Microinstruction Format

Computer Organization & Embedded System — Control and Central Processing Units, NEC licence examination syllabus (Nepal Engineering Council).

Microinstruction Format: decoding the control word

The exact bit-layout that makes a CPU tick.

If control memory is a lookup book, the microinstruction format is the grammar of every sentence in that book. In Mano's classic design, each microinstruction is split into distinct fields.

🔤 The Five Fields

F1, F2, F3Each selects one microoperation — like a small sentence with three verbs that can fire simultaneously.

CDCondition — which status bit to test for a branch.

BRBranch — what kind of jump to perform.

ADAddress — where to jump to.

The magic of splitting F1/F2/F3 into three separate fields is that up to three microoperations can execute in the very same clock cycle — as long as they don't conflict (e.g., you can't move two different values into the same register at once).

Example symbolic microinstruction: ADD: AC ← AC + DR (F1 field, an ALU microoperation) PC ← PC + 1 (F2 field, a control microoperation) → next address (AD field)

📏 Horizontal vs Vertical microprogramming

HorizontalA long control word with one bit per control signal. Very fast (no decoding needed) but uses lots of memory.

VerticalEncodes control signals compactly (like a mini opcode), needing a decoder to expand them. Saves memory, but adds a decoding delay.

💡 A guaranteed question type: "convert this symbolic microinstruction to its binary microinstruction" using a given field-encoding table. Practice at least 3-4 of these from Mano's textbook.

Syllabus points

Fields: F1, F2, F3, CD, BR, AD
Symbolic → binary microinstruction
Horizontal vs vertical microprogramming

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