Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!nosc!ucsd!ames!umd5!purdue!decwrl!ucbvax!BRAHMS.BERKELEY.EDU!cotner
From: cotner@BRAHMS.BERKELEY.EDU (Carl Cotner)
Newsgroups: comp.sys.ibm.pc
Subject: What speed memory seek..
Summary: How come a 12Mhz machine can use 100ns chips?
Message-ID: <8807270024.AA17149@brahms.berkeley.edu>
Date: 27 Jul 88 00:24:41 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: brahms.berkeley.edu!cotner@brahms.berkeley.edu (Carl Cotner)
Lines: 30


Am I having trouble dividing, or is there something else that I'm missing?

Here's what I'm referring to:  I have a 12MHz AT motherboard (clone), and the
paltry instruction manual that came with it stated that at 12Mhz, I can use
100ns chips and get 0 wait states.  In that same manual it states that the
10Mhz motherboard can achive 0 wait states by using 80ns chips.  This seems
somewhat fishy to me.  

Since 	1/12Mhz = 83ns,

I would expect that in order to do a memory seek in one machine cycle, I would
need RAM chips faster than 83ns, i.e. 80ns or less.  Similarly, since

	1/10Mhz = 100ns,

The 10Mhz board must use 100ns chips.  How come they can get away with 80 and 
100ns? 

BTW, I was also wondering if any of you out there have ever tried to install
slower memory chips on a turbo motherboard.  Since RAM prices are kind of high
right now, I thought I'd salvage some 120ns RAM from a spare AT board and
install them on my 6/12MHz motherboard.  I set the jumper on the board so that
it would boot up in 6 MHz mode rather than 12.  Despite this slower setting,
I received several memory parity errors during setup.  Do I have a defective
motherboard, or is the clock still operating at 12MHz for some portions
of the board? The motherboard is called a SAM/12, if that's of any help.

Just a few thoughts.  I'd appreciate any answers though.

cc