Re: Application of parity flag

From: TS (spamtrap_at_crayne.org)
Date: 10/22/04 

Date: Fri, 22 Oct 2004 01:10:35 +0000 (UTC)

>About the only practical use that I can think of testing parity of
>communications data (which is usually handled by hardware).
>
> mov al,<byte to be tested>
> test al,al
> jp <somewhere> ; byte has even parity
> ; byte has odd parity

I guess that creating checksums (Parity=XOR of all bits) is the main
purpose for the parity flag.

There is another useage for the parity flag, which has nothing to do
with parity: FPU comparations (e.g. FCOMI) set PF whenever the
operands are unordered and clear it if they can be compared.

However, you can also use it for creating semi-random numbers or
interesting patterns: I´ve used it for calculating cellular automata
once, and the paritiy of an ever incremented byte also gives a rather
interesting binary sequence...

You might even use PF to check the corresponding bit in a value which
has been loaded into EFLAGS via LAHF, POPF, IRET, a task switch or
whatever.

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