In a computer with the contrasting von Neumann architecture (and no cache), the CPU can be either reading an instruction or reading/writing data from/to the memory. Both cannot occur at the same time since the instructions and data use the same bus system.
In a computer using the Harvard architecture, the CPU can both read an instruction and perform a data memory access at the same time, even without a cache. A Harvard architecture computer can thus be faster for a given circuit complexity because instruction fetches and data access do not contend for a single memory pathway.
The Modified Harvard architecture is very much like the Harvard architecture but provides a pathway between the instruction memory and the CPU that allows words from the instruction memory to be treated as read-only data. This allows constant data, particularly text strings, to be accessed without first having to be copied into data memory, thus preserving more data memory for read/write variables. Special machine language instructions are provided to read data from the instruction memory. Most modern computers that are documented as Harvard Architecture are, in fact, Modified Harvard Architecture.
