Modern micro-controllers are basically Systems on a Chip (SOC) with memory integrated into the package as well. The lack of external memory buses allows for smaller packages and pin counts. It also greatly simplifies the use of these parts.

Many smaller micros (PIC16, PIC24, AVR) are setup in what is known as the Harvard Architecture. This means that the executable memory and data memory are completely separated. Instructions cannot be executed from the data memory, and the different memory sections can be addressed in completely different ways.

Usually the executable memory is on Flash or ROM (rarely now days). While the processor is able to directly fetch instructions from Flash, there is typically no way to directly read variables from it. Usually special retrieval instructions or address remapping are needed. Some compilers will automatically use the constructs for any constant variable reads.

Furthermore, Flash and RAM don’t even need to have the same line size. The PIC16F series has 14 bit instruction lines and 8 bit data lines. The PIC24F series has 24 bit instruction lines and 16 bit data lines. The AVR has 16 bit instruction lines and 8 bit data lines. The odd flash word sizes can often make constant storage complicated. The compilers for the two PICs usually store constants in just the bottom 8 or 16 bits of a line thus wasting big chunks of Flash if you use lots of constants. If you want to make full use of the storage, you must use the Flash peripheral registers instead of the simpler address remapping that the compiler uses. While the alignments are much cleaner on the AVR, all Flash data reads have to be done with a special load instruction. So it falls in between the two PIC methods in complexity.

Details on these operations are a topic for a future post. Next time I will cover Peripheral Registers, Flash Pages, and Memory Banks.