Harvard architecture

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The Harvard architecture is a computer architecture with physically separate storage and signal pathways for instructions and data. The term originated from the Harvard Mark I relay-based computer, which stored instructions on punched tape (24 bits wide) and data in electro-mechanical counters. These early machines had limited data storage, entirely contained within the central processing unit, and provided no access to the instruction storage as data. Programs needed to be loaded by an operator, the processor could not boot itself.

Today, most processors implement such separate signal pathways for performance reasons but actually implement a Modified Harvard architecture, so they can support tasks like loading a program from disk storage as data and then executing it.

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Memory details

In a Harvard architecture, there is no need to make the two memories share characteristics. In particular, the word width, timing, implementation technology, and memory address structure can differ. In some systems, instructions can be stored in read-only memory while data memory generally requires read-write memory. In some systems, there is much more instruction memory than data memory so instruction addresses are wider than data addresses.

Contrast with von Neumann architectures

Under pure von Neumann architecture 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 .

Also, a Harvard architecture machine has distinct code and data address spaces: instruction address zero is not the same as data address zero. Instruction address zero might identify a twenty-four bit value, while data address zero might indicate an eight bit byte that isn't part of that twenty-four bit value.

Contrast with Modified Harvard architecture

A modified Harvard architecture machine is very much like a Harvard architecture machine, but it relaxes the strict separation between instruction and data while still letting the CPU concurrently access two (or more) memory buses. The most common modification includes separate instruction and data caches backed by a common address space. While the CPU executes from cache, it acts as a pure Harvard machine. When accessing backing memory, it acts like a von Neumann machine (where code can be moved around like data, a powerful technique). This modification is widespread in modern processors such as the ARM architecture and X86 processors. It is sometimes loosely called a Harvard architecture, overlooking the fact that it is actually "modified".

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