What is the difference between Harvard and modified Harvard architecture?

Answer 1

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 by contrast with a Von Neumann architecture computer, in which both instructions and data are stored in the same memory system and (without the complexity of a 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. However, with entire computer systems being 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 applies.

Modified Harvard architecture:

A pure Harvard architecture computer suffers from the disadvantage that mechanisms must be provided to separately load the program to be executed into instruction memory and any data to be operated upon into data memory. Additionally, modern Harvard architecture machines often use a read-only technology for the instruction memory and read/write technology for the data memory. This allows the computer to begin execution of a pre-loaded program as soon as power is applied. The data memory will at this time be in an unknown state, so it is not possible to provide any kind of pre-defined data values to the program.

The solution is to provide a hardware pathway and machine language instructions so that the contents of the instruction memory can be read as if they were data. Initial data values can then be copied from the instruction memory into the data memory when the program starts. If the data is not to be modified (for example, if it is 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).