Friday, February 19, 2016

Modified Harvard Architecture

Modified Harvard Architecture Model

Modified Harvard Architecture

True Harvard architecture - instruction memory and data memory are separated.

Modified Harvard architecture - instruction memory and data memory are the same though their buses are separated.

Modified Harvard architecture - the instruction and data separated caches of which data sources would be the same memory.

A pure Harvard architecture has disadvantage:  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, read-only technology for the instruction memory 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).

Nowadays most CPU has Von Neumann like unified address space and also separate instruction and data caches as well as memory protection, making them more Harvard-like.

Modern general-purpose high-performance processors use separate instruction and data caches, and so could be classified as modified Harvard even though they use a unified address space.

Related topics:
Von Neumann Architecture   |   Harvard Architecture   |   Von Neumann Vs Harvard Architecture   |   Microprocessor Vs Microcontroller   |   Choosing a Microcontroller

List of topics: Microcomputer

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