Memory - Upgrading and Repairing PCs - page 327

independently test and sort them by how fast they run. These are then packaged into modules with
unofficial designations and performance figures that exceed the standard ratings. Table 6.6 shows the
popular unofficial speed ratings I've seen on the market. Note that because the speeds of these
modules are beyond the standard default motherboard and chipset speeds, you won't see an advantage
to using them unless you are overclocking your system to match.
Table 6.6. Overclocked (Non-JEDEC) DDR Module (184-Pin DIMM) Speeds and Transfer
Rates
Most chipsets that support DDR also support dual-channel operation—a technique in which two
matching DIMMs are installed to function as a single bank, with double the bandwidth of a single
module. For example, if a chipset supports standard PC3200 modules, the bandwidth for a single
module would be 3,200MBps. However, in dual-channel mode, the total bandwidth would double to
6,400MBps. Dual-channel operation optimizes PC design by ensuring that the CPU bus and memory
bus both run at the same speeds (meaning throughput, not MHz) so that data can move synchronously
between the buses without delays.
The cycle time in nanoseconds (billionths of a second) matches the base clock speed, but DDR
modules transfer twice per cycle, so the bus speed frequency is always equal to double the clock
frequency. The throughput or bandwidth is simply the bus frequency times the width, which gives the
rate at which data can be read from or written to the module.
Typically, you can find DDR modules rated CL 2, 2.5, or 3.
With DDR, it is generally okay to install a module that is faster than the system requires, but you
should not install a slower module than the motherboard requires. For example, you can usually
install PC2700 memory even if the system requires only PC2100 or even PC1600, but if the system
requires PC2700, you should not install the slower PC2100 or PC1600 modules.
DDR2 SDRAM
DDR2 is a faster version of DDR memory. It achieves higher throughput by using differential pairs of
signal wires to allow faster signaling without noise and interference problems. DDR2 is still double
data rate just as with DDR, but the modified signaling method enables you to achieve higher clock
speeds with more immunity to noise and crosstalk between the signals. The additional signals
required for differential pairs add to the pin count—DDR2 DIMMs have 240 pins, which is more
than the 184 pins of DDR. The original DDR specification officially topped out at 400MHz (although