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From: jfh@killer.UUCP (John Haugh)
Newsgroups: comp.sys.m68k
Subject: Re: 68000 shift/rotate timings
Message-ID: <1065@killer.UUCP>
Date: Mon, 29-Jun-87 11:57:04 EDT
Article-I.D.: killer.1065
Posted: Mon Jun 29 11:57:04 1987
Date-Received: Sat, 4-Jul-87 13:07:22 EDT
References: <19497@ucbvax.BERKELEY.EDU>
Organization: The Unix(tm) Connection, Dallas, Texas
Lines: 16
Summary: Because you can only shift one bit in memory. Thats all.

In article <19497@ucbvax.BERKELEY.EDU>, robinson@renoir.Berkeley.EDU (Michael Robinson) writes:
> The Signetics S68000 Users Guide (which is just a photo-reduction of 
> the Motorola manual) shows the cycle timings to shift a word in a register
> as 6+2n (where n is the number of bits to shift), but gives the cycle
> timing to shift a word in memory as 8 plus effective address overhead.
>  
> Is this a misprint?  If not, why is the time to shift a register a 
> function of n, but the time to shift memory a constant?

The memory shift operations are single bit shifts only.  The register
variety can shift more than one at a time, so it takes longer (unless
you have a really neat barrel shifter.)

That answer this question?

- John.