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[ The PC Guide | Systems and Components Reference Guide | System Memory | Memory Speed, Access and Timing ]

DRAM Speed, System Timing and Overall Memory Speed

It is important to understand the relationship between the two main factors that control the true speed that your system memory runs at. The two factors are:

  • Memory Timing Settings: The memory's real speed is determined by the timing that the system is told to use, often via settings in the BIOS setup program. These settings control how quickly the system will try to read or write to the memory.
  • DRAM Speed: This is the minimum access time that your DRAM can physically handle, and is rated in nanoseconds (ns). The speed of the DRAM sets the limits for how fast your memory timing can be set. Note that newer SDRAM modules are sometimes rated in MHz (frequency) instead of access time.

The relationship between these two is as follows. The faster the physical DRAM is, the faster the system timing can be set. If you increase the system timing (by reducing the number of clock cycles required to access the memory using the appropriate BIOS settings) then the system will run faster--but if you set them too fast for the DRAM you are using, errors will result. The speed of the DRAMs does not directly control the speed of the memory system. It just sets the upper limit.

An analogy would be that the DRAM speed represents your car's maximum speed, how fast your car can go if there is no traffic on the road. You can increase how fast you drive (real speed) as long as you don't exceed this limit. However, if the traffic flow is going at 40 MPH then you are stuck going at 40 MPH. If you replace your current car with a faster one, it won't make any difference because you still can't go faster then 40 MPH in this situation. (I'm ignoring speed limits, driving on the shoulder and other matters of public safety here. :^) )

What this all means is that if you replace your system's 70 ns DRAM with 60 ns DRAM, the system will not run faster unless you also increase the system timing speed so that it tries to access the faster memory, faster. Conversely, replacing faster memory with slower memory won't cause the system to run any slower unless the system timing is decreased; however, if the new slower memory is too slow for the timing settings, memory errors (crashes, lockups) will result.

Note: Some systems automatically set the memory timing based on the speed of the memory, which they can detect. This in fact causes some of the confusion in this matter, because with this type of system putting faster DRAMs in will automatically cause the system to run faster. However, the principle still holds: the system timing is what is making the memory run faster. It's just that the system timing is being increased automatically and transparently to the user.

Next: DRAM Speed Ratings


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