: Belgium
ouadji

@Tyler
@Teehee


LEA ... pointless ?
i'm not agree with you.
a very useful instruction and much more powerful than it seems at first sight!
Code:
.next_dd_w:       mov     edi,[BIU_dword + ecx]
               lea     ebx,[ID_ITUN_01u + (ecx*8)]     ;<
               lea     esi,[ebx+14]                    ;<
.next_write:       lea     eax,[ecx*8]                     ;<
               neg     eax     
            lea     edx,[ebx-3000+eax]              ;<
               mov     eax,BST_CHECKED
             bt      edi,edx
             jc      @F    

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Post 21 Jun 2011, 09:25
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Picnic



Joined: 05 May 2007
Posts: 1288
Location: behind the arc
Picnic
xleelz wrote:
what is std and cld good for?


Moving, copying, traversing data, in conjunction with a REP prefix and movsb, stosb, cmpsb instructions. Quite useful.
Post 21 Jun 2011, 10:21
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sinsi



Joined: 10 Aug 2007
Posts: 693
Location: Adelaide
sinsi
Try writing 64-bit code without LEA.
Post 21 Jun 2011, 10:37
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idle



Joined: 06 Jan 2011
Posts: 359
Location: Ukraine
idle
BSWAP
CPUID
CMPXCHG
CMPXCHG8B
XADD

as those are not in i386 set i regret much about
clever thoughts come afterwards, crying or very sad
Post 21 Jun 2011, 11:15
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Teehee



Joined: 05 Aug 2009
Posts: 568
Location: Brazil
Teehee
@ouadji: pointless != useless, doesn't? Smile at least in my suck english it is Razz

i preffer to call it LEV (Load Effective Value) lol Razz
Post 21 Jun 2011, 12:08
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idle



Joined: 06 Jan 2011
Posts: 359
Location: Ukraine
idle
Quote:

LEV

in our native = lion
Post 21 Jun 2011, 13:06
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bitshifter



Joined: 04 Dec 2007
Posts: 764
Location: Massachusetts, USA
bitshifter
What about esc, not useless, but maybe least used?

As for any others mentioned, i think they ALL are very useful.
Some people have great big toolbox full of tools and do everything with just a hammer Smile
Post 21 Jun 2011, 13:34
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Enko



Joined: 03 Apr 2007
Posts: 678
Location: Mar del Plata
Enko
bitshifter wrote:

Some people have great big toolbox full of tools and do everything with just a hammer Smile

Not if you are Jemie from Mythbusters, he does have a great toolkit and he actually use it.
http://dsc.discovery.com/videos/mythbusters-raw-jamies-toolkit.html


My thought is that all the instructions at some point are usefull. For example, some "exotic" intructions can be used to bother noob crackers.
Post 21 Jun 2011, 13:52
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MazeGen



Joined: 06 Oct 2003
Posts: 975
Location: Czechoslovakia
MazeGen
My random favorites:

FNOP - I wonder when it can be really useful

ARPL - have anyone ever used it?

INVLPG in real mode - cool, but what for?
Post 22 Jun 2011, 09:24
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ouadji



Joined: 24 Dec 2008
Posts: 1081
Location: Belgium
ouadji

INT3 (0xCC) ... ok

but INT 3 (0xCD 0x03) ??

Just a matter of microprocessor design.
It would be foolish to disable this instruction.
But in practice, this instruction is completely useless.

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Post 22 Jun 2011, 09:56
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vid
Verbosity in development


Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia
vid
CD03 behaves bit differently from CC. I don't recall exact details, but you should be able to find them in manuals.
Post 22 Jun 2011, 10:19
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7724
Location: Kraków, Poland
Tomasz Grysztar
Post 22 Jun 2011, 10:20
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Feryno



Joined: 23 Mar 2005
Posts: 454
Location: Czech republic, Slovak republic
Feryno
Personally, I don't find any useless instruction.
Even found some usage for instructions/combinations of instructions looking "useless" like REPZ LODSD (a0C.exe in self_dbg in fdbg package) - MazeGen likes the REP LODSB, right?
Remember that if you find some "useless" instruction or some exotic combination of instructions, it may be an antidebug trick.
Don't reject anything only because currently you think it's useless, good idea opening your eyes may come in the feaure and may color the world !

idle wrote:
CMPXCHG
CMPXCHG8B
XADD
I used them frequently in drivers and hypervisors programming (atomic memory operations in SMP), it may also be very useful in multithread application (in common - everytime when more "things" want to read/write shared memory at the same time)

MazeGen wrote:
INVLPG
I used it frequently when hypervisor maps memory page of guest into its virtual memory space (older versions of virtualization don't support virtualization of memory so hypervisor must walk through paging tables of guest from guest CR3 to last level of paging tables to find physical memory page and then map it into itself)

db 0CCh / db 0CDh, 03h - again an antidebug trick, even microsoft windows exception handler doesn't count with CD03 choice and reports wrong exception address inside CD03 instruction (ms assumes every int3 comes from db 0CCh, int3 is trap type of exception, so ms handler subtracts 1 and reports that as origin of instruction leading into int 3, then you must think of some correction algorithm if you develop a debugger)
Post 22 Jun 2011, 13:01
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Enko



Joined: 03 Apr 2007
Posts: 678
Location: Mar del Plata
Enko
MazeGen wrote:
My random favorites:

FNOP - I wonder when it can be really useful

The date of the post is November 06, 2003 at 12:16:58.
8 years no reply?
Post 22 Jun 2011, 13:42
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MazeGen



Joined: 06 Oct 2003
Posts: 975
Location: Czechoslovakia
MazeGen
Feryno wrote:
MazeGen wrote:
INVLPG
I used it frequently when hypervisor maps memory page of guest into its virtual memory space
As I wrote, INVLPG is allowed in real mode, but what for?
Post 22 Jun 2011, 13:52
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DOS386



Joined: 08 Dec 2006
Posts: 1901
DOS386
Tyler wrote:
lea is pretty pointless.


NO : http://board.flatassembler.net/topic.php?p=111985#111985 Shocked

Quote:
The most useless instruction


VERR and VERW Sad
Post 10 Jul 2011, 02:17
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yoshimitsu



Joined: 07 Jul 2011
Posts: 96
yoshimitsu
I guess by "useless" instructions you mean those, which actually consist of two operations and were implemented to provide some higher level of code to draw simplicity but ended up being much slower and are therefore negatively connotated

examples are loop, enter, lods, stos, movs
on most processors they are considerably slower than their two instruction-pendants

kind of compareable to inc eax vs add eax,1 as the latter is often faster on newer processors
while 'inc' is considered more an 'exotic' instruction like the high-level instructions mentioned above

however, there are some exceptions like leave or rep lods/stos/movs, which are pretty fast
Post 13 Jul 2011, 01:53
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idle



Joined: 06 Jan 2011
Posts: 359
Location: Ukraine
idle
Quote:

kind of compareable to inc eax vs add eax,1

there is special opcode for shifts when count=1
Post 13 Jul 2011, 03:14
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17278
Location: In your JS exploiting you and your system
revolution
idle wrote:
there is special opcode for shifts when count=1
Well I can guess what you mean here, but actually the opcode is the same. There is a special short form binary encoding though. Wink
Post 13 Jul 2011, 04:02
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