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Harvard architecture is a computer architecture with physically separate storage and signal pathways for instructions and data. In Computer engineering, computer architecture is the conceptual design and fundamental operational structure of a Computer system Computer data storage, often called storage or memory, refers to Computer components devices and recording media that retain digital 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 (23 digits wide). The IBM Automatic Sequence Controlled Calculator ( ASCC) called the Mark I by Harvard University, was the first large-scale automatic digital Punched tape or paper tape is a largely obsolete form of Data storage, consisting of a long strip of paper in which holes are punched to store data These early machines had limited data storage, entirely contained within the data processing unit, and provided no access to the instruction storage as data, making loading and modifying programs an entirely offline process.

Contents

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 Computing, " word " is a term for the natural unit of data used by a particular computer design In Computer science, a memory address is an identifier for a memory location at which a Computer program or a hardware device can store a piece of data Instruction memory is often wider than data memory. 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 much wider than data addresses.

Contrast with other computer architectures

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. The von Neumann architecture is a design model for a stored-program Digital computer that uses a processing unit and a single separate storage structure In Computer science, a cache (kæʃ like "cash") is a collection of data duplicating original 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. The Modified Harvard Architecture is a variation of the Harvard computer architecture that allows the contents of the instruction memory to be accessed as if it were 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.

Speed

In recent years, the speed of the CPU has grown many times in comparison to the access speed of the main memory. Care needs to be taken to reduce the number of times main memory is accessed in order to maintain performance. If, for instance, every instruction run in the CPU requires an access to memory, the computer gains nothing for increased CPU speed — a problem referred to as being memory bound.

It is possible to make extremely fast memory but this is only practical for small amounts of memory for both cost and signal routing reasons. The solution is to provide a small amount of very fast memory known as a CPU cache which holds recently accessed data. As long as the memory that the CPU needs is in the cache, the performance hit is much smaller than it is when the cache has to turn around and get the data from the main memory. Cache tuning is an important aspect of computer design.

Modern high performance CPU chip designs incorporate aspects of both Harvard and von Neumann architecture. On-chip cache memory is divided into an instruction cache and a data cache. Harvard architecture is used as the CPU accesses the cache. In the case of a cache miss, however, the data is retrieved from the main memory, which is not divided into separate instruction and data sections. Thus, while a von Neumann architecture is presented to the programmer, the hardware implementation gains the efficiencies of the Harvard architecture.

Uses

Harvard architectures are also frequently used in:

See also

The von Neumann architecture is a design model for a stored-program Digital computer that uses a processing unit and a single separate storage structure The Modified Harvard Architecture is a variation of the Harvard computer architecture that allows the contents of the instruction memory to be accessed as if it were data
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