What's differenc between rambus and ddr

[sancheuz]

I two questions. First, which is faster ddr400 or rambus pc 800 or pc 1024. And secondly why do they call ddr 400 sometimes pc 400 and pc 3200, or ddr 333 pc 2700 or pc 333?

 

[KimB]

One of the major differences is that Rambus only has one chip active at a time, while DDR has all chips active at a time. This means that whenever the computer needs data from a different chip, Rambus will have to spend time powering up the new chip.

I'm sure you could imagine how this could be bad if the computer needed simultaneous access of many different locations in memory.

Anyway, DDR400 and PC800 Rambus have the same bandwidth, but DDR400 has lower latency, and thus should be faster in the majority of situations.

As for the naming, yes, it's confusing.
PC3200 was to be the new term by the DDR marketting folk. They decided it was better to use the bandwidth of the module instead of the clock speed.

PC400 comes from the older practice of naming the memory by the clock speed.

DDR400 comes from people that would rather describe the memory by clock speed, but understand that PC400 should no longer be used.

 

[Doomtrooper]

Sdram transfers data in 32-64 bits wide while Rambus sends data in 18 bit chunks but alot more frequently, hence the higher clock speed. Rambus has higher latency and runs very hot, two drawbacks to Rambus that I know of since I am a AMD zealot

DDR333 refers to million cycles per second (MHz)
PC2700 refers to million bytes per second (for DDR memory)
64-bit memory (the current standard for PC memory) = 8 bytes per transfer
333MHz x 8 bytes = approximately 2700 million bytes per second

or another Example: DDR266, 266 million x 8 = 2100 million (PC2100).

 

[mat]

Anyway, DDR400 and PC800 Rambus have the same bandwidth

only when you use Dual Channel Rambus and single "Channel" DDR

 

[KimB]

Only when you use Dual Channel Rambus and single "Channel" DDR

Right. Which corresponds to current implementations.

 

[Saem]

DDR can burst high enough to saturate the bus, but it doesn't sustain it as long as DRDRAM.

One other thing to consider is that DRDRAM's latency linearly scales down with frequency, however, for reasons I can't go into at this time, DDR doesn't.