Hardware Reference
In-Depth Information
Caution
In general, PC133 memory is considered to be backward compatible with PC100 memory.
However, some chipsets or motherboards had more specific requirements for specific types of
100MHz or 133MHz chips and module designs. If you need to upgrade an older system that
requires PC100 memory, you should not purchase PC133 memory unless the memory is
specifically identified by the memory vendor as being compatible with the system. You can use
the online memory-configuration tools provided by most major memory vendors to ensure that
you get the right memory for your system.
Typically, you find SDRAM modules rated CL 2 or CL 3.
DDR SDRAM
DDR SDRAM memory is a JEDEC standard that is an evolutionary upgrade in which data transfers
twice as quickly as standard SDRAM. Instead of doubling the actual clock rate, DDR memory
achieves the doubling in performance by transferring twice per transfer cycle: once at the leading
(falling) edge and once at the trailing (rising) edge of the cycle (see
Figure 6.4
)
. This effectively
doubles the transfer rate, even though the same overall clock and timing signals are used. To
eliminate confusion with DDR, regular SDRAM is often called
single data rate
(SDR).
DDR SDRAM first came to market in the year 2000 and was initially used on high-end graphics cards
because there were no motherboard chipsets to support it at the time. DDR finally became popular in
2002 with the advent of mainstream supporting motherboards and chipsets. From 2002 through 2005,
DDR was the most popular type of memory in mainstream PCs. DDR SDRAM uses a DIMM module
design with 184 pins.
Figure 6.6
(later in this chapter) shows the 184-pin DDR DIMM.
DDR DIMMs come in a variety of speed or throughput ratings and normally run on 2.5 volts.
Table
are designated by their speed in megatransfers per second, whereas the modules are designated by
their approximate throughput in megabytes per second.
Table 6.5. JEDEC Standard DDR Module (184-Pin DIMM) Speeds and Transfer Rates
The major memory chip and module manufacturers normally produce parts that conform to the official
JEDEC standard speed ratings. However, to support overclocking, several memory module
manufacturers purchase unmarked and untested chips from the memory chip manufacturers and then
