Reading a memory location creates a charge well whether or not one was previously there, destroying the original contents of that location, and so any read has to be followed by a rewrite to reinstate the original data. In some systems this was accomplished using a second electron gun inside the CRT that could write to one location while the other was reading the next. Since the display would fade over time, the entire display had to be periodically refreshed using the same basic method. As the data is read and then immediately rewritten, this operation can be carried out by external circuitry while the central processing unit (CPU) was busy carrying out other operations. This refresh operation is similar to the memory refresh cycles of DRAM in modern systems.
