Memory
Technology in Brief
Although it has been very
poorly publicized, the first thing to know about
RDRAM is that there are actually three different
speed grades that are being used in PCs. Although
PC800 is mentioned in the media and in Intel
promotional material as if it is the only type of
RDRAM, it is in fact the fastest, most expensive,
and rarest grade of three different speed levels
of RDRAM. At the bottom of the performance ladder
is PC600, which is also, as you would expect, the
cheapest. It is so slow that it will probably not
be used extensively by major system manufacturers
in any computer systems.
In the middle of this RDRAM
pack is PC700, which is a standard that was only
introduced after RDRAM producers pressured
Intel to accept this compromise grade. The reason
this happened was because many of those producers
were finding it next to impossible to reach
acceptable yields at PC800. These producers also
feared that falling to PC600 would result in the
memory being rejected as too slow by their
customers. Even though this standard was adopted a
year ago, memory manufacturers are still having
trouble reaching the PC800 standard today. Because
of this, PC800 RDRAM is almost unattainable and
most of the current systems are quietly shipping
out with the slower PC700 RIMMs.
Bandwidth
over Clock-speed
The second important thing to
understand about RDRAM is that clock-speed cannot
always be trusted. Yes, the fastest grade of PC800
does run at 800 MHz, but the bus width (which
determines the amount of data that can be
transferred at any instant in time) is only 16
bits (8 bits = 1 byte so 16 bits = 2 bytes) wide.
This means that PC800 RDRAM is capable of
transferring 2 bytes x 800 MHz = 1.6 GB/sec. The
number 1.6 GB/sec is called the
"bandwidth" for PC800 and represents the
maximum possible rate that data can be transferred
using this technology.
By the way RDRAM's bus is
double-pumped meaning that it transfers data on
the rising and falling edges of clock pulses.
Accordingly PC800 is fed by a 400 MHz clock. Less
intuitively., as stated in a
recent article, PC700 actually runs at 712
MHz double-pumped from a 356 MHz clock. To insure
that there is no logic to RDRAM speed ratings,
PC600 real speed is 532 MHz triggered by 266 MHz
clock.
SDRAM has a much larger 64-bit
(8 bytes) bus width. Currently the fastest DIMMs
(sticks of SDRAM) run at 133 MHz. This gives PC133
(SDRAM running at 133 MHz) a bandwidth of 8 bytes
x 133 MHz = 1.064 GB/sec. What should be apparent
is that due to their differing bus widths, RDRAM
must run four times faster than SDRAM to provide
the same bandwidth. So when comparing these
memory technologies, be sure to take this
important fact into consideration.
Positioned by Micron and many
other manufacturers to replace SDRAM, DDR SDRAM
also has a 64-bit bus, but like RDRAM, DDR SDRAM
transfers data at the rising and falling edges of
clock signals, effectively doubling its bandwidth.
Therefore DDR SDRAM feed by a 133 MHz clock has a
bandwidth of 2 x PC133 bandwidth = 2 x 1.064
GB/sec = 2.128 GB/sec. According to Micron, DDR
SDRAM should be priced similarly to current SDRAM
making it greatly cheaper than RDRAM. Although DDR
SDRAM is currently in certain Nvidia graphics
cards, it has not been released yet for use as
main memory. DDR SDRAM should make its widespread
debut around July and should first appear in
systems built around AMD's Athlon.
Following is a simple chart
comparing the bandwidths of these competing memory
technologies.
For Intel's Coppermine based
systems, there is one final important issue to
consider in regard to bandwidth. Coppermine's data
bus is 64-bits wide, but only operates at 133 MHz.
This means that it has the same effective
bandwidth as PC133 SDRAM of 1.064 GB/sec. Because
of this bottleneck, much of the bandwidth
advantage that RDRAM has over SDRAM is lost. There
is still a theoretical advantage when DMA (Direct
Memory Access) requests and AGP transfers are
issued simultaneously with processor requests, but
our tests show that this is not enough to help
RDRAM above its other problems.
Memory
TypeDesignation
Bus
Width
(Bytes)External
Clock-speedEffective
Clock-speedBandwidth
RDRAM
PC800
2
Bytes
400
MHz
800
MHz
1.6
GB/sec
RDRAM
PC700
2
Bytes
356
MHz
712
MHz
1.424
GB/sec
RDRAM
PC600
2
Bytes
266
MHz
532
MHz
1.064
GB/sec
SDRAM
PC133
8
Bytes
133
MHz
133
MHz
1.064
GB/sec
SDRAM
PC100
8
Bytes
100
MHz
100
MHz
0.8
GB/sec
DDR
SDRAM
PC266
or DDR133
8
Bytes
133
MHz
266
MHz
2.128
GB/sec
DDR
SDRAM
PC200
or DDR100
8
Bytes
100
MHz
200
MHz
1.6
GB/sec
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