computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...

, a tagged architecture is a particular type of

computer architecture In computer engineering, computer architecture is a description of the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, t ...

where every

word A word is a basic element of language that carries an semantics, objective or pragmatics, practical semantics, meaning, can be used on its own, and is uninterruptible. Despite the fact that language speakers often have an intuitive grasp of w ...

of memory constitutes a

tagged union In computer science, a tagged union, also called a variant, variant record, choice type, discriminated union, disjoint union, sum type or coproduct, is a data structure used to hold a value that could take on several different, but fixed, types. O ...

, being divided into a number of bits of data, and a ''tag'' section that describes the type of the data: how it is to be interpreted, and, if it is a reference, the type of the object that it points to. In contrast, program and data memory are indistinguishable in the

von Neumann architecture The von Neumann architecture — also known as the von Neumann model or Princeton architecture — is a computer architecture based on a 1945 description by John von Neumann, and by others, in the ''First Draft of a Report on the EDVAC''. The ...

, making the way the memory is referenced critical to interpret the correct meaning. Notable examples of American tagged architectures were the

Lisp machine Lisp machines are general-purpose computers designed to efficiently run Lisp as their main software and programming language, usually via hardware support. They are an example of a high-level language computer architecture, and in a sense, the ...

s, which had

tagged pointer In computer science, a tagged pointer is a pointer (concretely a memory address) with additional data associated with it, such as an indirection bit or reference count. This additional data is often "folded" into the pointer, meaning stored inline ...

support at the hardware and

opcode In computing, an opcode (abbreviated from operation code, also known as instruction machine code, instruction code, instruction syllable, instruction parcel or opstring) is the portion of a machine language instruction that specifies the operat ...

level, the

Burroughs large systems The Burroughs Large Systems Group produced a family of large 48-bit mainframes using stack machine instruction sets with dense syllables.E.g., 12-bit syllables for B5000, 8-bit syllables for B6500 The first machine in the family was the B5000 in ...

, which have a data-driven tagged and descriptor-based architecture, and the non-commercial Rice Computer. (mostly written in r before1994, and archived by the

Wayback Machine The Wayback Machine is a digital archive of the World Wide Web founded by the Internet Archive, a nonprofit based in San Francisco, California. Created in 1996 and launched to the public in 2001, it allows the user to go "back in time" and see ...

on a date indicated y "20080224"in the URL) Both the Burroughs and Lisp machine are examples of

high-level language computer architecture A high-level language computer architecture (HLLCA) is a computer architecture designed to be targeted by a specific high-level programming language (HLL), rather than the architecture being dictated by hardware considerations. It is accordingly al ...

s, where the tagging is used to support types from a

high-level language In computer science, a high-level programming language is a programming language with strong abstraction from the details of the computer. In contrast to low-level programming languages, it may use natural language ''elements'', be easier to use, ...

at the hardware level. In addition to this, the original Xerox Smalltalk implementation used the least-significant bit of each 16-bit word as a tag bit: if it was clear then the hardware would accept it as an aligned memory address while if it was set it was treated as a (shifted) 15-bit integer. Current Intel documentation mentions that the lower bits of a memory address might be similarly used by some interpreter-based systems. In the Soviet Union, the Elbrus series of supercomputers pioneered the use of tagged architectures in 1973.

References

Computer architecture {{comp-sci-stub

See also

References