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From: maniac@arrakis.nevada.edu (ERIC SCHWERTFEGER)
Newsgroups: comp.sys.amiga
Subject: Re: Fix that baby! (RAM Board clarifications)
Keywords: memory bus
Message-ID: <840@unsvax.NEVADA.EDU>
Date: 22 Sep 89 18:46:37 GMT
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Reply-To: maniac@arrakis.nevada.edu.uucp (ERIC SCHWERTFEGER)
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Organization: Univ of Nevada System Computing Services - Las Vegas
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	Having done designs for a 68000 based single board computer with
DRAM memory (years ago), I can add to this discussion on what speed memory
is needed on the amiga,
	While it is true that the memory cycle on the amiga (for the cpu)
is 560 ns, the CPU doesn't give the memory that much time to read.  First
of all, the address isn't made available until half way through the 
first clock cycle, and doesn't say that that is a valid address until
the end of the first clock cycle (you can, however work around this).
Now, the data is expected in by the start of the fourth clock cycle.  This
gives the memory 2 clock cycles, or 280 ns to respond.  Now, you can figure
on another 100 ns of overhead for memory decoding, buffering, etc.  Remember
that light only travels less than a foot in 1 ns, and those paths through
the chips are hardly straight, or at full speed.  The overhead gets even
worse when you have to deal with an expandable system like the Amiga.
	Now that all that is done, you have about 180 ns for a memory
access, unless the memory board is poorly designed or some other design
consideration took precidence.  In the Ram board I have (EXP -1000),
I have a mix of 150 ns and 120 ns memory, and unless I pull all my
150 ns chips, I need to let my Amiga warm up for a few minutes in order
to boot.  I have heard of one memory expansion/hard disk unit (can't remember
the name), that requires 80 ns rams, due to higher overhead.
	Hope this information helps.
 
Eric Schwertfeger, UNLV, maniac@arakis.nevada.edu