Harvard architecture

The term is often stated as having 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 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 initialize itself.

The concept of the Harvard architecture has been questioned by some researchers. According to a peer-reviewed paper on the topic published in 2022,

• "The term 'Harvard architecture' was coined decades later, in the context of microcontroller design" and only "retrospectively applied to the Harvard machines and subsequently applied to RISC microprocessors with separated caches";

• "The so-called 'Harvard' and 'von Neumann' architectures are often portrayed as a dichotomy, but the various devices labeled as the former have far more in common with the latter than they do with each other";

• "In short [the Harvard architecture] isn't an architecture and didn't derive from work at Harvard".

Modern processors appear to the user to be systems with von Neumann architectures, with the program code stored in the same main memory as the data. For performance reasons, internally and largely invisible to the user, most designs have separate processor caches for the instructions and data, with separate pathways into the processor for each. This is one form of what is known as the modified Harvard architecture.

Harvard architecture is historically, and traditionally, split into two address spaces, but having three, i.e. two extra (and all accessed in each cycle) is also done, while rare.

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 for pre-programmed tasks 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

In a system with a pure von Neumann architecture, instructions and data are stored in the same memory, so instructions are fetched over the same data path used to fetch data. This means that a CPU cannot simultaneously read an instruction and read or write data from or to the memory. 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 is not 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, which is a powerful technique). This modification is widespread in modern processors, such as the ARM architecture, Power ISA and x86 processors. It is sometimes loosely called a Harvard architecture, overlooking the fact that it is actually "modified".

Another modification provides a pathway between the instruction memory (such as ROM or flash memory) and the CPU to allow words from the instruction memory to be treated as read-only data. This technique is used in some microcontrollers, including the Atmel AVR. This allows constant data, such as text strings or function tables, to be accessed without first having to be copied into data memory, preserving scarce (and power-hungry) data memory for read/write variables. Special machine language instructions are provided to read data from the instruction memory, or the instruction memory can be accessed using a peripheral interface. (This is distinct from instructions which themselves embed constant data, although for individual constants the two mechanisms can substitute for each other.)