Can't move memory to insert something without STD. It's either STD or moving memory in small steps - ineffective.

Moving, copying, traversing data, in conjunction with a REP prefix and movsb, stosb, cmpsb instructions. Quite useful.

Try writing 64-bit code without LEA.

BSWAP
CPUID
CMPXCHG
CMPXCHG8B
XADD

as those are not in i386 set i regret much about
clever thoughts come afterwards, crying or very sad

@ouadji: pointless != useless, doesn't? at least in my suck english it is

i preffer to call it LEV (Load Effective Value) lol

in our native = lion

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

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.

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?

CD03 behaves bit differently from CC. I don't recall exact details, but you should be able to find them in manuals.

You can find those details here: http://board.flatassembler.net/topic.php?p=32983#32983

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 !

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)

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)

The date of the post is November 06, 2003 at 12:16:58.
8 years no reply?

As I wrote, INVLPG is allowed in real mode, but what for?

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



VERR and VERW

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

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.