W
e invariably refer to the video
memory in modern videocards as
GDDR, diff erentiating it only by
version (GDDR2, GDDR3, GDDR4, and now
GDDR5), but the technology’s full acronym
is actually GDDR SDRAM, which stands for
Graphics Double Data Rate Synchronous
Dynamic Random Access Memory.
“Double data rate” describes the
memory’s capacity for double-pumping
data: Transfers occur on both the rising and
falling edges of the clock signal. This endows
memory clocked at 800MHz with an eff ective
data-transfer rate of 1.6GHz. “Synchronous”
refers to the memory’s ability to operate in
time with the computer’s system bus. This
allows the memory to accept a new instruc-
tion without having to wait for a previous
instruction to be processed, a practice known
as instruction pipelining.
GDDR2 memory was never a very
popular solution among GPU manufacturers:
The technology required 2.5 volts to power
its input buff ers and core logic (i.e., VDD
voltage), which is the same as GDDR. GDDR2
operated at much higher clock speeds than
its predecessor, however, which produced
a tremendous amount of heat. The fact that
GDDR2’s VDDQ voltage requirement (the
electricity needed to power the memory’soutput buff ers) was only 1.8 volts didn’t com-
pensate for this problem.SURVIVAL OF THE FITTEST
GDDR3—an open standard developed by ATI
in conjunction with the standards organiza-
tion JEDEC Solid State Technology Asso-
ciation—is the most widely used graphics
memory technology in use today. Ironically,
Nvidia introduced the fi rst
graphics processors designed
to use GDDR3: The GeForce
FX 5700 Ultra, followed by the
GeForce 6800 Ultra. ATI didn’t
deploy a GDDR3 solution until
it shipped the Radeon X800.
GDDR3 improved on previ-
ous GDDR designs by supporting higher
clock speeds while requiring less power.
These chips consume less electricity, so they
produce less heat and can rely on simpler
cooling hardware (GDDR3’s VDD and VDDQ
voltage requirements are both 1.8 volts).
GDDR3 also has separate read and write data
strobes, which contributes to a much faster
read-to-write ratio (meaning the turnaround
from a read operation to a write operation
occurs much more quickly) than GDDR2 sup-
ported. GDDR3 chips have a hardware reset
feature that can wipe their memory cleanto start receiving new data should such an
operation be necessary.
ATI and Nvidia (in conjunction with
JDEC) both had a hand in establishing the
specifi cation for the next generation of
graphics memory, GDDR4, but Nvidia has so
far decided not to use the new technology in
any of its reference designs. ATI, meanwhile,
incorporated the new memory fi rst in itsRadeon X1950 XTX cards and subsequently
in several models of its Radeon HD 2000,
3000, and 4000 series.EVOLUTIONARY DEAD END
GDDR4’s improvements over GDDR3 were
mostly incremental. It seemed to off er a
power advantage in that it could operate with
just 1.5 volts, compared to GDDR3’s 1.8 volts.
Board designers, however, quickly discovered
that they needed 1.8 volts anyway to ensure
stability at higher clock rates.
Two other GDDR4 enhancements areGDDR Memory Features Compared
AT A GLANCEGDDR3 GDDR4 GDDR5
DRAM Density 256Mb to 1Gb 512Mb 512Mb to 2Gb
Data Rate 1.0Gb/s to 2Gb/s 1.6Gb/s to 3.2Gb/s 3.6Gb/s to 6.0Gb/s
VDD/VDDQ Voltage 1.8v/1.8v 1.5v/1.5v or 1.8v/1.8v 1.5v /1.5v
Pre-Fetch Scheme 4 bit 8 bit 8 bit
Burst Length 4 or 8 bit 8 bit 8 bit
Real-World Memory Nvidia GeForce GTX 280 AMD Radeon HD 3870 AMD Radeon HD 4870
Bandwidth Example (1GB frame buffer with a 512-bit (512MB frame buffer with (512MB frame buffer with
memory interface): 142GB/s 256-bit interface): 72GB/s a 256-bit interface): 115GB/sGDDR5 PROMISES TO DELIVER
TWICE THE MEMORY BANDWIDTH
OF GDDR3 RUNNING AT THE
SAME CLOCK FREQUENCY.70 |MAMAMAXIMXIMXIMXIMUUUUMMPPPCC|OCT 08 |www.maximumpc.com
WHITE PAPER
R&D^
EXAMINING TECHNOLOGY AND PUTTING IT TO USETracking the ongoing evolution of graphics memory —MICHAEL BROWN
GDDR Memory
WHITE PAPER
GDDR Memory
WHITE PAPER