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From: nickroberts@blueyonder.co.uk (Nick Roberts)
Newsgroups: comp.lang.ada
Subject: Re: Calculate and set Parity
Date: Tue, 17 Sep 2002 22:05:50 GMT
Organization: AdaOS
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Date: 2002-09-17T22:05:50+00:00
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On Mon, 16 Sep 2002 18:42:00 +0200, "Sebastian"
<sebastian.madunic@avionics.saab.se> strongly typed:

>Hi,
>
>I need a subroutine that calculates the parity of a record type with the
>size of 16 bits. Is there some nice way
>to do this? I currently have a solution were I use Unchecked_Conversion to
>convert my variable to an
>array booleans, then I calculate every true etc. Is there some way to do
>this without using Unchecked_Conversion.

>From the Intel documentation:

PF (bit 2) Parity flag. Set if the least-significant byte of the result
contains an even number of 1 bits; cleared otherwise.

This flag is set by any arithmetic instruction.

Assuming your 16-bit number is in DataHi:DataLo, the following code on any
IA machine should leave the answer in DL:

   MOV AL, DataLo  ; load a byte
   AND AL, AL      ; set PF
   LAHF            ; load flags byte
   SHR AH, 2       ; shift PF into bit 0
   AND AH, 1       ; mask off all other bits
   MOV DL, AH      ; result into DL
   MOV AL, DataHi
   AND AL, AL
   LAHF
   SHR AH, 2
   AND AH, 1
   XOR DL, AH      ; parities must be XORed to get cumulative result

I think most other architectures provide something similar.

I'll leave the details as an exercise ;-) HTH

-- 
Nick Roberts
Per Ardua ad Disastra