Week 28: TAOCP 1.3.1 Description of MIX

MIX, Knuth's imaginary 1960s-style computer: its words made of bytes and signs, registers, memory, comparison indicator, and the full instruction set. Every program in the original volumes is written for it.

Time About 7 hours this week

Why it matters

Reading and writing assembly for a simple machine teaches what high-level code costs. Even though modern hardware is different, the habit of counting memory references and instruction cycles is timeless.

Before you start

1.1 Algorithms
Some exposure to assembly or the TIS-100/EXAPUNKS games

How to approach it

Install a MIX simulator first (GNU MDK is the common choice). Keep the instruction summary chart open. For every instruction family, write a two-line program and run it to see the registers change. Field specifications like (0:5) and (1:3) confuse everyone at first; spend time on them.

Exercises

Do the 00 to 15 exercises to make sure you can read MIX fluently. Then write and actually run at least two of the 20-ish programming exercises in a MIX simulator (GNU MDK works). Reading assembly is not the same as writing it; the bugs you hit are the lesson.

Traps

Assuming bytes are 8 bits. A MIX byte holds at least 64 values and programs must not depend on its exact size.
Misreading field specs (L:R). Test them in the simulator.
Wondering whether to skip MIX for MMIX: Knuth is replacing MIX with MMIX (see the Vol 1 Fascicle 1 and his MMIX page), but the 3rd-edition text you are reading uses MIX, so learn MIX now.