Hard
Cider Index
 By
Sharon Aker
A Little Beyond the Basics
Pseudo, and so on. The basic RAM chips that make up
your computer's memory is more completely referred to as DRAM,("D-ram")
with the D standing for dynamic, because its contents change.
The phrase comes in handy, since sometimes we need to differentiate
between this kind of memory and the other kinds of memory
chips in your computer:
*VRAM. ("V-ram"). Video RAM is specialized
high-speed RAM used - you guessed it- for the video display.
(Early Macs grabbed screen memory from the DRAM, which wasn't
such a problem back when Mac screens were black-and-white,
9-inch displays.)
*PRAM. ("P-ram"). Parameter RAM is a small
amount of memory in your Mac that's powered by an internal
battery. It keeps track of various control panel settings
like the mouse tracking speed, the virtual memory setting
and the time and date. (That's how the Mac remembers what
time it is even if you unplug it.)
*Static RAM, SRAM. ("S-ram"). Static RAM
doesn't need constant refreshing to deep its information;
it holds it until you change it.
*Pseudo-static RAM. Pseudo-static RAM takes only a
very little electrical power to keep its switches set in place,
so it's used in PowerBooks, where power draw is always a concern.
It's also a lot less expensive than true static RAM.
*Flash RAM. Despite its name that connotes a short-lived
flash-in-the-pan attitude, flash RAM actually holds its information
even without any electrical current available - for as long
as ten years. Flash RAM is available for some PowerBooks,
on insertable cards that serve as extremely fast hard drives.
EDO and EDON'T. There are two types of DRAM DIMMs.
EDO (pronounced as separate letters) is extended data output
and is five to twenty-five percent faster than the other kind.
The "other" kind, which is often just referred to
as "standard" or "regular" DIMMs, is fast
page mode, or FPM. Some PowerMacs need EDO DIMMs; others can't
use them; still others can use either - but may not benefit
from the extra EDO speed. The biggest caveat: the PowerMac
7200 can be damaged if you use EDO DIMMs in it.
And now, SDRAM. You know all about a computer's clock
speed - the metronome that keeps things moving through the
processor. But memory access has always had a different timing
scheme, based on the memory chips themselves. Because the
memory is asynchronous - it's timing has nothing to do with
the speed or needs of the processor's cycles - there are periods
of inactivity ("wait states") for the processor.
Standard DRAM, as described two entries ago, is asynchronous.
But synchronous DRAM, or SDRAM, uses a clock that synchronizes
with the processor's clock and eliminates the wait state,
resulting in speeds up to twenty percent faster than EDO RAM.
SDRAM is used in the G3 Macs.
Translation. If you're looking at memory specs, or
at the S/DIMMs themselves, you may see marking like 1x 8-40
or 4 x 32-80. First of all, the pronunciation is "one
by eight, forty." Next, it's relatively easy to interpret
the numbers.
The first two numbers - a x b - describe the
total amount of memory on the board, with a being the
amount of memory in each chip, and b the number of
chips. But of course it's not quite that straightforward since
the chip memory is described in terms of megabits. With binary
numbers, mega actually stands for 1,048,576, so that's how
many bits there are in a megabit. You can divide that by 8
to find out how may bytes there are - 131,072 - and divide
that by 1024 to find that a megabit is 128K. So, for a 1 x
8, there are 8 chips of one megabit each: a total of 1,024K,
or one meg. For the 4 x 32, the chips are four megabits each
(512K); with 32 of them, that's 16,384K, or 16 megs.
The second number also provides a clue as to whether the
module is a SIMM or DIMM, and how many pins it has: 32 is
a 72-pin SIMM, and 64 is a DIMM. The number after the hyphen
is, as you've probably guessed, simply the memory's speed.
Refresh Rates. You may see specs on a DIMM that refer
to its refresh rate. It all harks back to the issue of RAM,
or DRAM, needing a constant supply of electricity to keep
its switches set - to "remember" things. That's
not exactly accurate, although it's the party line because
nobody wants you to think the RAM contents can survive a shutdown.
But the truth is, the chips don't need a constant flow of
electricity; they work perfectly well with little zaps at
regular intervals. RAM chips (the real chips, the tiny things
inside that black block we also call a "chip") are
arranged in tiny rows and columns; each row gets a new zap
of electricity (or is "refreshed"), and then the
next row gets zapped, and so on.The refresh rate refers to
the number of zaps it takes to get all the rows juiced - in
other words, how many rows can be zapped in a single "refresh
cycle". A 2K refresh rate handles 2000 rows of chips
in a single cycle. Which, of course, is measured in small
fractions of a second. So, if you can turn off your Mac and
get it back on again in that time frame, your RAM contents
will be there waiting for you.
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Sharon
Aker is the author of The Macintosh Bible, 7th
Edition published by Peachpit
Press. from which this article is excerpted..
Sharon Zardetto Aker is the author of over a dozen
Macintosh books. She was the author of The Macintosh
Bible, 3rd Edition and contributing editor to the fourth,
fifth, and sixth editions. Sharon has written hundreds
of articles for many different magazines over the years,
designs Web sites and databases, and teaches classes
on all Macintosh-related topics. |
The Material above is copyrighted and appears
here courtesy of the authors and Peachpit Press
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