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From: gnu@hoptoad.uucp (John Gilmore)
Newsgroups: net.micro.atari16,net.micro.amiga,net.micro.68k
Subject: Re: 68000 Memory Managment
Message-ID: <1045@hoptoad.uucp>
Date: Sun, 31-Aug-86 06:20:39 EDT
Article-I.D.: hoptoad.1045
Posted: Sun Aug 31 06:20:39 1986
Date-Received: Sun, 31-Aug-86 10:03:34 EDT
References: <508@elmgate.UUCP> <64@mit-prep.ARPA> <510@elmgate.UUCP> <4422@gatech.CSNET>
Organization: Nebula Consultants in San Francisco
Lines: 28
Keywords: 68000 atari amiga 68k mmu
Xref: mnetor net.micro.atari16:1778 net.micro.amiga:4480 net.micro.68k:1183

In article <4422@gatech.CSNET>, jeff@gatech.CSNET (Jeff Lee) writes:
>>                                                         ...contiguous
>>column addresses in a DRAM chip are no longer contiguous in 68K's
>>address space.
>If you are talking about nibble-accessible rams...

There are many types of RAMs where accessing columns is faster than
accessing rows.  Nibble mode, where you can toggle CAS and get out 4
bits; page mode, where you can drop CAS, set up a new column address,
and enable CAS again; and static column mode, where you can just change
the column address on the fly, wait the column access time, then look
at the data.  There are also video RAMs which scan data out serially by
columns.
 
>                 ...does anyone know of a REAL (ie - available off the
> shelf, now) system that actually uses the nibble mode of some of the
> newer rams...

I do not remember if the new cached Sun uses nibble mode (or other
fast column mode) RAMs.

Part of the problem is that designers don't want to require nibble mode
RAMs, since they usually cost more than "any old RAMs".  Designing to
the least common denominator in RAMs [modulo what speed you need]
is very useful for reducing prices.
-- 
John Gilmore  {sun,ptsfa,lll-crg,ihnp4}!hoptoad!gnu   jgilmore@lll-crg.arpa
		     May the Source be with you!