Custom Single-Purpose Processor Design

Computer Organization & Embedded System — Hardware–Software Design Issues on Embedded System, NEC licence examination syllabus (Nepal Engineering Council).

Custom Single-Purpose Processor Design: building your own mini-CPU

The GCD calculator every embedded systems textbook uses — and for good reason.

What if, instead of buying a general CPU and writing software for it, you built a tiny circuit that does ONE calculation and absolutely nothing else — no operating system, no instruction fetch overhead, just pure dedicated logic? That's a single-purpose processor.

It's built from an FSMD (Finite State Machine with Datapath): the datapath does the actual computation (registers, comparators, adders), and the controller (an FSM) sequences when each datapath operation happens.

Worked example: designing hardware for GCD(x, y)

Algorithm (Euclid's method): while (x != y) { if (x > y) x = x − y; else y = y − x; } // result is in x (or y) when they become equal

Datapath for this needs: two registers (x, y), a comparator (x==y? and x>y?), and a subtractor. Controller (the FSM above) decides, each clock cycle, whether to compare or subtract, based on the current state.

💡 This exact GCD example (or a similar simple algorithm) is commonly asked as "design an FSMD for the given algorithm" — practice drawing both the state diagram AND the datapath block diagram.

Syllabus points

Combinational + sequential logic basis
Datapath + controller (FSMD)
Design steps: algorithm → FSMD → datapath + control

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