Refreshing speeds ................................................................................
Another specification for memory is its refresh speed, which tells you how
often the system writes and rewrites bits of data to individual memory loca-
tions. Think of it as a rejuvenation process.Remember that dynamic memory only holds data when it is receiving
power; over time — or when the power is turned off — the contents of
dynamic memory are lost forever. This doesn’t mean you are going to lose
the manuscript for your Great American Novel or the design for your personal
intergalactic space probe when you turn off your laptop — just be sure to save
any work you want to keep on your machine’s hard drive. The storage on a
hard drive is more or less permanent once data has been written with an elec-
trical pulse to a metallic surface that can hold a magnetic charge. You explore
hard disks, and what I mean by “more or less permanent,” in Chapter 7.Memory’s refresh speed is just one element of a computer’s overall speed.
Refresh rates are measures in nanoseconds (ns); the lower the number, the
more often the memory is rewritten per second. But a memory chip or
module with a fast refresh rate can be hamstrung by a slow bus speed or a
limited bandwidth. If you think of bus speed as the speed limit and band-
width as the number of lanes on a superhighway, you can see how you need
both speed and bandwidth in order to move vast quantities of data rapidly.Feeling special with ECC memory ......................................................
A special class of RAM is called ECC (Error Checking and Correcting)memory.
This type of circuitry continually checks the data in search of garbled or lost
data; in some circumstances it can repair the error by itself or at least report
the problem to the user.Who might need ECC memory? If your computer is controlling a life-support
system in a hospital or managing the New York Stock Exchange, in few or no
situations is an “Oops” acceptable. For the rest of us, the very rare instance
where a computer hangs up or corrupts memory is not such a big deal, and
very few consumer laptops use ECC memory.Although ECC offers some significant advantages to certain users and applica-
tions, unless the PC is properly designed, the use of error-correcting memory
may actually slow down the computer’s operations somewhere between a tiny
bit and a whole bunch.90 Part II: Explaining What Could Possibly Go Wrong