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From: olson@rochester.ARPA (Thomas J. Olson)
Newsgroups: net.micro.68k
Subject: Re: The Motorola 68030 (mem access time)
Message-ID: <21166@rochester.ARPA>
Date: Mon, 29-Sep-86 09:44:49 EDT
Article-I.D.: rocheste.21166
Posted: Mon Sep 29 09:44:49 1986
Date-Received: Wed, 1-Oct-86 01:10:30 EDT
References: <8609262336.AA01693@cory.Berkeley.EDU>
Reply-To: olson@rochtest.UUCP (Thomas J. Olson)
Organization: U of Rochester, CS Dept, Rochester, NY
Lines: 23
Keywords: ripple mode


Matt writes : 

>        100ns access time @ 40 Mbytes/sec (remember: 32 bit data bus).  I 
>think that the quickest transfer is 64 Mbytes/sec, which would come out to
>about 62ns access time.

  I believe that the 64 Mb per sec figure is for a rather bizarre special 
addressing mode, which is designed to take advantage of "ripple mode"
memory devices.  In these devices you present one address, then clock
four times.  Each clock gives you another bit.  
  For the 68030,  the utility is this : you present a paragraph address
(bottom 7 bits zero).  Then you clock four times, loading four thirty-two
bit words into cache.  You have just saved most of the work involved in
your next three memory cycles.  As far as memory access time goes, lots of
ripple mode devices can clock out ripple bits at better than 62.5 ns/bit.
  Somebody who has a real live data sheet, is this right?

--Tom Olson (olson@rochester)


"In *real* machines, the CPU *never* reads memory.  *Cache* reads memory!"
-- Peter Dibble