MMIX

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

MMIX (pronounced em-mix) is a 64-bit 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). In Knuth’s own words:

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.

MMIX is a big-endian 64-bit RISC, with 256 64-bit general-purpose registers, 32 64-bit special-purpose registers, fixed-length 32-bit instructions and a 64-bit virtual address space. The MMIX instruction set comprises 256 opcodes, one of which is reserved for future expansion. MMIX uses IEEE 754 floating-point numbers.

All instructions have an associated mnemonic. For example, instruction #20 (32) is associated with ADD. Most instructions have the symbolic form "OP X,Y,Z", where OP specifies the sort of instruction, X specifies the register used to store the result of the instruction and the rest specify the operands of the instruction. Each of these fields is eight bits wide. For example, ADD $0,$1,3 means "Set $0 to the sum of $1 and 3."

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