MMIX

Designer	Donald Knuth
Bits	64-bit
Introduced	1999
Design	RISC
Encoding	Fixed
Branching	Condition Code
Endianness	Big
Open	Yes, and royalty free
Registers
32 special-purpose registers
General-purpose	256

MMIX (pronounced em-mix) is a 64-bit reduced instruction set computing (RISC) architecture designed by Donald Knuth, with significant contributions by John L. Hennessy (who contributed to the design of the MIPS architecture) and Richard L. Sites (who was an architect of the Alpha architecture). Knuth has said that,

MMIX is a computer intended to illustrate machine-level aspects of programming. In my books The Art of Computer Programming, it replaces MIX, the 1960s-style machine that formerly played such a role… I strove to design MMIX so that its machine language would be simple, elegant, and easy to learn. At the same time I was careful to include all of the complexities needed to achieve high performance in practice, so that MMIX could in principle be built and even perhaps be competitive with some of the fastest general-purpose computers in the marketplace."^[1]

Knuth started the design of MMIX in 1999, and released the stable version of the design in 2011.^[2] The processor is numbered as "2009" with Knuth explaining that this is the arithmetic mean from the numbers of other computer architectures; as well as being "MMIX" in Roman numerals.^[3]