Re: Question About RAM

["Paul Camp" <pjcamp@coastal.edu>]

I know more about PC's than Macs (somebody else can comment about 
those) but here is some general information.

The speed rating (70 ns vs. 80 ns) is how much time is required for 
one memory read/write cycle. It use to be more important than it is 
these days since you wanted to choose memory capable of keeping up 
with your CPU (eg 1/(70ns) = 14 MHz). This is no longer such a big 
issue since DRAM is WAY slower than any modern CPU but it still has a 
small effect on total system performance. These days, the much faster 
(and more expensive) SRAM is used as a cache between the CPU and DRAM 
to help keep up but even it is slower than most CPUs.

To help with speeding up memory accesses, SIMMS are usually installed 
in banks of 2, interleaved so that one can be read while the other is 
refreshing. This reduces the amount of time the CPU spends twiddling 
its thumbs but it also requires you to install SIMMS in matched 
pairs -- same size, same speed. You CAN mix different speeds across 
different banks but it isn't generally a very good idea since all the 
SIMMS will run at the slowest of the two speeds so you basically 
wasted the faster ones.

You need to make sure you have a free bank available before ordering. 
If not, you will have to remove some SIMMS (in pairs) to install the 
new ones and this will of course affect your purchasing decision. 
Their are companies which will buy used SIMMS for remanufacturing and 
this will reduce your expenditure a bit should you have to replace 
SIMMS rather than just adding new ones. You can find ads for them in 
the back of publications like MacUSER, PC Magazine and so on. Their 
is one company, Autotime, which will take your old SIMMS and make 
new, bigger ones out of them by pulling off the chips and soldering 
them to a new board. I don't recommend them since my experience with 
changing some 30 pin SIMMS to 72 pin was dreadful. They do not assume 
any responsibility for insuring the safe return of your property.

It is plenty safe enough for you to install the memory if you are 
careful. I recommend buying an antistatic wrist band (about $5-10) 
and making sure you are well and truly grounded -- static is the 
death of electronics. Once you are grounded, though, your fingers are 
fine for holding the SIMM and you don't really need any additional 
tools. The little piece of plastic serves more as an insulator than a 
holder and you would be better served by the wrist band. Just be 
careful not to force them in since the SIMM will crack if you give it 
a chance.

Also, you might want to call around to several of the companies you 
are looking at. Memory prices are pretty volatile (they're traded 
like pork bellies these days) so the price in the ad is undoubtedly 
out of date. Fortunately, the price has been on a steady downward 
spiral the past couple of months so you should be able to get off 
without too much expense.

> One thing that would help is if I knew the difference between 70 ns and 
> 80 ns memeory.  One catalog advertises 70 ns and the other 80 ns.  I know 
> this somehow relates to the speed, but how much of a difference is it?  
> Must I use a certain one or can I choose?  If I can choose, what do you 
> suggest?
>
> One last thing.  Do you think it is safe enough for me to install the 
> memory?  Some ofthe catalogs are trying to sell a $10 piece of plastic to 
> hold the chip.  Do I need that?
>
> Any help and advice would be greatly appreciated!  Thanks in advance!
>
> Jonathan Gillis

Paul J. Camp                   "The Beauty of the Universe
Assistant Professor of Physics  consists not only of unity
Coastal Carolina University     in variety but also of 
Conway, SC   29526              variety in unity.
pjcamp@csd1.coastal.edu             --Umberto Eco
(803)349-2227                         The Name of the Rose
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