Datasheet

Identifying Purposes and Characteristics of Memory 45
falling edges of the clock signal. This obtains twice the transfer rate at the same FSB clock
frequency. It’s the increasing clock frequency that generates heating issues with newer com-
ponents, so keeping the clock the same is an advantage. The same 100MHz clock gives a
DDR SDRAM system the impression of a 200MHz clock in comparison to an SDR SDRAM
system. For marketing purposes and to aid in the comparison of disparate products (DDR
vs. SDR, for example), the industry has settled on the practice of using this effective clock
rate as the speed of the FSB.
Module Throughput Related to FSB Speed
There is always an 8:1 module-to-chip (or module-to-FSB speed) numbering ratio because
of the 8 bytes that are transferred at a time with 64-bit processors. The following formula
explains how this relationship works:
FSB in MHz (cycles/second)
X 8 bytes (bytes/cycle)
throughput (bytes/second)
Because the actual clock speed is rarely mentioned in marketing literature, on packag-
ing, or on store shelves for DDR and higher, you can use this advertised FSB frequency
in your computations for DDR throughput. For example, with a 100MHz clock and two
operations per cycle, motherboard makers will market their boards as having an FSB of
200MHz. Multiplying this effective rate by 8 bytes transferred per cycle, the data rate
is 1600MBps. Now that throughput is becoming a bit trickier to compute, the industry
uses this final throughput figure to name the memory modules instead of the actual fre-
quency, which was used when naming SDR modules. This makes the result seem many
times better (and much more marketable), while it’s really only twice (or so) as good, or
close to it.
In this example, the module is referred to as PC1600, based on a throughput of 1600MBps.
The chips that go into making PC1600 modules are named DDR200 for the effective FSB fre-
quency of 200MHz. Stated differently, the industry uses DDR200 memory chips to manufac-
ture PC1600 memory modules.
Let’s make sure you grasp the relationship between the speed of the FSB and the name for
the related chips as well as the relationship between the name of the chips (or the speed of the
FSB) and the name of the modules. Consider an FSB of 400MHz, meaning an actual clock
signal of 200MHz, by the way—the FSB is double the actual clock for DDR, remember. It
should be clear that this motherboard requires modules populated with DDR400 chips, and
that you’ll find such modules on the PC3200 “rack.”
86498book.indb 45 7/22/09 5:37:42 AM