Each word contains a header and a data field. There are several important pointers into main memory. There is a pointer to the most recently defined word, which is used to start searches back through memory when compiling words. There is a pointer to the top of the return stack. There is a pointer to the current end of the dictionary, used while compiling.
Immediate instructions do something significant when they are used.
Normal instructions compile a pointer to their executable part onto the end of the dictionary. We would like to implement these words as well. As the highest priority thing, we will build control flow structures first. Once we have control structures, we can write recursive routines that terminate, and we are ready to tackle tasks like parsing, and the building of a command mode. The first thing we have to do is give the symbols for our built-ins.