RDRAM <b><font color=red><i>does</i></font color=red></b> run with a bit on the rising edge and falling edge of the signal. It runs at double data rates.
Look at bandwidth calculations...
You know that PC800 has 1.6GB/s bandwidth. How is that calculated???
800MHz x 2 bytes per clock (or bytes per cycle {MHz = Cycles/Second}) = 1600MB/s = 1.6GB/s (or 12,800,000,000 bits per second.)
The <A HREF="http://www.intel.com/design/chipsets/850/index.htm?iid=ipp_browse+chpsts_850dsk&#mch" target="_new">clock generator runs at 400MHz</A>. 400MHz x 2 (DDR) = 800MHz. (The cycle has two bits per wave.) It is "double pumped." (For more documentation on the system clock in the MCH, Memory Controller Hub, see <A HREF="http://www.intel.com/design/chipsets/datashts/290691.htm" target="_new">here for the Intel 82850 MCH Datasheet</A>.)
There are two, 2, bits per cycle. One happens to be on the rising edge and one is on the falling edge.
<A HREF="http://www.hardwarecentral.com/hardwarecentral/print/1686/" target="_new">"<b>RDRAM vs. SDRAM</b>
RDRAM promises a bandwidth twice that of PC100; well, true to some extent, but only valid if comparing PC800 RDRAM with PC100 SDRAM. PC800? PC100? Confusing to say the least, as that would suggest that PC800 is 8X the speed of PC100. Upon closer examination, RDRAM uses a 2 byte (16 bit) wide databus versus SDRAM's 8byte (64 bit) wide databus.
Furthermore, the PC800 rating is a bit confusing, as PC800 RDRAM is actually a double-pumped module operating at 400 MHz clockspeed. Double-pumped simply means data is transferred to the RDRAM on both the rising and falling edge of the clock, often referred to as double data rate (DDR), creating an effective 800 MHz memory rating. PC100 SDRAM is referred to as single data rate (SDR) and operates at 100 MHz clockspeed; it can only transfer data on the rising edge of the clock, thus having an effective 100 MHz memory rating.</A>
Does this finally settle confusion about the incorporation of the two technologies? RDRAM already runs as DDR.
The major differences are...
1. RDRAM is serial and DDR SDRAM is parallel.
2. The data buses. (SDRAM is 64 bits wide and RDRAM is 16 bits wide.)
3. The latencies are different. (Architecture is different and the timing, signals, and strobes are different. PC100 SDRAM has a latency of about 90ns (nanoseconds) and RDRAM PC800 has a latency of about 67.5ns. (DDRSDRAM is a little higher than SDRAM, clock for clock that is, because of the additional timing sets.)
Number of channels, ECC, Registered, and most of the other arrangements are practically the same.
<b>"I put instant coffee in the microwave and almost went back in time" - Steven Wright</b> :lol:
Edit:
The number of bits in my previous calculation was incorrect. The corrected number is listed above.
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