Many programs need only temporary storage for certain variables. That is, a variable may only be used for a limited time and then “thrown away”. It would be inefficient to allocate permanent space for this sort of variable. In its place, many systems use a stack. Ordinarily, an application is split into two parts, a code section and a data section. The data section contains the “permanent” (global) data. As these two will not consume the entire memory map, the remainder of the memory is often used for temporary storage via a stack. The stack starts at the opposite end of the memory map and grows toward the code and data sections. It is called a First-In-Last-Out stack or FILO stack. It works like a stack of trays in a cafeteria. The first tray placed on the stack will be the last one pulled off and vice versa. When temporary variables are needed, this memory area is used. As more items are needed, more memory is taken up. As our code exits from a function, the temporary (
auto) variables declared there are no longer needed, and the stack shrinks. If we make many, many function calls with many, many declared variables, it is possible for the stack to overrun the code and data sections of our program. The system is now corrupt, and proper execution and functioning of the program are unlikely.
Above is a memory map example of a system with 64k bytes of memory (k=1024 or 210). Individual memory slots are not shown. Only the general areas are shown.
It is worthwhile to note that in some systems, code and data are in a common area as shown (Von Neumann architecture) while in others they are physically split (Harvard architecture). Whether split or not, the basic concepts remain. So, why would we want to split the two areas, each accessed via its own memory bus1? Simple, separating the code and data allows the processor to fetch the next instruction (code) using a memory bus that is physically separate from the data bus it is currently accessing. A shared code/data memory bus would require special timing to coordinate this process as only one thing can be on the bus at any given time. Having two separate memory buses will speed execution times.
- A bus typically refers to a collection of wires or connections upon which multiple data bits (or address bits) are sent as a group.