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From: gnu@hoptoad.uucp (John Gilmore)
Newsgroups: comp.sys.m68k
Subject: Re: 68000 shift/rotate timings
Message-ID: <2337@hoptoad.uucp>
Date: Sat, 27-Jun-87 23:37:11 EDT
Article-I.D.: hoptoad.2337
Posted: Sat Jun 27 23:37:11 1987
Date-Received: Sun, 28-Jun-87 06:42:19 EDT
References: <19497@ucbvax.BERKELEY.EDU>
Organization: Nebula Consultants in San Francisco
Lines: 15

robinson@renoir.Berkeley.EDU (Michael Robinson) wrote:
>                              why is the time to shift a register a 
> function of n, but the time to shift memory a constant?

Because the 68000 (and 68020) can only shift a memory location by a
single bit.  It uses the "X" bit for carrying the shifted bit between
words, e.g.  if you are shifting a 256-bit string, you can do it with a
dbra loop over a roxl instruction, but only one bit at a time.
Shifting long strings by multiple bits requires more complicated logic,
e.g. a funnel shifter rather than a barrel shifter, and a register to
hold the intermediate result as it propagates along the string.  Motorola
didn't get that fancy.
-- 
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