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From: terryl@tekcrl.UUCP
Newsgroups: comp.unix.questions
Subject: Re: Re: /dev/null: The final frontier
Message-ID: <1718@tekcrl.TEK.COM>
Date: Thu, 4-Jun-87 14:31:15 EDT
Article-I.D.: tekcrl.1718
Posted: Thu Jun  4 14:31:15 1987
Date-Received: Sat, 6-Jun-87 11:40:30 EDT
References: <7488@brl-adm.ARPA> <1705@umd5.umd.edu> <311@xios.XIOS.UUCP>
Reply-To: terryl@tekcrl.tek.com
Organization: Tektronix, Inc., Beaverton, OR.
Lines: 32

In article <311@xios.XIOS.UUCP> greg@sdn.UUCP (Greg Franks) writes:
>In article <1705@umd5.umd.edu> zben@umd5.umd.edu.UUCP (Ben Cranston) writes:
>>It has been proposed that the bit-bucket hardware of most computers be
>>enhanced to sort the zero-bits and one-bits into separate buckets.  If
>>this be done, these buckets can be the source for new bits needed by the
>>major bit-consuming instructions of the computer, namely the shifting
>>instructions (note rotate instructions do not need a source of new bits).
>
>Au contraire - Rotate instructions can be both a source and a sink of
>bits.  The ROR and ROL instructions of the MC680*0 machines use the
>carry flag as a sink and a source of excess bits.  (Note that these
>micro processors also have a ROXL and ROXR instuctions for multi-word
>(whatever) rotates).  

     Double au contraire (i.e. you're wrong, Bucko). I quote from the
M68000 16/32-bit Microprocessor Manual (Fourth edition):

	"For ROL, the operand is rotated left; the number of positions shifted
	 is the shift count. Bits shifted out of the high order bit go to both
	 the carry bit and back into the low order bit.  The extend bit is not
	 is not modified or used."

      So rotates use the carry bit as a sink, but not as a source. Perhaps you
were thinking of the ROX[L,R] instructions which do use the extend bit as both
a sink and a source????

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