Part 09 of 12

Instructions and control

Define a tiny processor language and make every instruction physically executable.

Six connected lessonsA diagram per chapterStart from first principles

The story before the details

A general-purpose computer needs commands represented in exactly the same bits that it uses for numbers. This is possible because a pattern has meaning only in context. When fetched into the instruction register, fields of bits are interpreted as an opcode and perhaps operands. When fetched as data, the same pattern might be an integer.

An instruction set architecture is the rulebook specifying what those encodings do. Control circuitry implements that rulebook by producing select, enable, load and write signals. Some simple designs use ROM lookup tables to decide the signals for each opcode and microstep; other designs use hardwired logic. Both can obey the same ISA.

Timing is part of correctness. Signals do not propagate instantaneously. It would be a mistake to capture a RAM result before the address settles or to activate two incompatible bus drivers simultaneously. A clean control timeline makes the machine behave reproducibly, one action after another.

Microcoded control, diagram of connected labeled components — Microcoded control. Follow the arrows to identify where information travels and what each block contributes.

What you will discover

These six lessons are arranged so each one answers a question created by the previous one. Do not rush to the last lesson. If a concept feels opaque, use the checkpoint before continuing.

Keep this in mind

Define a tiny processor language and make every instruction physically executable. You should be able to explain each concept using only an input, a rule, an output, and any state that must be retained.

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