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In [[computer architecture]], '''4-bit''' [[Integer (computer science)|integer]]s, or other [[data (computer science)|data]] units are those that are 4 [[bit]]s wide. Also, 4-bit [[central processing unit]] (CPU) and [[arithmetic logic unit]] (ALU) architectures are those that are based on [[processor register|register]]s, or [[Bus (computing)|data bus]]es of that size. [[Memory address]]es (and thus [[address bus]]es) for 4-bit CPUs are generally much larger than 4-bit (since only 16 memory locations would be very restrictive), such as 12-bit or more, while they could in theory be 8-bit. A group of four bits is also called a [[nibble]] and has 2<sup>4</sup> = 16 possible values.
Some of the first [[microprocessor]]s had a 4-bit word length and were developed around 1970. Traditional (non-quantum) 4-bit computers are by now obsolete, while recent [[quantum computer]]s are 4-bit,<ref>{{Cite web|title=Qiskit/openqasm|url=https://github.com/Qiskit/openqasm|access-date=2020-11-16|quote=bigadder.qasm: Quantum ripple-carry adder. 8-bit adder made out of 2 4-bit adders from adder.qasm.|website=GitHub|language=en}}</ref><ref>{{cite arXiv|last1=Cross|first1=Andrew W.|last2=Bishop|first2=Lev S.|last3=Smolin|first3=John A.|last4=Gambetta|first4=Jay M.|date=2017-07-13|title=Open Quantum Assembly Language|class=quant-ph|quote=The only storage types of Open QASM (version 2.0) are classical and quantum registers, which are one-dimensional arrays of bits and qubits, respectively.|eprint=1707.03429}}</ref> but also based on [[
The first commercial microprocessor was the [[binary-coded decimal]] (BCD-based) [[Intel 4004]],<ref name="Mack_2005"/><ref name="Hofstra_History"/> developed for calculator applications in 1971; it had a 4-bit word length, but had 8-bit instructions and 12-bit addresses. It was succeeded by the [[Intel 4040]].
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