What does PC<number> mean? I know PC2700 goes along with the 333 MHZ bus.
What does it really mean and how do you match up the appropriate RAM with
the bus speed?
What happens if I put PC2000 RAM in a 333 MHZ bus? I know its slower, but
will the RAM burn out? What if I put faster RAM in the motherboard?
What happens if I put say PC2700 and PC3500 RAM in the same motherboard?
PC rating is the theoretical throughput when the module is running at the
max speed spec'd stable by manufacturer. In other words, PC2100 is spec'd
to be stable up to 133MHz at Double Date Rate...
133.3 x 2 x 8 = 2133, rounded off to (PC)2100.
166.7 x 2 x 8 = 2667, rounded off to (PC)2700
200.0 x 2 x 8 = 3200, no rounding needed, (PC)3200
If you put PC2100 module in a system with DDR333 FSB, you'd have to
either:
1) set the bios (or jumpers) to asynchronous mode, meaning the memory bus
clock rate is slower than the FSB clock rate. That significantly hurts
performance.
2) leave the memory bus in synchronous mode, memory at DDR333, 166MHz
clock rate, is overclocking it. If you're lucky (or possibly by slowing
down some bios memory timings) it will be stable, but that is not
guaranteed by memory manufacturer, you run a very high risk of memory
errors... extensive testing would be needed before such an overclocked
memory should even be booted to the operating system. Best practice is to
not o'c memory at all.
You can mix PC2700 and PC3500, but the system is then only expected to be
stable up to speed of slower module. However, the system doesn't change
it's operating speed (frequency) based on what modules are installed (with
rare exceptions like filling a 3rd memory bank on certain boards) rather
the motherboard will choose the memory bus speed regardless of the
modules, rather based on the CPU installed, the default settings for
whether the memory bus will run in synchronous or asynchronous mode, or
instead by user's manual setting of bios parameters or changing jumpers.
For an Athlon based system the best choice is usually to run memory at
same, synchronous speed to the FSB. For example an XP2500 Barton uses
DDR333 FSB, which is 166MHz clock rate, so it should have at least PC2700
memory. For a P4 there can be performance gain from running asynchronous
memory bus FASTER than the FSB, so a P4 with 800 "MHz" (actually a 200MHz
FSB Quad Data Rate), might be paired with PC3200 for synchronous memory or
PC3700 for asynchronous memory, but PC3700 is still pretty expensive, most
people feel the cost too high for the relative gain and the faster the
memory the more difficult to use multiple modules for dual channel mode
(or even in single channel mode for that matter). Dual channel mode is a
whole 'nuther thread though...
