--- Amber Rhea <[EMAIL PROTECTED]> wrote:
> I've often wondered what would happen if one tried
> SLOWER SIMMs that those
> listed as the minimum requirement. Everymac.com has
> various Macs noted as
> requiring 80ns, 70ns, 60ns, et al. My curiosity
> hasn't been so burning that
> I've bought slower SIMMs for this purpose and tried
> them in a Mac, but it
> would be interesting to know why this SIMM speed
> requirement is important,
> and why it varies so from Mac to Mac, even among
> Macs made around the same time.
On the RAM bus, the memory controller sets the
timing of reads and writes. When the bus is set
for a read or write, it waits a certain amount
of time for the RAM to be ready to read or write.
If you use too slow of RAM it won't be ready to
be read or written before the time for the operation
to take place ends. Then you get a memory error and
bad stuff happens. To some extent older computers
can be sped up by using faster RAM than the slowest
specified. The reason is that a computer that
requires at least 80ns RAM will read or write
data the very instant the RAM is ready. Older systems
generally used a longer "window" of time for
RAM operations to take place. Popping 60ns RAM
into a system specced for 100ns will allow most
memory operations to take place right at the start
of the time window, thus giving a mild speedup.
10-15 years ago when I was using 8088/80286/80386
PCs it was quite easy to see the speed difference
when I upgraded a box to 60ns or 70ns RAM. The
RAM test part of bootup would fly by with a
BRRRRZZZZTTT! instead of me being able to hear
a distinct "tick" for each K. Of course, loading
up a 286 with 12megs of those individual 256Kx1
DIP chips was horrendously expensive and used up
three of the ISA expansion slots. What was funny
was I didn't have all the RAM the same speed and
the memory test sounded like it was shifting gears
as it went from the faster RAM to the slower chips.
(I configured the 60ns stuff as conventional and
XMS and the 70ns as hardware EMS so the 70ns wouldn't
slow down the faster RAM.)
If you take a IIci and install 70ns SIMMs in one
bank and 60ns in the other, it will all run at
70ns. The same goes for PCs, except for the olden
days of memory cards on the ISA bus.
As computers got faster, the slight speed difference
from using faster RAM became imperceptible to
the user, and only a benchmarking program could
tell the difference, which in normal operation
doesn't amount to a hill of beans. :) The memory
timing cycles are so tight now that even a couple
of nanoseconds too slow will cause trouble, which
is why it pays to buy name brand RAM like PNY,
Micron etc.
=====
"The earth swarms with inhabitants. Why then should nature,
which is fruitful to an excess here, be so very barren in
the rest of the planets?" Bernard de Fontenelle, 1686
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