Modified Harvard Architecture - Modified Harvard Architecture

Modified Harvard Architecture

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 and 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.

Accordingly, some pure Harvard machines are specialty products. Most modern computers instead implement a modified Harvard architecture. Those modifications are various ways to loosen the strict separation between code and data, while still supporting the higher performance concurrent data and instruction access of the Harvard architecture.

The most common modification builds a memory hierarchy with a CPU cache separating instruction and data. This unifies all except small portions of the data and instruction address spaces, providing the von Neumann model. Most programmers never need to be aware of the fact that the processor core implements a (modified) Harvard architecture, although they benefit from its speed advantages. Only programmers who write instructions into data memory need to be aware of issues such as cache coherency and executable space protection.

Another change preserves the "separate address space" nature of a Harvard machine, just providing special machine operations to access the contents of the instruction memory as data. Because data is not directly executable as instructions, such machines are not always viewed as "modified" Harvard architecture:

• Read access ... initial data values can be copied from the instruction memory into the data memory when the program starts. Or, if the data is not to be modified (it might be a constant value, such as pi, or a text string), it can be accessed by the running program directly from instruction memory without taking up space in data memory (which is often at a premium).
• Write access ... a capability for reprogramming is generally required; few computers are purely ROM based. For example, a microcontroller usually has operations to write to the flash memory used to hold its instructions. This capability may be used for purposes including software updates and EEPROM replacement.