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From: spaf@gt-karloff.uucp (Gene Spafford)
Newsgroups: comp.lang.fortran
Subject: Re: Re:FORTRAN-77 XLA PLUS BITWISE OPERATORS
Message-ID: <15115@gatech.gatech.edu>
Date: Sun, 26-Apr-87 14:11:34 EDT
Article-I.D.: gatech.15115
Posted: Sun Apr 26 14:11:34 1987
Date-Received: Sun, 26-Apr-87 23:38:11 EDT
References: <12296045502.17.BEEBE@SCIENCE.UTAH.EDU>
Sender: news@gatech.edu
Reply-To: spaf@karloff.gatech.edu (Gene Spafford)
Organization: Software Engineering Research Center (SERC), Georgia Tech
Lines: 42

The following implements a bitwise logical AND operation in Fortran.
It is *NOT* portable, but the idea can be adapted fairly easily to
other representations.  The other functions (OR, XOR, NOT, etc)
can be implemented in a similar manner.

Assumptions for this version:
1) Integer representation is 2's complement, with sign bit in
most significant bit position.
2) Integer addition quietly ignores overflow
3) Parameter NUMBITS is set to number of bits in an integer.

Way it works: by doubling the number each time through the loop, you
are doing an effective shift left by one position.  If the sign bit
is set, the most significant bit is a one, and the number is negative.
By comparing bits, we know whether to introduce a 0 or 1 bit into
the result.  The result is shifted too, to bring the bits into
the right positions.


	integer function bitand (iarg1, iarg2)
	integer iarg1, iarg2
	integer loop, jarg1, jarg2, kret
	parameter (numbits = 32)

	kret = 0
	jarg1 = iarg1
	jarg2 = iarg2

	do 10 loop = 1, numbits
	   kret = kret + kret
	   if (jarg1 .lt. 0 .and. jarg2 .lt. 0) kret = kret + 1
	   jarg1 = jarg1 + jarg1
	   jarg2 = jarg2 + jarg2
   10	continue

	bitand = kret
	end

Gene Spafford
Software Engineering Research Center (SERC), Georgia Tech, Atlanta GA 30332
CSNet:	Spaf @ GATech		ARPA:	Spaf@Gatech.EDU
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