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DRAM Speed Ratings
There are two different ways that DRAM chips are rated for speed. Conventional asynchronous DRAM chips have a rated speed in nanoseconds (ns, or a billionth of a second), a speed which represents the minimum access time for doing a read or write to memory. This includes the entire access cycle. Most asynchronous memory in modern systems is 50, 60 or 70 ns in speed. Older systems (386 and earlier) use usually 70 or 80 ns RAM. Very old systems use even slower memory: 100, 120 or even 150 ns. Systems running with a clock speed of 60 MHz or higher generally require 60 ns or faster memory to function at peak efficiency. 70 ns is fine for 486 or older PCs.
Synchronous memory is much faster than conventional asynchronous RAM. It is usually rated at 12, 10 or even 7 nanoseconds; however you have to be careful here. An SDRAM module rated at 10 ns is not "5 times faster" than an EDO module rated at 50 ns. Since SDRAM is synchronized to the internal system clock, SDRAM speed ratings refer to the maximum speed at which the SDRAM module can burst data onto the bus. This does not include the addressing latency time the way asynchronous DRAM speed ratings do, which is why the numbers are much smaller. The core DRAMs inside the SDRAM module are usually not any faster than those of older technologies; the increase in usable speed is due to how the module is constructed and controlled. See this section on SDRAM for more details.
DRAM chips are usually marked with their speed via a suffix at the end of the part number. If you look at the chips themselves, you'll see something like "-6" or "-60". This usually means 60 nanosecond DRAM. The suffix found on SDRAM chips is often "-12", "-10" or "-07". Note that older memory running at 100 or 120 ns also used "-10" and "-12" sometimes. This memory hasn't been used in years so there really shouldn't be any confusion between the two types. However, 70 ns memory uses "-7" and this can be readily confused with 7 ns SDRAM memory if you are not careful.
Note: In addition to being
refered to using a nanosecond speed rating, SDRAMs are also often rated in terms of their
maximum frequency, in MHz. This is really the same thing, just expressed in a different
way: for example, an SDRAM module with a 10ns rating would be called instead a "100
MHz SDRAM". 100 MHz is 100 million cycles per second, which is the reciprocal of
10ns, one-hundred-millionth of a second per cycle. This MHz number is not the
same as saying that the SDRAM with that rating is designed for a system of that
speed. A 100 MHz SDRAM may not function in a 100 MHz system bus PC. See here for more.
The rated speed of the memory is a maximum. In theory, the memory cannot support memory timing that requires a faster speed of RAM. However, in practice many companies rate their DRAM conservatively, so that the memory will function at a higher speed than what is indicated. This is why many Pentium systems running on a 66 MHz bus will work with 70 ns memory, even when set to 60 ns timing. However, this is not reliable and cannot be counted on (in a way, it is a form of overclocking) and is not recommended. You can usually compensate for slower memory by turning down the system timing level, which will cause a small performance decrease but give you better reliability.
Next: Mixing DRAM Speeds