Modified Harvard architecture

The Modified Harvard architecture is a variation of the Harvard computer architecture that allows the contents of the instruction memory to be accessed as if it were data. Most modern computers that are documented as Harvard architecture are, in fact, Modified Harvard architecture.

The original Harvard architecture computer, the Harvard Mark I, employed entirely separate memory systems to store instructions and data. The CPU fetched the next instruction and loaded or stored data simultaneously and independently. This is in contrast to a Von Neumann architecture computer, in which both instructions and data are stored in the same memory system and (without the complexity of a CPU cache) must be accessed in turn. The physical separation of instruction and data memory is sometimes held to be the distinguishing feature of modern Harvard architecture computers. With microcontrollers (entire computer systems integrated onto single chips), the use of different memory technologies for instructions (e.g. flash memory) and data (typically read/write memory) in von Neumann machines is becoming popular. The true distinction of a Harvard machine is that instruction and data memory occupy different address spaces. In other words, a memory address does not uniquely identify a storage location (as it does in a Von Neumann machine); you also need to know the memory space (instruction or data) to which the address belongs.

A computer with a Von Neumann architecture has the advantage over pure Harvard machines in that code can also be accessed and treated the same as data, and vice versa. This allows, for example, data to be read from disk storage into memory and then executed as code, or self-optimizing software systems using technologies such as just-in-time compilation to write machine code into their own memory and then later execute it. Another example is self-modifying code, which allows a program to modify itself. A disadvantage of these methods are issues with executable space protection, which increase the risks from malware and software defects. In addition, in these systems it is notoriously difficult to document code flow, and also can make debugging much more difficult.

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