MIX Computer Overview and Fundamental Architecture

Key Insight

MIX is a mythical computer, designed to emulate 1960s and 1970s machines, offering a powerful yet simple machine language suitable for most algorithms. Its design aimed to preserve the simplest aspects of historic computers, making its characteristics easy to learn and assimilate for anyone with more than a casual interest in computing. Despite its utility, MIX is now obsolete and is slated for replacement in future editions by MMIX, a reduced instruction set computer (RISC) capable of 64-bit arithmetic, reflecting dominant machines of the 1990s.

A peculiar property of MIX, identified by the number 1009, is its simultaneous binary and decimal nature; programmers do not need to know the underlying arithmetic base, allowing algorithms to function with minimal changes across both types of machines. The basic data unit is a byte, capable of holding at least 64 distinct values (0 to 63 inclusive) and at most 100 values; this implies six bits on a binary computer or two digits on a decimal one. Programs are advised to assume no more than sixty-four values per byte and should function correctly regardless of specific byte size, with a computer word comprising five such bytes and an additional sign.

MIX features nine registers for computation and data management: the A-register (Accumulator) and X-register (Extension), both storing five bytes and a sign, often functioning as a 10-byte pair for arithmetic. Six I-registers (Index registers I1-I6) each hold two bytes and a sign, primarily used for counting and addressing. The J-register (Jump address) holds two bytes with an implicit positive sign, used for subroutine return addresses. Additional components include an overflow toggle, a three-state comparison indicator (LESS, EQUAL, GREATER), 4000 words of memory (numbered 0-3999, each five bytes and a sign), and various input-output devices.