How can DRAM be used as a computer memory if values quickly become zero?
The answer lies in a simple technique: devise a way to read a bit out of memory before
the charge has time to dissipate, and then write the same value back again. Writing a
value causes the capacitor to start again with a full charge. So, reading and then writing
a bit will reset the capacitor without changing the value of the bit.
In practice, computers that use DRAM contain an extra hardware circuit, known as
a refresh circuit, that performs the task of reading and then writing a bit. Figure 10.2
illustrates the concept.