Hardware Reference
In-Depth Information
DDR4 modules are available in speeds of 1,600MHz (effective) and higher, with speeds of up to
3,200MHz (effective) expected in the future. Just as with DDR, DDR2, and DDR3 the true clock
speed is half the effective rate, which is technically expressed in million transfers per second
(MTps).
Table 6.11
shows the official JEDEC-approved DDR4 module types and bandwidth
specifications.
Table 6.11. JEDEC Standard DDR4 Module (284-Pin DIMM) Speeds and Transfer Rates
The topology of DDR4 is technically not a bus as was used in the DDR3 and earlier memory
standards. DDR4 uses a point-to-point connection instead, where each channel in the memory
controller is connected to a single module.
RDRAM
Rambus DRAM (RDRAM) was a proprietary (non-JEDEC) memory technology found mainly in
certain Intel-based Pentium III and 4 systems from 2000 through 2002. Very few of these systems are
still in use today.
For more information about RDRAM and RIMM modules, see Chapter 6, “Memory,” in
Upgrading
and Repairing PCs
, 19
th
Edition.
Memory Modules
Originally, PCs had memory installed via individual chips. They are often referred to as
dual inline
package
(DIP) chips because of their physical designs. The original IBM XT and AT systems had 36
sockets on the motherboard for these individual chips—and more sockets could often be found on
memory cards plugged into the bus slots. I remember spending hours populating boards with these
chips, which was a tedious job.
Besides being a time-consuming and labor-intensive way to deal with memory, DIP chips had one
notorious problem—they crept out of their sockets over time as the system went through thermal
cycles. Every day, when you powered the system on and off, the system heated and cooled, and the
chips gradually walked their way out of the sockets—a phenomenon called
chip creep
. Eventually,
good contact was lost and memory errors resulted. Fortunately, reseating all the chips back in their
sockets usually rectified the problem, but that method was labor intensive if you had many systems to
support.
