In computer science, a tagged architecture is a particular type of computer architecture where every word of memory constitutes a tagged union, 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.
Two notable series of American tagged architectures were the Lisp machines, which had tagged pointer support at the hardware and opcode level, and the Burroughs large systems which had a data-driven tagged and descriptor-based architecture. Another "exemplary" instance was the architecture of the Rice Computer. Both the Burroughs and Lisp machine were examples of high-level language computer architectures, where the tagging was used to support types from a high-level language 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.