HARVARD ARCHITECTURE vs. VON NEUMANN

HARVARD ARCHITECTURE vs. VON NEUMANN

If you are new to Microcontrollers one of the arguments you are going to
hear bantered about is Harvard Architecture versus the Von Neumann
Architecture.


THE VON NEUMANN ARCHITECTURE

Most computers we are familiar with use an architecture called Von Neumann.
The term arose out of Neumann's 1945 draft report on the ADVAC computer. He
was not, however the original inventor of it.

A Von Neumann machine has one large monolithic RAM structure that contains
both program memory and data memory mixed together. Since both program steps
and data must be loaded from the same place, it can create a problem called
the Von Neumann Bottle-Neck.


THE HARVARD ARCHITECTURE

Most microcontrollers use a different system called Harvard architecture.
The larger program storage and the smaller data memory are separated. The
first such machine, the Harvard Mark I had it's programs hard-coded on
paper-tape and the volatile data was loaded into electric relays.


Harvard style machines allow program steps to be fetched at the same time as
data, thereby creating potentially faster through-put and less of a
bottle-neck. They also have the benefit that run away processes can't damage
the program stored in the non-volatile program area so they're more stable.
Many C programs lack proper boundary checking and a null pointer or an
over-run buffer can overwrite and crash a program that shares RAM with data.
If you are new to this architecture you need to keep this in mind. When
creating a routine that needs a few bytes of storage, I would normally
create that space within the routine itself. On a Harvard machine, those
bytes would not be in volatile RAM but part of the hard coded program memory
stored in ROM (or FlashRAM).

source :http://sci.tech-archive.net/Archive/sci.electronics.misc/2006-04/msg00057.html