Hi mishedlo; Re "Is there only 1 published spec for RDRAM
but multiple specs for DDRAM? If more than 1 spec for DDR - How many?"
This is a good question, and is a possible indication of your desire to learn something.
Rambus publishes a specification for RDRAM chips, but each manufacturer has the option to exceed those requirements. For example, different makers, using different processes, will be able to guarantee different power consumptions, and the parts will have different specifications for that. In addition, RDRAM parts are available in something like 4 different, incompatible, package / pinout types [i.e. regular, mirror, Micron's uBGA, and PS/2], and you have to be sure which part you are getting. (This does not apply to mom and pop, as the parts are conveniently soldered down onto RIMMs for you guys.) SDRAM and DDR are almost entirely shipped in the same packages. On the other hand, SDRAM and DDR are available in more widths than RDRAM.
DDR and SDRAM is similar to RDRAM, in that each manufacturer will publish a different specification. In addition, different manufacturers may produce parts with different added features. These features would include things like virtual channel, for instance. By doing this, the manufacturer gets to sell their parts into the standard DDR market, but also gets to provide a little extra performance for customers who want the virtual channel interface, for instance.
This is not a departure from standard industry procedure. SDRAM, for instance, is available with virtual channel features as well. The VIA chipsets support that feature and provide some handsome performance using it.
With SDRAM, there are minor differences in how the parts have to be initialized &c. Some of these differences show up in hardware, and the hardware designers take it into account. I know that this sounds like a death-defying act of bravado to mom and pop, but in practice it is really quite simple. You get copies of all the SDRAM specs from all the manufacturers and you read them. Then you put together an initialization routine that will satisfy all the requirements. Or, as with RDRAM, you make sure that your power supply can supply enough current for the worst power hungry RDRAM maker. This does not require superhuman skills, it is just the normal, day to day, engineering practice in memory design.
All this goes back to showing why it is that when another company makes a DDR DIMM, it has to be "qualified" to show it works in a given motherboard. RDRAM has the same problem, that is why Intel publishes a list of RIMM modules that are compatible on their website.
As I've said before, if you want an early DDR machine, you should buy it with all the memory already installed, it could save you a lot of headaches. All that stuff is likely to be worked out a couple quarters later. One of the worst (modern) problems is the serial PROM that holds data regarding the DIMM. SDRAM has had numerous problems with those. RDRAM has had a fairly easy life largely because it has not been available from very many memory makers. Compatibility problems magnify when you have large numbers of suppliers.
-- Carl |
