When you thought you knew something well....

[xandaz]

    Obviously by this time i should have had this well learned but it appears that i don't. I'm reffering to the way flags changes due to logical opperations.
    For instance:

Code Select Expand


_test  dd   11111111111111111111111111111100b

.code
       test    _test,100b


i was thinking that it would set CF or Clear ZF if the immidiate opperand were to be 0 but it doesn't. Can someone explain how this works. And what is the parity flag for?
thanks

[qWord]

Which flags and how they are affected can be read in the documentation (AMD, Intel).
TEST is the same as AND, except, that the destination operator is not modified - a barrow can not occur, thus the CF flags is allways set to 0 (the same applies to OR and XOR).
The parity flag is set, when the low order byte of the result holds an even number of '1'-bits:
e.g.:
AL = 011y -> PF=1
AL = 001y -> PF=0

In your example the result would be  100y, thus ZF, PF and SF are cleard.

[zemtex]

The parity flag is set if the number of set bits is even in the least significant byte of the result.

[dedndave]

the logical instructions, AND, OR, XOR, TEST, always clear the carry flag
they already covered PF
usually, the only flag of interest with TEST is the ZF   :P

[Farabi]

If you want to test spesific bit, I used to use "BT" instruction, it is lot simpler, and maybe tinier, dont know about the speed.

Code Select Expand


BT cx,1 ; Test bit 1 for set, if it is carry it is set, if not, it is not setted. Dead simple


[xandaz]

    Thanks guys.... i feel kinda stupid for the post but i'm thankful for the intel.
    BT? never heard of it. Oh i see it now. Damn i'm way behind with the oppcodes. Got to get my stuff together.

[jj2007]

...dont know about the speed.

http://www.masm32.com/board/index.php?topic=17294.msg145011#msg145011

[Farabi]

...dont know about the speed.

http://www.masm32.com/board/index.php?topic=17294.msg145011#msg145011

Thanks, new tech aquired. SHL  :cheekygreen:

[xandaz]

    yeah... i've been having a hard time to understand what seems too obvious. The opcode ref says of 'BT' :The destination bit indexed by the source value is copied into the Carry Flag.
   But then i have this code:

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or eax,1
test eax,1   ; no zero flag and it's alright
bt eax,1      ; CF keeps unset

why didnt CF set? The indexed bit0 isnt copied into CF and bit0 of eax==1... wtf
and then this:

Code Select Expand


or eax,1
bt eax,0  ; CF is set
why is this? bit0 of eax==0 and 0 AND 1 ==0 wtf?
Can someone explain this?

[xandaz]

    Alright... nevermind... i just saw a post by donkey explaining it. So the source allows a 256 bit zero based indexing possibilities.
    Sorry... it was that obvious. I should take care of my chi or something... loosing attention.
    Thanks and later