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Index > Main > F3 prexif for ins/outs ???

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b1528932



Joined: 21 May 2010
Posts: 287
b1528932
f2 = rep in case of ins/outs
f2 = repz in stos/lods/movs
f3 = repnz in scas/movs

ins/outs dont use f3 prefix be definition.


ollydbg and fasm allow encoding of f# prefix. is that a bug?
ollydbg doesnt allow repz, but allow repnz and rep, wtf?!
Post 22 Jun 2010, 18:46
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
fasm does not perform any kind of validation on prefixes, things like "lock fs rep xor eax, eax" are accepted and interpreted in the same way like this:
Code:
lock
fs
rep
xor eax, eax    


BTW, I think you have your prefixes wrong, F3 is for REP and REPE/REPZ, and F2 for REPNE/REPNZ
Post 22 Jun 2010, 19:39
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baldr



Joined: 19 Mar 2008
Posts: 1651
baldr
LocoDelAssembly wrote:
fasm does not perform any kind of validation on prefixes…
And it probably shouldn't. I've heard about "rep ret" AMD optimization, should FASM disallow it?
Post 22 Jun 2010, 20:00
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4633
Location: Argentina
LocoDelAssembly
I don't know, but I'm sure it is not a bug as I understand it as part of fasm's syntax. However, since you're allowed to insert prefixes as instructions (like DOS's Debug program), maybe it should and also ensure the instruction size is not exceeding the limit by introducing too many prefixes (not taking into account previous assembled lines that may have left prefixes with no instruction at the end).
Post 22 Jun 2010, 20:49
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b1528932



Joined: 21 May 2010
Posts: 287
b1528932
ok ur right

its very confusing, many mnemonics may correspont to 1 opcode, and many opcodes may correspont to 1 mniemonc.

ehh...
Post 22 Jun 2010, 21:17
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baldr



Joined: 19 Mar 2008
Posts: 1651
baldr
LocoDelAssembly wrote:
However, since you're allowed to insert prefixes as instructions (like DOS's Debug program), maybe it should and also ensure the instruction size is not exceeding the limit by introducing too many prefixes (not taking into account previous assembled lines that may have left prefixes with no instruction at the end).
I don't think that FASM should question sanity of the programmer. ;-)
b1528932 wrote:
its very confusing, many mnemonics may correspont to 1 opcode, and many opcodes may correspont to 1 mniemonc.
2+2 has the same numerical value as 4 does, isn't it confusing? Mnemonics are just devices to simplify coding, not to improve understanding of processor architecture.
Post 22 Jun 2010, 21:39
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