Hardware Reference
In-Depth Information
Unfortunately, refreshing the memory takes processor time away from other tasks because each
refresh cycle takes several CPU cycles to complete. In older systems, the refresh cycling could take
up to 10% or more of the total CPU time, but with modern systems running in the multigigahertz range,
refresh overhead is now on the order of a fraction of a percent or less of the total CPU time. Some
systems allow you to alter the refresh timing parameters via the CMOS Setup. The time between
refresh cycles is known as
tREF
and is expressed not in milliseconds, but in clock cycles (see
Figure
6.1
).
Figure 6.1. The refresh period dialog box and other advanced memory timings can be adjusted
manually through the BIOS Setup program.
It's important to be aware that increasing the time between refresh cycles (tREF) to speed up your
system can allow some of the memory cells to begin draining prematurely, which can cause random
soft memory errors to appear.
A
soft error
is a data error that is not caused by a defective chip. To avoid soft errors, it is usually
safer to stick with the recommended or default refresh timing. Because refresh consumes less than 1%
of modern system overall bandwidth, altering the refresh rate has little effect on performance. It is
almost always best to use default or automatic settings for any memory timings in the BIOS Setup.
Many modern systems don't allow changes to memory timings and are permanently set to automatic
settings. On an automatic setting, the motherboard reads the timing parameters out of the serial
presence detect (SPD) ROM found on the memory module and sets the cycling speeds to match.
DRAMs use only one transistor and capacitor pair per bit, which makes them dense, offering more
memory capacity per chip than other types of memory. Currently, DRAM chips are being prepared for
