1.6. A Closer Look at Memory Accesses: Register vs. Immediate

The examples so far presume that the programmer knows the exact memory location of every number that he or she wants to load and store. In other words, it presumes that in composing each program, the programmer has at his or her disposal a list of the contents of memory cells #0 through #255.

While such an accurate snapshot of the initial state of main memory may be feasible for a small example computer with only 256 memory locations, such snapshots almost never exist in the real world. Real computers have billions of possible locations in which data can be stored, so programmers need a more flexible way to access memory, a way that doesn’t require each memory access to specify numerically an exact memory address.