-----Original Message-----
What do the numbers PC-3200, PC-2700, PC-2100 stand for.
I dont really get the relation with System bus and chip speed, dual
channel...
If there is a place that explains IŽd be very grateful if someone lets me
know it.
TIA
Roberto
Hello, Roberto,
The numbers refer to the maximum possible data transfer rates of DDR400,
DDR333 and DDR266 memory in bytes per second. The data path from a DDR
module (like an SDR module) is 64 bits or 8 bytes wide. Since DDR modules
send out two bits per clock cycle, the memory bus speeds are doubled to give
the maximum data rate in bits per second per data bit (sorry for any
confusion...the terminology is kind of convoluted). As far as bus speeds
go, the following memory bus speeds are associated with DDR memory:
200MHz PC3200 DDR400
166MHz PC2700 DDR333
133MHz PC2100 DDR266
Just to make things a bit more confusing, due to physical and electrical
constraints on the way that memory works in a PC, your system will never
really come all that close to performing as well as the maximum data
transfer rates seem to indicate...maybe that's a topic for an article on the
FreePCTech web site.
System bus speeds (as in the front side bus for the processor) on newer
motherboards need not necessarily be the same as memory bus speeds. For
example, my Asus motherboard supports 100MHz, 133MHz and 166MHz front side
bus speeds for the AMD Athlon CPU and 133MHz, 166MHz and 200MHz bus speeds
for the memory.
Chip speed is generally measured in nanoseconds. You can get a feel for
what chip speed is associated with what DDR memory speed by taking the
inverse of the memory bus frequency and comparing it with the chip speed.
For example, the memory bus frequency of PC3200 (DDR400) is 200MHz. The
inverse of 200,000,000 is .000000005, or 5ns. Thus, you should be able to
look on the chips of a module and see some designator (usually -5) that
indicates that it is rated as at least a 5ns chip.
A dual channel memory bus, like Nvidia's NForce2, has two independent memory
controllers on the motherboard. This is particularly important for DDR400
memory because of the very short time period for data signals (5ns). The
more modules on a given memory channel, the slower they are to respond to
signals to/from the motherboard, primarily due to electrical properties like
capacitance and inductance. With dual channel boards, it's possible to
minimize the number of modules per channel, yet still be able to have at
least two DIMMs (and in Intel's new chipset as many as six) in a system.
There's really no relationship between dual channel memory controllers and
the speed of DDR memory, but there is something of an informal one with the
system bus and that is that you should operate your memory at the highest
bus speed that does not exceed the bus speed of the CPU. The idea is to
match the data transfer rates of the two buses as closely as possible.
Slower memory becomes a bottleneck because the processor bus can handle data
faster than the memory bus can provide it. Running the memory bus faster
than the processor bus is gains virtually no performance improvement and
opens the possibility of data errors due to speed issues, particularly if
you have more modules than you have memory channels.
I hope this helps!
Drew Dunn
The NOSPIN Group
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