Sentence:  This concept can be extended further by allowing a primary <WSD>overlay</WSD> to call a secondary overlay, and so on. However, we will limit ourselves here to the use of primary overlays. Before proceeding further, you need to understand the memory map of the computer, which is a diagram showing the use to which the memory of the computer is put. 
Meaning of the word marked with <WSD> tags:  A block of program code that is loaded over something previously loaded, so as to replace the functionality.