Datasheet

48 Chapter 1
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Personal Computer System Components
special licensing considerations and royalties from Rambus. The contract ran from 1996
until 2002. In 1999, Intel launched the first motherboards with DRDRAM support. Until
then, Rambus could be found mainly in gaming consoles and home theater components.
DRDRAM did not impact the market as Intel had hoped, and so motherboard manufactur-
ers got around Intel’s obligation by using chipsets from VIA Technologies, leading to the
rise of that company.
Although other specifications preceded it, the first motherboard DRDRAM
model was known as PC800. As with non-DRDRAM specifications that use this nam-
ing convention, PC800 specifies that, using a faster 400MHz actual clock signal and
double-pumping like DDR SDRAM, an effective frequency and FSB speed of 800MHz
is created. DRDRAM was originally named in a dissimilar fashion to other forms of
SDRAM, instead based on the FSB speed. You might recall, for those memory types,
that the FSB speed was used to name the actual chips on the modules, not the modules
themselves. PC800 DRDRAM, then, features a double-pumped 800MHz FSB. Newer
modules, such as the 32-bit RIMM 6400, are named for their actual throughput,
6400MBps, in this case. The section “RIMM” in this chapter details the physical
details of the modules.
There are only 16 data pins per channel with DRDRAM, versus 64 bits per channel
in other SDRAM implementations. This fact results in a 16-bit (2-byte) channel. A 2-byte
packet, therefore, is exchanged during each read/write cycle, bringing the overall transfer
rate of PC800 DRDRAM to 1600MBps per channel. DRDRAM chipsets require two
16-bit channels to communicate simultaneously for the same read/write request, creating a
mandatory 32-bit dual-channel mode. Two PC800 DRDRAM modules in a dual-channel
configuration produce transfer rates of 3200MBps. In motherboards that support 32-bit
modules, you would use a single RIMM 3200 to achieve this 3200MBps of throughput,
using the same actual 400MHz clock and 800MHz FSB and transferring 4 bytes at a time.
Despite DRDRAM’s performance advantages, it has some drawbacks that keep it from
taking over the market. Increased latency, heat output, complexity in the manufacturing pro-
cess, and cost are the primary shortcomings. The additional heat that individual DRDRAM
chips put out led to the requirement for heat sinks on all modules. High manufacturing costs
and high licensing fees led to triple the cost to consumers over SDR, although today there is
more parity between the prices.
In 2003, free from its contractual obligations to Rambus, Intel released the i875P
chipset. This new chipset provides support for a dual-channel platform using standard
PC3200 DDR modules. Dual-channel DDR transfers 16 bytes (128 bits) per read/write
request, giving PC3200 a total throughput rate of 6400MBps. As a result, and because
of the advent of DDR2 and DDR3, DRDRAM no longer holds any performance
advantage.
To put each of the SDRAM types into perspective, consult Table 1.2, which sum-
marizes how each technology in the SDRAM arena would achieve a transfer rate of
3200MBps, even if only theoretically. For example, PC400 doesn’t exist in the SDR
SDRAM world.
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